Developing a S ystem to Study the Dynamics of the Heterolysis of PSubstituted Radicals in terms of Magnetic Field Effects
by
Elaine K. Adams
Submitted in partial fulfiiIlment of the requirements for the degree of Master of Science
Dalhousie University Halifax, Nova Scotia September, 1998
@ Copyright by Elaine K. Adams, 1998 National hirary Bibliothèque nationale du Canada Acquisitions and Acquisiins et Biliograpfii Services seMces bibliographiques
The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seli reproduire, prêter, distriiuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de micro fi ch el^ de reproduction sur papier ou sur fonnat électronique.
The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fkom it Ni la thése ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. Table of Contents
List of Figures ...... vi ... List of Tables ...... xm
1.1 General Introduction ...... -1 1.2 Introduction to the Radical Pair Model ...... 2 1.2.1 GeneraI MiceUar Characteristics ...... 4 1.3 Observation of Magnetic Field Effects ...... 6 1-4 Magne tic Field Effec ts on PHeterolysis Reac tions ...... 12 1-5 Scope of Thesis...... 15 Chapter 2 Heterolytic Cleavage for p-Substituted Radicals ...... 16 2.1 General Introduction ...... 16 2.2 Laser Photolysis of 2-Chloro- 1-(4-methoxyphenyl)ethyl acetate...... 18 2.3 Laser Photolysis of 2-Chloro- 1-(4-methoxypheny1)propyl acetate...... -21 2.4 Laser Photolysis of 2-Bromo- 1-(4-methoxypheny1)ethyl acetate...... -23 2.5 Laser Photolysis of 2-Bromo- 1-(4-methoxypheny1)propyl acetate ...... 25 2.6 Discussion ...... 27 Chapter 3 Reactivity of Radical Cations with Radicals ...... 33 3- 1 GeneraI Introduction ...... 33 3.2 Laser Photolysis of Anethole in Acetonitrile ...... 35 3.2.1 Laser Photolysis with Benzyl bromide ...... 36 3.2.2 Laser Photolysis with 3-Cyanobenzyl brornide ...... 42 3.2.3 Laser Photolysis with Diphenylmethyl chloride...... 46 3.2.4 Identification of the Transient at 345 nm ...... 49 3.3 Laser Photolysis of Anethole in SDS ...... 56 3.3.1 Laser Photolysis with Benzyl bromide ...... 57 3.3.2 Laser Photolysis with 3-Cyanobenzyl bromide ...... 61 3.3.3 Laser Photolysis with Toluene and 3-Cyanotoluene...... 64 3.4 Laser Photolysis of Anethole in CTAB ...... 69 3.4.1 Laser Photolysis with Benzyl bromide ...... -70 3.5 Laser Photolysis of p-Methoxystyrene in Acetonibile ...... 72 3.5.1 Laser Photolysis with Benzyl bromide ...... 74 3.5.2 Laser Photolysis with 3-Cyanobenzyl bromide ...... 78 3.5.3 2-Bromo- 1-(4-meth0xyphenyl)ethyl Radical Formation ...... 80 3.6 Irradiation of ~Methoxystyrenein SDS...... 83 3.6.1 Laser Photolysis with Benzyl bromide ...... 84 3.6.2 Laser Photolysis with 3-Cyanobenzyl bromide ...... 88 3.6.3 Laser Photolysis with Toluene and 3-Cyanotoluene ...... 91 3.7 Irradiation of p-Methoxystyrene in CTAB ...... 94 3.8 Discussion...... 97 3.8.1 Homogeneous media: Acetonitrile ...... 97 3.8.2 Heterogeneous medi a...... IO1 Chapter 4 Preliminary Work in Radical Pair Generation ...... 105 4.1 General Introduction ...... -105 4.2 Addition of -CH2Br and -CH20COPh to Dibenzylketone...... -106 4.3 Addition of Formaldehyde to Dibenzyketone ...... 107 4.4 Preparation of 1,3~bis(4.methoxyphenyl)~2~propanone...... IO8 4.4.1 Addition of -CH2Br and -CH20COPh ...... 109 4.4.2 Addition of Formaldehyde ...... 109 4.5 Laser Expe~hents...... Il0 4.5.1 Magnetic Field Trial ...... 111 4.6 Future Work ...... 114 Chapter 5 Experimental ...... 115 5.1 Materials ...... 115 5.2 General Instmtmention ...... 115 5.3 Laser Flash Photolysis...... 116 5.3.1 Equipment ...... -116 5.3.2 Data Processing ...... 117 5.4 Sample Preparation...... 117 5.4.1 p-Heterolysis studies ...... 117 5.4.2 Photoionkation studies ...... 118 5.5 Laser Photolysis with an Applied Magnetic Field ...... 120 5.5.1 Sample Preparation ...... 120 5.5.2 Apparatus ...... -121 5.6 Syuthetic Procedures ...... 121 References ...... 126 List of Fimes
Figure 2- 1. Transient absorption spectnim of 2-chloro- 1-(4-methoxypheny1)e thyl acetate in nitrogen-saturated 0.1 M SDS recorded 76 (a), 128 (O) 224 (a),and 368 (Ci) ns after the laser pulse. Inset shows the tirne-resolved growth at 370 nrn under nitrogen (a) and oxygen-saturated (O)conditions...... 20
Figure 2-2. Transient absorption spectrum of 2chloro- L -(4-methoxypheny1)ethyl acetate in nitrogen-saturated 0.05 M CTAB recorded 2 10 ns (a),540 ns (O), 920 ns (a), and 1.86 ps ((O) after the laser pulse. Inset shows the tirne-resolved growth at 370 nm under nitrogen (8)and oxygen-saturateci (O) conditions...... -20
Figure 2-3. Transient absorption spectrum of 2cbioro- 1-(4-methoxypheny1)propyl acetate in nitrogen-saturated 0.1 M SDS recorded 70 (@), 148 (O), 232 (m), and 564 (0)ns afier the laser pulse. Inset shows. . the tirne-resolved growth at 390 nrn under nitrogen (a) and oxygen-saturated (O) conditions...... 22
Figure 24. Transient absorption spectmm 2chloro- 1-(4-methoxypheny1)propyl acetate in nitrogen-saturated 0.05 M CT'AB recorded 208 ns (a), 552 ns (O), and 1.29 ps (i)after the laser pulse. Inset shows. . the the-resolved growth at 390 nrn under nitrogen (a) and oxygen-saturated (O) conditions...... 22
Figure 2-5. Transient absorption spectnim of 2-bromo- 1-(4-methoxypheny1)ethyl acetate in nitrogen-saturated 0.1 M SDS recorded 120 (a), 308 (O), and 548 (i)ns after the laser pulse. Inset shows decay trace at 370 nm under N2 (a) and 02(O) conditions...... 24
Figure 2-6. Transient absorption spectrum of 2-bromo- 1-(4-methoxypheny1)ethyl acetate in nitrogen-saturated 0.05 M CTAB recorded 130 (a),300 (O), and 670 (i) ns after the laser pulse. Inset shows decay trace at 350 nm under N;! (e)and 02 (0)conditions.. ... 24
Figure 2-7. Transient absorption spectrum of 2-bromo- 1-(4-methoxypheny1)propyl acetate in nitrogen-saturated 0.1 M SDS recorded 60 (a), 192 (O), 408 (W), and 640 (0)ns after the laser pulse. Inset shows the decay trace rnonitored at 390 nm under nitrogen (a) and oxygen-saturated (O) conditions...... 26
Figure 2-8. Transient absorption spectnim of 2-bromo- 1-(4-methoxypheny1)propyl acetate in nitrogen-saturated 0.05 M CTAB recorded 130 (a),30 (O), and 670 (i) ns after the laser pulse. Inset shows .the . decay trace monitored at 390 nm under nitrogen (a) and oxygen-saturated (O)conditions...... 26 Figure 3- 1. Transient absorption specûum generated upon 308-nrn irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitde recorded 560 ns (a), 1.88 ps (O), 5.72 ps (W), and 12.6 ps (O) after the laser pulse. Inset shows decay traces monitored at 600 nm under nitrogen (a) and oxygen-saturated (O) conditions...... 36 Figure 3-2. Transient absorption spectmm generated upon 308-nm irradiation of anethole (0.42 mM) in nitmgen-saturateci acetonitriie with benzyl bromide (12.9 mM) recorded 560 ns (O), 1.88 ps (O), 6.12 ps (m), and 12.6 1s ((O) after the laser pulse. Inset shows decay traces monitored at 3 18 nm under nitrogen (0)and oxygen-saturated (O) conditions...... 39 Figure 3-3. Transient absorption specûum generated upon 308-~iinadiation of anethole (0.42 mhî) in oxgyen-saturateci acetonitrile with benzyl bromide (12.9 mM) recorded 560 ns (e),1.88 ps (O), 6.12 ps (H), and 12.6 ps (0)after the laser pulse. Inset shows decay traces monitored at 345 nm under nitrogen (0)and oxygen-saturated (O) conditions...... 39
Figure 3-4. Decay traces monitored at 3 18 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-sahirated acetonitrile in the presence of benzy1 bromide at concentrations of O (a),3.2 (O), 6.5 (a),9.7 (O), and 12.9 (*)rnM ...... 40 Figure 3-5. Plot of the optical density monitored at 318 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitriIe vernis the concentraiion of benzyl bromide...... ,.,, ...... 40
Figure 3-6. Decay traces monitored at 600 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile in the presence of benzyl bromide at concentrations of O (a) and 12.9 (O) mM...... 41 Figure 3-7. Plot of the opticai density monitored at 345 (O) and 600 (a) nm after 308-nm irradiation of anethole (0.42 mM) in oitrogen-sanirated acetonitrile venus the concentration of benzyl bromide...... -41 Figure 3-8. Decay traces monitored at 345 nm upon 308-11. irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile in the presence of benzyl bromide at concentrations of O (a), 3.2 (O),6.5 (m), 9.7 (D), and 12.9 (+) mM ...... 42 Figure 3-9. Transient absorption spectrurn generated upon 308-nrn irradiation of methole (0.42 mh.0 in nitrogen-saturated acetonitrile with 3-cyanobenzyl bromide ( 12.3 mM) recorded 560 ns (a), 1.88 ps (O), 6.12 ps (8),and 12.6 ps ((O) after the laser pulse. Inset shows decay traces monitored at 330 nm under N;! (0)and 02(O) conditions...... 44
Figure 3- 10. Transient absorption spectnun generated upon 308-nm irradiation of anethole (0.42 mM) in oxygen-sahuated acetonitrile with 3-cyanobenzyl bromide ( 12.3 mM) recorded 560 ns (O), 1.88 ps (O), 6.12 ps (m), and 12.6 ps (O)after the laser pulse. hset shows decay traces monitored at 345 nm under N;! (a) and 02(O) conditions...... -44 Figure 3-1 1. Decay traces monitored at 345 nrn upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile in the presence of 3-cyanobenyl bromide at concentrations of O (O),2 (O), 6 (i),and 12.3 (0)mM...... 45
Figure 3-12. Plot of the optical density monitored at 345 (O) and 600 (a) nm after 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile versus the concentration of 3cyanobenzyl bromide...... -.-..-45
Figure 3- 13. Decay traces monitored at 600 MI upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturateci acetonitrile in the presence of 3-cyanobenzyl bromide at concentrations of O (a) and 12.3 (0) mM...... -46 Figure 3-14. Transient absorption spectmm generated upon 308-nm irradiation of anethole in nitrogen-saturated acetonitde with diphenyhethyl chloride (0.9 mM) at 360 os (a),840 ns (O), 7.68 ps (W), and 12.9 p (O) after the laser puise...... -48
Figure 3-15. Decay traces monitored at 330 nm upon 308-nm irradiation of anethole (0.42 mM) in nitmgen-sa~uatedacetonitrile in the presence of diphenyhethyl chloride at concentrations of O (O), 0.8 (O), 1.7 (m), 2.5 (a),3.3 (*),and 4.1 (A) mM...... -48 Figure 3-16. Plot of the optical density monitored at 330 (0)and 390 (a) mn upon 308- nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile versus the concentration of diphenyimethy1 chloride ...... -49 Figure 3- 17. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile with benzyi chioride (12.9 mM) recorded 560 ns (a),1.88 ps (O), 6.12 ps (m), and 12.6 ps ((O) after the laser pulse ...... 54
Figure 3-18. Decay traces monitored at 600 nm for the anethole radical cation upon 308- nm irradiation of anethole (0.42 mM) with benzyl bromide (12.9 mM) in nitrogen-saturated acetonitrile with O (a),4 (O), and 12 (m) x10-5 M of chlonde anion added...... 54 Figure 3-19. Transient absorption specmgenerated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturateci acetonitde with ammonium bromide (0.12 mM with 0.2% water) recorded 480 ns (e), 1.80 ps (O), 6.04 ps (i),and 12.6 ps (O)after the laser pulse. Inset shows the decay traces monitored at 345 nm under nitrogen (a)and oxygen- saturated (O) conditions...... -55 Figure 3-20. Plot of observed fit-order decay rate constants (O)for the anethole radical cation monitored at 600 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen- saturated acetonitde versus the concentration of bromide anion...... 55 Figure 3-2 1. Transient absorption spectm generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS recorded 560 ns (a),2.76 p.s (O), 7.60 ws (a), and 13.4 ps (O)after the laser puise...... 57 Figure 3-22. Transient absorption specmim generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with benzyl bromide (7.94 mM) recorded 560 ns (O), 2.76 ps (O), 7.60 ps (i),and 13.4 ps (CI) after the laser pulse. Inset shows the decay traces monitored at 3 18 nm in the presence of benzyl brornide at concentrations of O (0)and 7.94 (.) rnM ...... *...... 59
Figure 3-23. Decay traces monitored at 700 nm upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-sahirated 0.1 M SDS with O (e), 1.98 (O), and 7.94 (m) rnM of benzyl bromide added...... 59
Figure 3-24. Plot of the optical density monitored at 390 (0)and 700 (e) nm upon 308- nm irradiation of anethoIe (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide...... 60 Figure 3-25. Plot of observed first-order decay rate constants monitored at 700 nm (O) for the electron upon 308-nm inadiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide...... 60 Figure 3-26. Transient absorption speceum generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with 3-cyanobenzyl bromide (7.76 mM) recorded 440 ns (e),2.32 ps (O), 6.96 p (i),and 12.8 p (O) after the laser pulse. ... 62
Figure 3-27. Decay traces monitored at 700 nm upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS in the presence of 3cyanobenzyl bromide at concentrations of O (a), 1.94 (O),and 7.76 (m) mM...... 62
Figure 3-28. Plot of the optical density monitored at 390 (0)and 700 (a)nm upon 308- nm irradiation of ifnethole (0.4 mM) in nitrogen-sanirated 0.1 M SDS versus the concentration of f-cyanobenzyl bromide...... 63
Figure 3-29. Plot of observed fmt-order decay rate constants monitored at 700 nm (0)for the electron upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3-cyanobenzyl bromide...... 63 Figure 3-30. Transient absorption spectrurn generated upon 308-MI irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with toluene (8.8 mM) recorded 440 ns (a), 2.32 ps (O),6.96 ps (i),and 12.8 ps (m) after the laser pulse...... 66 Figure 3-31. Plot of the opticai density monitored at 390 (O)and 700 (a)nm upon 308- nm inadiation of anethole (0.4 mM) in aitrogen-saturated 0.1 M SDS versus the concentration of toluene...... -66 Figure 3-32. Plot of observed fmt-order decay rate constants monitored at 700 nm (O) upon 308-nm Uradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of toluene...... -67 Figure 3-33. Transient absorption spectrum generated upon 308-MI irradiation of anethole (0.4 rnM) in nitrogen-saturated 0.1 M SDS with 3-cyanotoluene (5.6 mM) recorded 560 ns (a), 2.76 ps (O), 7.60 ps (M), and 13.4 ps (O) after the laser pulse. Inset shows decay traces monitored at 320 nm under nitrogen (O) and oxygen-saturated (0)conditions...... 67 Figure 3-34. Plot of observed fmt-order decay rate constants for the electron monitored at 700 nrn (O) upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3cyanotoluene ...... -68
Figure 3-35. Plot of the optical density monitored at 320 (a)and 390 nm (O) upon 308- nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3-cyanotoluene...... 68 Figure 3-36. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.27 mM) in nitrogen-saturated 0.05 M CTAE3 recorded 400 ns (a), 1.60 ps (O), 4.48 ps (m), and 10.8 ps (O) after the laser pulse...... 70 Figure 3-37. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.27 mM) in nitrogen-sanirated 0.05 M CïAB with benzyl bromide (7.94 mM) recorded 40us (a), 1.60 ps (O), 4.48 ps (a), and 10.8 ps (U) after the laser pulse. Inset shows decay traces monitored at 3 18 nm in the presence of benzyl brornide at concentrations of O (O) and 7.94 (0)mM...... -71
Figure 3-38. Plot of the optical density monitoring at 390 (@) and 700 (0) nm upon 308- nm irradiation of anethole (0.27 mM) in nitrogen-saturated 0.05 M CTAB versus the concentration of benzyl bromide...... 72 Figure 3-39. Transient absorption spectnim generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in oitrogen-saturated acetonitrile recorded 320 ns (a), 1-30 ps (O), 3.80 ps (w), and 10.1 p (O) after the laser puise...... 74
Figure 340. Transient absorption spectnim generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with benzyl bromide (12.9 mM) in nitmgen-saturated acetonitrile recorded 320 ns (a). 1.36 p (O), 3.80 ps (i)), and 10.1 ps @) after the laser pulse. Inset shows 3 18 rn traces under N2 (a)and 02(O) conditions...... 76 Figure 341. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with benzyl bromide (12.9 mM) in oxygen-saturated acetoniaile recorded 320 ns (O), 1.36 ps (O), 3.80 p (W), and 10.1 ps (O) after the laser pulse. Inset shows 330 nm traces under N2 (a) and 02(0) conditions...... 76 Figure 3-42. Plot of the optical density monitored at 3 18 nm upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitmgen-sanuated acetonit.de versus the concentration of benzyl bromide...... 77
Figure 3-43. Plot of the optical density monitored at 330 (O)and 600 (@) nm upon 308- nrn irradiation of p-methoxystyrene (0.75 mM) in nitrogen-saturated acetonitrile versus the concentration of benzyl bromide...... 77 Figure 344. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with 3cyanobenzyl bromide ( 12.3 mM) in nitrogen-saturated acetonitrîle recorded 320 ns (a)),1.36 ps (O).3.64 p (i), and 9.68 ps (O) after the laser pulse. Inset shows 330 nrn traces under N2 (@) and 02(O) conditions...... 79 Figure 3-45. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 rnM) with 3cyanobenzyl bromide (12.3 mM) in oxygen-saturated acetonitrile recorded 320 ns (a), 1.36 ps (O), 3.64 ps ((., and 9.68 ps (O) after the laser pulse. Inset shows 370 nm traces under N;! (e)and 02 (0)conditions...... -79 Figure 3-46. Plot of the optical density monitored at 600 nm upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-sanirateci acetonitrile versus the concentration of 3- cyanobenzyl bromide...... 80
Figure 3-47. Decay traces monitored at 600 nm for the radical cation upon 308-nm irradiation of p-methoxystyrene (0.75 mM) and beozyl bromide (12.9 mM) in nitrogen- saturated acetonitriie with O (e),6 (O), and 12 (m) x 10-~M of chloride anion...... 82 Figure 3-48. Transient absorption spectnim generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated acetonîtde with ammonium bromide (0.47 mM) recorded 320 ns (a), 1.36 p (O), 3.80 ps (i),and 10.1 ps ((O after the laser pulse. Inset shows 330 nm haces under N;! (e)and 02 (0)conditions...... 82
Figure 349. Plot of observed fmt-order decay rate constants monitored at 6ûû nm (O) for the radical cation upon 308-nm irradiation of p-methoxystyrene (0.75 mM) in nitmgen- saturatecl acetonitrile versus the concentration of bromide anion...... -83 Figure 3-50. Transient absorption spectrum generated upon 308-nm Vradiation of p- methoxystyrene (0.75 mM) in nitrogen-sahirated 0. l M SDS recorded 320 ns (a), l. 12 ps (O), 2.88 ps (m), and 6.92 p ((O) after the laser pulse...... 84 Figure 3-51. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS with ben@ bromide (7.94 mM) recorded 320 ns (a), 1.12 p (O), 2.88 ps (m), and 6.92 pi (O)after the laser pulse.
Figure 3-52. Decay traces monitored at 700 nm upon 308-nrn irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS in the presence of benzyl brode at concentrations of O (O), 2 (O), and 7.9 (W) mM...... 86
Figure 3-53. Plot of the opticai density monitored at 700 (e)and 370 (0)nm upon 308- nm irradiation of p-methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide...... 87
Figure 3-54. Plot of observed fmt-order decay rate constants monitored at 700 nm (O) for the eiectron upon 308-nm inadiation of p-methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide...... 87 Figure 3-55. Transient absorption spectmm generated upon 308-am irradiation of p- methoxystyrene (0.75 mM) with 3cyanobenzyl bromide (10.9 mM) in nitrogen-saturated 0.1 M SDS recorded 320 ns (a), 1.12 vs (O), 2.88 ps (m), and 6.92 ps (O) after the laser pulse...... 89
Figure 3-56. Decay traces monitored at 700 nrn upon 308-m irradiation of p- methoxystyrene (0.75 mM) in nitrogen-sahuated 0.1 M SDS in the presence of 3- cyanobenzyl bromide at concentrations of O (a), 2.72 (O),and 10.9 (a)M...... 89
Figure 3-57. Plot of the optical density monitored at 700 (a)and 370 (O) nm upon 308- am irradiation of p-methoxy styrene (0.75 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3cyanobenzyI bromide...... -90 Figure 3-58. Plot of the observed fmt-order decay rate constants monitored at 700 nm (O) for the electron upon 308-nm irradiation of p-methoxystyrene (0.75 mM) in nitmgen- sahirated 0.1 M SDS versus the concentration of 3-cyanobeql bromide...... 90 Figure 3-59. Transient absorption specm generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with toluene (8.8 mM) in nitrogen-saturated 0. l M SDS recorded 320 ns (a),1.12 p (O), 2.88 ps (a), and 6.92 ps (Ci)after the laser pulse. .. -92 Figure 3-60. Transient absorption spectnun generated upon 308-nrn irradiation of p- rnethoxystyrene (0.75 mM) in nitmgen-saturated 0. l M SDS with 3cyanotoluene (5.63 mM) recorded 320 ns (a), 1.12 ps (O), 2.88 ps (i), and 6.92 p (D) after the laser pulse. Inset shows decay traces monitored at 320 nm under N2 (O) and 02 (O) conditions.. ... -93
Figure 3-61. Plot of the optical density monitored at 320 (e) and 370 (O) nm upon 308- nm irradiation of pmethoxystyrene (0.75 mM)in nitrogen-saturated 0.1 M SDS versus the concentration of 3cyanotoluene...... -93 Figure 3-62. Plot of observed fmt-order decay rate constants monitored at 700 nm (0)for the electmn upon 308-nm irradiation of p-methoxystyrene (0.75 mM) in nitrogen-sanirated 0.1 M SDS versus the concentration of 3cyanotoluene...... 94 Figure 3-63. Transient absorption spectnim generated upon 308-nm irradiation of p methoxystyrene (0.75 mM) in nitmgen-saturated 0.05 M CTAB recorded 320 ns (a), 1.36 ps (O), 4.08 ps (i),and 10.9 ps (O) after the laser pulse. Inset shows the decay trace rnonitored at 600 m...... 96 Figure 3-64. Transient absorption spectnim generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in oxygen-saturated 0.05 M CTAB recorded 320 ns (a), l .36 ps (O), 4.08 ps (i), and 10.9 ps ((O) after the laser pulse. lnset shows the decay mce monitored at 370 nm under nitrogen (a) and oxygen-saturated (0) conditions...... 96 Figure 4- 1. Transient absorption specûum generated with 266-nm irradiation of 13- dihydroxy-2,4-diphenyl-3-pentanone(0.85 mM)in nitrogen-saturated 0.1 M SDS recorded 4.40 (O), 17.6 (O), 47.6 (m), and 123 (0)ps after the laser pulse. hset shows decay traces monitored at 320 nm under nitrogen (O) and oxygen-saturated (O) conditions.. ...Il 1 Figure 4-2. Decay traces monitored at 3 18 nm upon 308-nm irradiation of dibenzyketone (3.6 mM) in nitrogen-saturated 0.1 M SDS in the absence (O) and presence (a) of a magnetic field...... -113 Figure 4-3. Decay traces monitored at 320 nm upon 308-MI irradiation of 1,5-dihydroxy- 2.4-diphenyl-3-pentanone (0.85 mM) in nitmgen-saturated 0.1 M SDS in the absence (O) and presence (O) of a magnetic field...... -113 Figure 5- 1. The general layout of a typical nanosecond laser flash photolysis apparatus.
Figure 5-2. Transient traces obtained with laser flash photolysis where the transient is shown to decay (a) or grow-in @) after the laser pulse...... Il7 List of Tables
Table 1- 1. The fodaand name of some common synthetic surfactants...... 6
Table 2-1. The observed first-order rate constants obtained for the the-resolved growth of the radical cations from B-substituted radicals generated upon 266-nm irradiation of the appropriate precursors in niaogen-saturated 0.1 M SDS and 0.05 M CTAB and water. ..27
Table 3-1. Free energy change associated with electron -fer (AGET) nom the olefuis to benzyl bromide and 3-cyanobenzyl bromide calculated using the Weiler equation...... 99
Table 5-1. Typical volumes of stock solutions used in experiments for fadicd cation generaîion fkom photoionization of olefh derivatives in the presence of radical precursors and toluene in miceUar media...... -120 Magnetic field effects have been weU-established in chemical reactions involving radical pairs. However, few s~idieshave focused on radical systems of biological significance. With a possible link between electrornagnetic fields in the environment and health effects, the need to snidy this type of systern is emphasized. The P-heterolysis of P-substituted radicds is an important reaction due to the occurrence of $-substituted radicals in biological processes. Thus, B-heterolysis is an ided reaction to probe the possible iink of exposure to electromagnetic fields and adverse health effects. In order to investigate the influence of magnetic fields on the heterolysis reaction, appropriate substituted diarylketones have to be prepared From these precursors, a radical pair will be generated upon laser irradiation in which one mernber contains an akyl moiety with a leaving group in the p-position. This shidy wiU encompass an investigation of heterolytic rate constants and the reactivity of radical cations with radicals as well as initial work in synthetic preparation. Several fbsubstinited radicals have been incorporated into miceuar media and the heterolytic reaction monitored with rate constants on the average of 106- IO* s-1 king measured. It was determined that Cl- was a better leaving group than Br for the study of magnetic field effects on the heterolysis reaction due to the fact that the rate constants could only be resolved on the nanosecond laser system with Cl- as the leaving group. In addition, the reactivity of radical cations with radicals has been investigated in both homogeneous and heterogeneous solutions. Photoionization of olefms generates an electron which cm be scavenged by arylallcyl halide compounds to facilitate a dissociative cleavage of the C-halide bond to generate a radical. In acetoniaile, the halide anion generated with the radicals upon trapping of the electron reacted with the oleh radical cation to generate the corresponding phalide radical. In aqueous micella. solution, the changes associated with the radical cation are less dramatic. It is concluded that the confinement of the radical cation and radical withn the micellar cavity does not hhder the observation of the radical cation; therefore, the yield of radical cation in the presence and absence of a magnetic field should not be influenced by a radical partner. Preliminary synthetic work has been carried out. The preparation of 1,5-dihydroxy-2,4-diphenyl-3- pentanone has been accomplished to date and prelimuiary work with magnetic fields indicate that the effects are similar to dibenzylketone.
xiv Abbreviations
Singlet = S Triplet = T k, = rate constant for escape hmthe micellar cavity kIsc = rate constant for intersystem crossing T+,T-, T, = the triplet subleveIs B = external magnetic field SDS = sodium dodecyl sulfate crAB = cetyltnmethylammoniurn bromide kd-~ = decay rate constant for a nidical pair DBF = 2.3,6,7-dibenzyofluorene AC = alterna- current DC = direct current EAL = ethanolamine ammonia lyase ~doCblm= adenosylcob(m)alamin Co0= cob(lï)aiamin radical HRP = horseradish peroxidase khe, = rate constant for heterolytic cleavage An = Cmethoxyphenyl Anethole = 1-(4-methoxypheny1)propene DBK = dibenzylketone THF = teaahydroh Dr. F. L. Cozens - for ailowing me the opportunity to join her laboratory and for comments and advice in the preparation and writing of this thesis. Dr. N. P. Schepp - for aU bis assistance and suggestions.
Dalhousie University for fimding of the graduate program undertaken here.
Dr. L Pincock and Dr. D. Arnold - a thank you for their assistance with laboratory needs and for agreeing to be on the thesis cornmittee.
To Melanie, Wendy, Leslie, Christa, and Suzanne - labmates, present and past, that have be there in dark and not so dark times! Good luck.
1also have to mention a few names that have given tremendous support and help that has dowed me to get through this: Natasha and the entire Moxon family for all their support and love. And Scott for his support, love, and fnendship. And £inally to everyone, fnends and family, who have played a part in getting me to this point.. .a th& you Cha~ter1 Introduction
1.1 General Introduction
In the 20th century, our world has become one in which we are surrounded by technology. Televisions, cellular phones, and computers aUow us to obtain information at our hngertips and appliances such as microwaves are commonplace. However, the same technology places sources of electromagnetic radiation in our environment. It is therefore with a vested interest that people assume low levels of exposure to magnetic fields in the environment are safe and the benefits to manufacturers of such an assumption are enormous. However, the potential Iink of electromagnetic fields to adverse health effects is the subject of great controversy. 1-3 Media attention of reported "cancer clusters" has fueled speculation that there may indeed be a link to living next to high sources of electromagnetic radiation, such as power hes, and the occurrence of ceriain cancers. However, obtaining scientfic data and establishg a causal relationship between exposure to electromagnetic fields and health problems remah difficdt. With a wide range of health issues and implications, such as preventive rneasures for cancer and stricter safety standards for technology, the need for a sound model by which magnetic fields can interact with biological processes is ernphasized. In fact, the fadt with many epidemiological studies is the Iack of ciear models from which interpretations and theories can be tested.2~4~5One possible mechanisrn for magnetic field effects is described by the radical pair model (vide infra). The radical pair model is weli understood4-1l and links magnetic field effects to the influence on chernical reactions involving '%e radicals" .45vl Free radicals are known to be prevalent in biological processes ranghg from cancer to aging.5 Therefore, physiological processes Uicluding enymatic systems which have fiee radicals as intermediates may be subject to magnetic field effects under this rnodel.12 The examination of systems with biological devance is needed to evaluate the scientifïc validity of the possible link between exposure to electromagnetic fields in the environment and heakh pro blems.
1.2 Introduction to the Radical Pair Mode1
The focus of the radical pair mode1 is the influence of magnetic fields on the dynamics of radical pairs. Each member of a radical pair has a spin associated with the lone electron. When the radicals are paired in close spatial contact, the spins becorne correlated. A radical pair that is spin-comlated cm either be in the singlet (S) or triplet O configuration which would denote antiparallel or parallel spins, respectively. Within the triplet state, product
formation is spin forbidden; however, singiet radicai pairs can interact to form products. In homogeneous solutions, radical pair interactions are short-lived. However, within conhed media such as miceilar or in vivo, the duration of the radical-radical encowiters is pr~longed.~The extended iifetime of the mdical pair provides an oppominity for a cornpetition between product formation and escape £iom the pair to be established. Therefore, within a heterogeneous environment such as micelles, a triplet radical pair cm convert to a singlet radical pair by intersystem crossing, thereby enabling product formation, or separate by the escape of one member of the radical partnenhip, Scheme 1- 1.4~5~13The decay rate constant of the radical pair has a component for both escape, b,,, and intersystem crossing, kIsc Escape of one member of the radical pair generates '% radicals" whereas intersystem crossing results in prodüct formation by radical-radical coupling. Scheme 1-1
Triplet
I ] = miceUar confmement
The idluence of an extemal magnetic field on the radical pair depends on how the field affects the rate constant for intersystem crossing of the triplet pair to a singlet pair, k~sc-The conversion of the triplet to the singlet manifold is known to be slowed with the introduction of a magnetic field.46-9-l However, there is no apparent eEect on the rate constant of escape fiom the heterogeneous system. With respect to the radical pair model, slowing down intersystem crossing wiIl increase the lifetime of the triplet radical pair. The length of time that the radical pair is unreactive with respect to product formation is increased; therefore, the probability of escape will increase. The increased probability of escape will increase the concentration of free radicals that remain after the geminate processes which may ultimately be hamiful. Furthemore, the combined decay rate constant for the triplet radical pair wiIl be reduced reflecting the decrease in the intersystem crossing component upon the application of an applied magnetic field.
The explanation for why magnetic fields of moderate strength can slow down the rate of intersystem crossing is offered by Zeeman splining4-1l of the energies of the triplet sublevels (T-,T,, Tc),Scheme 1-2. In the absence of an extemal magnetic field (B), the triplet sublevels are degenerate. However, upon the application of a magnetic field, the triplet sublevels differentiate in ierms of energy. As the separatioa between Tc (or T.) and T, increases, intersystem crossing slows down and eventudy shuts off completely fiom these triplet sublevels. Scheme 1-2
However, the energy gap between T, to S remains unchanged which dows intersystem crossing to proceed but ody fiom T, triplets. Consequentiy, the number of triplets that can
convert to the singiet manifold is dramaticaIly reduced. Due to the substantial decrease in intersystem crossing, a reduction in product formation and a subsequent increase in free radical concentration is expected To observe a magnetic field effect, the radical pairs must be spatiaiiy confined in order for a cornpetition between intersystem crossing and escape to be established.4J4J5 If either
of these processes occurs more rapidly than the other, the innuence of applied magnetic fields may be too minor to observe experimentally. The confimement of fadical pairs in
miceilar media is often employed for this purpose and provides an oppomuiity to mimic in vivo conditions.16 This is aiso advantageous in researching the link between magnetic fields and adverse health effects.
1.2.1 General Micellar Characteristics
Surfactants are the monomeric version of coiloidal particles known as rnicelles.~6J7 Surfactants have two distinct moieties which characterize their nature. An allcyl chah of considerable length (e.g. octyl or decyl) establishes the hydrophobie nature of the surfactant while an ionic, nonionic or zwitterionic component, dubbed the head group, is responsible for the property of hydrophilicity. The duality between the hydrophobic and hydrophilic segments in the composition of the surfactant is displayed in the tendency of the surfactant to form micelles when dissolved in a polar solvent where water is often the preferred choice. The characteristics of the surfactant are retained within each individual micelle. Therefore, the micelles which are often depicted in a sphencal manner have two main structural regions designated the interior and Stem Iayer, Scheme 1-3.
Scheme 1-3
monomeric surfactant
-hydrophobic chah miceUe + fkee surfactant t O hy drophilic head group 0 counterion
The interior of the miceile is formed by the hydrophobic aiky1 chains of the surfactant. Due to their inherent aversion to water, the aikyl chains prefer to avoid interaction with the polar solvent and tend toward self-association. The radius of the miceile is therefore linked to the Iength of the surfactant -1 chain.16 Furthemore, the ai@[ chains of the surfactant will pack randomly and exposure to water is minimized but not eliminated. In contrast, the polar head groups tend toward the surface due to their strong affinity for water creating an interface between the intenor of the micelle and the bulk of the solvent. The interface is referred to as the Stern layer whose thickness is proportional to the size of the particular head group associated with the surfactant.16 If the polar component is ionic in nature, the repulsion between the charged groups will be offset by corresponding counterions of the surfactant In addition, these oppositely charged ions associated with the hdgroups within the Stem layer will p&dy neutralize the charge associated with the micelles. The types of surfactant are diverse and variety is found both in the length of the aikyl chah and the particular head group wbich defines the surfactanfl6 Table 1-1.
Table 1- 1. The fodaand name of some common synthetic surfactants. Formula Surfactant Name C 12mOS03-Na+ sodium dodecylsulfate, SDS C 14H290S03-Na+ sodium tetradecylsulfate C 1&9N+(CH3)3Br tetradecyltrimethylammonium bromide C 1&3N+(CH3)3Br cetyltrimethylammonium bromide, C'T'Al3
Due to the creation of a hydrophobic "packet" within the polar solvent of water, micellization enhances the solubiiity of nonpolar solutes. The incorporation of solutes within the confines of the micellar aggregate is iinked to the abiiity of micelles to affect the rate of reactions and equilibria In addition, dose confinement of rnolecular species is the micellar characteristic exploited in research that investigates the potential link between extemal magnetic fields and heaith effects.
1.3 Observation of Mapetic Field Effects
Much of the research involving magnetic field effects has focused on systerns where a triplet radical pair is easily formed photochemical1y.g- 1*13.15*18-24 The two most common methods for radical pair generation are the Nomsh type 1 reaction of, for example, dibenzyiketone denvati~es,~3*15*~o**1*2~equation 1- 1, and the photoreduction of ketones, such as benzophenone, 1 8919v22*23 equation 1-2. hydrogen donor (HD)
Such systems have demonstrated that magnetic fields can indeed modulate the behaviour of geminate radical pairs. The research indicates that the major innuence of an applied magnetic field on a radical pair involves the decay rate constant for the geminate radical pair and the hction of radicals which escape. The effects can be predicted and explained under the parameters of the radical pair model. A typical example is provided by a study of the radical pair photochernically generated nom diben~yllcetone.~~The decay rate constant, kdecay, for the benzyl-phenylacetyl radical pair, equation 14, is an additive expression of both kIsc and ksc.As predicted under the model, the decay rate constant is altered by magnetic fields with an observed decrease at low field which is subsequently recovered as the field strength is Merincreased. With the application of a magnetic field, the triplet sublevels of T+ and T. begin to split from T, which slows intersystern crossing between T+ (or T-)and T,. Since krsc, but not bSc,is altered by the applied magnetic field, the decrease in Layat low fields is due to the slow component which represents the intersystem crossing from T, (or T-)to T,. However, when TI are split to such an extent as to make intersystem crossing between T+(or T-)to T, energetically unfavourable, singlet radical pairs will only be generated with intersystem crossing fiom T, to S. The conversion of T, to S is not affected by the applied magnetic field; therefore, the value of kdecayreturns to the value measured at zero applied field. In terms of the number of radicals that escape, a measurable increase in the absorption magnitude due to the benzyl radicals is observed with increasing applied magnetic field. The greater probability for escape resdts from the extended duration of the triplet radical pair due to the decrease in intersystem crossing. Few studies of reactions with biologicai importance have been conducted in the presence of magnetic field~.~~W8Vitamin E (a-tocopherol) has ken used as a hydrogen donor in the photoreduction of n-butyrophenone." The ketyl-phenoxyl radical pair generated was observed to be influenced by an applied magnetic field in the same rnanner as described above. An increase in the number of fiee radicals as well as a decrease in the observed decay rate constant for the radical pair, which was subsequently recovered at higher appiied fields, was experimentally measured. In addition, work has been conducted with melatonin, a hormone produced in alI mammals by the pineal gland? Laser excitation of benzophenone io the presence of melatonin as the hydrogen donor generated a triplet radical pair. The effect of an applied extemal field on the ketyCmeIatonin radicai pair was a 128% increase in the fraction of radicals which escaped the micelle with respect to that at zero magnetic field. Recently, radical pairs denved photochemically from 2,3,6,7- dibenzofluorene (DBF), which is reported to be an ideai candidate for DNA intercalation, have been e~amined.~~The radical pair generated from the DBF triplet oxidation of ascorbyl-6-palmitate, an oil soluble synthetic antioxidant, was demonstrated to be influenced by the presence of an external magnetic field. The decay rate constant for the geminate radical pair was observed to decrease with increasing applied field strength. The studies of the systems of vitamin E, melatonin and DBFfascorbyl-&palmitate did include biologically based molecules but did not utilize them in terrns of biological systems. Environmenrally based questions such as the influence of oscillating fields, radical pairs initidy formed in the singlet state, and the effects on processes of biological signifcance have been underrepresented in the literature. However, a study on the effects of both altemahg current (AC) and direct current @C) fields has suggested that radical behaviour is identicdy influenced regardles of the type of field applied, provided the two types of magnetic fields are of the same instantaneous strength.28 In addition, the photocleavage of 1-benzoloxy-2,2,6,6-tetramethylpiperidine has ken studied where the 2,2,6,6tetramethylpiperidine N-oxide and benzyl radical pair are generated in the singlet state.14 In contrast to triplet denved radid pairs, an iucrease in the decay rate constant of the radical pair was observed in the presence of an applied field In temof the radical pair model, singlet radical pairs cm dso undergo intersystem crossing and escape. However, unme an initially fomed triplet pair, the singlet radicai pairs cm undergo direct coupiing to
form products. In this sense, intersystem crossing prevents reaction. Therefore, the triplet
state acts as a "safe-haven" for the radical pair. With an applied field* the population of radical pairs in the singlet state will not be depleted as efficiently due to the decrease in intersystem crossing by Zeeman splitting. Therefore, the increase in the decay represents the prevention of singlet radical pairs fiom undergoing intersystem crossing to the
ufveactive triplet state which enhances product formation. a spin-aiiowed process from the singlet state only.
Recent work on enzymes by Gnssom et a112-29-33 pmvides examples of studies that attempt to address the need for biologicaliy relevant systems in research on mgnetic field effects. Both the enzymatic systems of ethanolamine ammonia lyase*9*31J* and horseradish peroxidase33 have been investigated. The B 12 dependent enzyme ethanolamine ammonia lyase (EAL) catalyzes the conversion of ethanolamine to acetaldehyde and ammonia by facilitating a 1,2-migration of the amine gr0up.~~Jl3*Every B 12 enzyme that functions in this manner utilizes the cofactor adenosylcob(III)aiamin (~do~bl*). Homolysis of the carbon-cobalt bond in ~do~bl*serves as a radical source to initiate catalysis. A generakd version of the known feaiures of the mechanism" by which ~do~blmmediates the enymatic reactioa is shown in Scheme 1-4. The fmt step is the cleavage of the weak carbon-cobalt bond to form the Cob(II)alamin, Co(ll), and 5'-deoxyadenosyl radicals upon enzyme binding of both the cofactor and substrate. The substrate donates a hydïogen to the 5'-deoxyadenosyl radical, to give the substrate radical that then undergoes rearrangement to form a product radical. The product raàical abstracts a hydrogen to reform the 5'-deoxyadenosyl radical. The dissociation of the product dows another substrate to bind and the cycle repeats. The Co(II) and 5'-deoxyadenosyl radical pair is initially generated as a singlet pair.29~31-32 Therefore, the nonproductive recombination of the enzyme bound Srdeoxyadenosyl and Co0radicais is predicted to be the rnagnetic field sensitive step. In facf the rate of appearance of Co@) decreases by 1796 in the presence of an applied magnetic field. The results with EAL open the gates to the possibility that enzymes with radical intermediates can have rnagnetic spin dependent chemistry.
Scheme 1-4 S-H A
S-H A
where S-H= HOCH2CH2NH2,S = HOCHCH~NH~,P. = d&CH2@, P = HCCH,il
More recently, the sensitivity to magnetic fields of horseradish peroxidase (HRP), one of a family of heme-containhg enzymes which catalyze the reduction of peroxides, has been in~estigated.3~The catalytic cycle of horseradish peroxidase begins with a twc~lectron oxidation of native-HRPFe(III)J to ferryl-HRPm(IV)] (HRP-1) and the prophyrin radical cation upon which peroxides are reduced to water. The recovery of native-= involves two one-electron reduction steps in which electron donation fiom two organic substrates fm converts HRP-1 to HRP-II and then HRP-II to native-HRP to cornpiete the cycle, Scheme 1-5. Scheme 1-5 Native HRP HRP-1
HRP-II A decrease of 35% in the rate of conversion of HRP-II to native-HRP via an organic substrate was measured in the presence of an applied field. Since HRP-II is a paramagnetic species. the electron transfer from a diamagnetic substrate wiii produce a triplet radical pair.
Only the triplet state cm undergo back electron transfer and stül conserve the total anguiar momentum of the system. However, both the triplet and singlet forms of the reduced HRP-II and substrate radical pair can convert to native-HRe. By slowing down intersystern crossing with the application of the magnetic field, the Netirne of the triplet radical pair increases. This increased Metirne of the initiaLiy formed triplet radcal pair increases the iikeiihood of back electron tramfer. The singlet reservoir is removed with the applied field and this results in the observed reduction in the rate of conversion of HRP-II to native-HRP.
The work of Grissom et al on enzymatic systems has provided experimental examples of biological reactions in vino which have ken hfiuenced by extemal magnetic fields. One biologicdy relevant reaction suitable for similar magnetic field effects which has not been investigated is the bheterolysis of &substituted radicals.
1.4 Mapetic Field Effects on f3-Heterolysis Reactions
Radicals as intermediates in enzyme-catalyzed reactions are gaining increased recognition in mechanistic interpretati0ns.3~Js For example, radicals with phosphate, hydroxy or amino groups in the fbposition formed in severai enzymatic systems are postdateci to undergo heterolysis of the carbon-heteroatom bond to generate a radical cation, equation 1-3.
R R )-& -x- a)-& OI I R R - R R X = O -P(OR), , OH, +NH3
The rearrangement of substrate radicals to product radicals in the enzymatic systems of di01 dehydrase and ethanolamine ammonia lyse is thought to occur through a radical cation intermediate." Ribonucleotide reductases, which are enzymes that duce ribonucleotides to deoxyribooucleotides responsible for DNA synthesis, is postulated to involve a mechanism with an intermediate sugar radical cation formed through the heterolysis of a Psubstituted sugar radical? In addition, radical induced DNA damage is accounted for with a radical cation interme~iiate.3~J8The sugar radical formed upon the abstraction of hydrogen by a radical source from a DNA strand is argued to undergo heterolysis of the carbon-oxygen bond linking it to the phosphate backbone, equation 1-4, generatuig a radical cation intermediate. The fact that P-heterolysis is prevaient in biological processes, such as radical induced DNA damage, makes it an important reaction to understand in terms of magnetic field effects. I I OP-O
DNA cleavage products (14)
The proposed research to be discussed in the following chapters highlights initial work into developing a system to study whether magnetic fields can aiter the chernical yield of heterolysis products of psubstituted radicals. The yield of radical cation, formed fkom heterolysis of a P-substituted radical (kh,3, is dependent upon the cornpetition with k~sc which leads to geminate recombination as shown in Scheme 1-6.
Scheme 1-6
The triplet radical pair initiaily formed wiU be infiuenced by magnetic fields by a predicted reduction in the rate constant for intersystern crossing. However, the rate constant for heterolysis, khet, as well as &, wiU be unaffected by the magnetic field. Therefore, when 14 bctis in cornpetition with kIsC, the introduction of a magnetic field should increase the yield fkom heterolysis as a result of the complementary decrease in intersystern crossing caused by the extemal magnetic field The particular system to be investigated involves substituted diaryketones. A triplet radcd pair wiii be generated through photochemicdy induced decarbonylation of the triplet state of the ketone precursor. The precmon will be synthetically designed so that one member of the radical pair will contain an aky1 moiety with a leaving group X in the p-position, Scheme 1-7.
Scheme 1-7
The requirements for magnetic field effects to be observed in such a system are threefold. Fist, the P-substituted radical formed must have an appropriate rate constant for heterolysis of the C-X bond. Therefore, the heterolytic rate constants for various fbsubstituted radicals need to be examined to evaluate the Muence of miceIiar incorporation on the heterolysis reaction and detennine the appropriate leaving group, X, for the study of magnetic field effects. In a more generai sense, the absolute reactivity of radical cations in micellar media needs to be examined and compared with homogeneous solution. Second, the system outlined wiil result in the formation of a radical cation and radical confined within micelles. If the radical and radical cation react with each other then the yield of radical cation will be influenced which may complicate the observation of magnetic field effects on the rate constant of heterolysis. Therefore, it is necessary to determine whether are not radicai cation and radical transients when generated within miceifar cavities will react with one other. Third, a triplet radical pair that indudes one radical that is capable of a hetedytic cleavage in the p-position has to be readily formed in order for a competition between the processes of he teroly sis and intersystem crossing to be established. Therefore, synthetic preparation of appropriate precursors wiU be required.
1.5 Scope of Thesis
The following chapten will outline work that has been completed in developing a system to study magnetic field effects on P-heterolysis. To date, rneasurement of p-heterolysis rate constants for P-substituted radicais in miceliar media and the reactivity of radicals toward radical cation species in both homogeneous and heterogeneous solutions have been examined. As well, preliminary synthetic work to generate appropriate precursors has also ken canied out
In the future. the synthefc path discovered through trial and enor can provide a method io generate the substituted diarylketones and whether heterolytic yields cm be dtered by the presence of extemal magnetic fields can be determined. Cha~ter2 Heterolvtic Cleavaue for 8-Substituted Radicals
2.1 GeneraI Introduction
p-Substituted radicals are known to undergo p-heterolysis39 to generate the
corresponding radical cation which can be trapped by a nucleophile or the solvent, equation 1-3. $-Substinited radicals also undergo other reactive pathways including homoly sisa of the carbon-leaving group bond to generate an aikene and a 1,2-1nigration~~to form a more stable radical, equation 2- 1.
As mentioned with respect to magnetic field effects, B-heterolysis is represented in mechanistic interpretations of enzymatic routes. Product studies22 and measurement of absolute rate constane3 for the Pheterolysis reaction have substantiated the importance of this process. Recently, a more extensive study of the dynamics of the Pheterolysis of radicals in solution has been conducted.39 The ability of the p-substituent as a leaving group, the ioegability of the solvent, and substituent changes were noted to influence the rate constant for the heterolysis of Psubstituted radicals. The conclusion was that &heterolysis is Muenced by these variables in the same manner as other SNI reactions. Therefore, the stabilization of the positive charge tbrough medium or substituent effects and improvernents in leaving group ability enhanced the heterolytic rate constant Heterolysis of p-substituted radicals has been noted to proceed rapidly. In comparing the rate of hydrolysis of aUql compounds with leaving groups in the p-position to those of P-substituted alkyl radicals, the rates are several orders of magnitude faster for the radicaL44 The same accelerated rate trend for radicais was seen in solution for fbsubstituted arylakyl radicals where reasoning was based on the influence of the radical moiety.39 As shown in Scheme 2-1. as the leaving group. X begins to dissociate, a partial positive charge is placed on the p-carbon. In the case of a radical; however, the charge is not localized but shared with the haIf-füed p-orbital of the adjacent radical center.
Scheme 2-1
In order to examine the innuence of magnetic fields on the heterolytic reaction for P-substituted radicals, the rate constant for heterolysis must be similar to those for escape and intersystem crossing. Therefore, rate constants for the heterolysis of B-substituted radicals have ken measured in the micellar media of SDS and CTAB. Specifically, substituted (4-methoxyphenyl)alky1 acetate denvatives (la-d) were incorporated into micellar media Upon irradiation with a 266-nm laser, these precursors undergo homolytic cleavage of the acetoxy group to generate bsubstituted radicals (2a-d). The heterolytic cleavage of the C-X bond of the radical (2a-d) can then be measured by monitoring the appearance of the respective radical cations (3a-d), Scheme 2-2. Scheme 2-2 O
la-d 2a-d 3a-d
2.2 Laser Photolysis of 2-Chloro-1-(4-methoxypheny1)ethyl acetate
The micelles formed by the surfactant known as sodium dodecylsulfate (SDS, Cl2H250SO3-Na+) are anionic with the sulfate head groups balanced by sodium cations. The transient absorption specinim generated upon 266-nm irradiation of 2thloro- 1-(4- methoxypheny1)ethyl acetate (la) in 0.1 M SDS is dominated by absorption near 370 and 600 nm, Figure 2-1, which is characteristic of the p-methoxystyrene radical cation
(3a).39*45*4 As shown in the inset of Figure 2- 1, the formation of the radical cation is not complete during the laser pulse. hstead, the radical cation monitored at 370 nrn grows-in with a fmt-order rate constant of (6.635).1)x 106 s-1. The the-resolved growth is quenched with the presence of oxygen, inset of Figure 2-1. Oxygen quenching of the growth is consistent with 3a king formed by heterolytic cleavage of the C-CI bond of the
2sNoro- 1-(4-methoxypheny1)ethyl radical (2a) generated upon irradiation of la, equation As show in the inset of Figure 2-1, some prompt radical cation formation takes place even under oxygen-saturated conditions. The same observation is made upon laser irradiation of la in neat water." The prompt radical cation is thought to be caused by a rapid two- photon process. The radical 2a produced by the fmt photon undergoes photoheterolysis with the absorption of a second photon to generate the radical cation within the same laser pulse. This process takes place too rapidly to be quenched by oxygen. The miceHe formed with the surfactant cetyItrimethylammonium bromide (CTAB, CH3(CH2) 5N+(CH3)3Br) is cationic with the head group of trimethylammonium and bromide as the countenon. The transient absorption spectrum, Figure 2-2, generated upon irradiation of la in 0.05 M CI'AB is typical for the p-rnethoxystyrene radical cation with bands at the characteristic wavelengths of 370 and 600 nm. As in SDS, a time-resolved growth is clearly observed in CïAB at 370 nm. The growth fits well to a fint-order expression giving a rate constant of (9.6k 1.0)~105 s- l. The tirne-resolved growth at 370 nm is quenched by oxygen, inset of Figure 2-2, conflfming that heterolysis with the radical (2a) to give the radical cation (3a) occurs within CïB micelles, equation 2-2. In addition, there is a sharp peak centered around 300 nm which could not be characterized when the heterolysis reaction was carried out in SDS. Figure 2-1. This absorption is characteristic of the 2-c Moro- 1-(4methoxyphenyl)ethyl radical which is known to absorb at 300 nm.39 The lack of radical absorption in SDS is amibuteci to a large amount of fluorescence in the region of the radical maximum on the short time scales required to observe the growth of the radical cation. In the case of CTAB, the slower growth rate as compared to that in SDS ailows longer the windows to be use and the radical is observed.
A slower reaction has been observed in cationic micelles as opposed to anionic micelles previously with respect to SNI type reactions (vide i~i@a).~8 Wavelength / nm
Figure 2- 1. Transient absorption spectnim of 2chloro- 1-(4methoxyphenyl)ethyl acetate in nitrogen-saturated 0.1 M SDS recorded 76 (O), 128 (0) 224 (R), and 368 (0)ns after the laser pulse. Inset shows the the-resolved growth at 370 nm under nitrogen (a)and oxygen-saturated (0)conditions.
Wavelength / nm
Figure 2-2. Transient absorption spectnim of 2chloro-1-(4methoxyphenyl)ethyl acetate in nitrogen-sahuated 0.05 M CïB recorded 210 ns (a), 540 ns (O), 920 ns (m), and 1.86 ps (O) after the laser pulse. Inset shows the thne-resolved growth at 370 nm under nitrogen (0) and oxygen-saturated (0)conditions. 2.3 Laser Photolysis of 2-Chloro-1-(4-methoxyphenyl)propyl acetate
Photolysis of 2-chlore-L(4-methoxypheny1)propyl aœtate (lb) in 0.1 M SDS generates a transient spectrum characteristic of radical cation f0rmation39~4s~~6with absorption bands at 390 and 600 nm, Figure 2-3. The 1-(4-rnethoxyphenyl)propene radical cation (3b) grows-in with a fmt-order rate constant of (1.0+0.03)~107 s-1. The tirne-resolved growth at 390 nm is quenched with the introduction of oxygen, inset of
Figure 2-3, indicating that the formation of the radical cation involves heterolytic cleavage of the C-Ci bond in the 2-chioro-l-(4-rnethoxyphenyl)ethyl radical (2b), equation 2-3.
O
SimiIarly, when lb is incorporated into 0.05 M CTAB, the transient spectrum generated upon laser irradiation is dominated by absorption due to the radical cation (3b),
Figure 2-4. A time-resolved growth with a rate constant of (5.7kO.l)xlO6 s-1 is observed at 390 nm, inset of Figure 2-4. Radical cation formation is again quenched in the presence of oxygen which supports generation of 3b through heterolytic cleavage of the C-CI bond of 2b within CTAB,equation 2-3. In both SDS and CTAB, no signincant radical (2b) absorption is detected due to strong fluorescence which prohibits its observation at 3 10 ndg - 300 350 400 450 500 550 600 650 Wavelength / nm
Figure 2-3. Transient absorption spectrum of 2shloro-1-(4-methoxypheny1)propyl acetate in nitrogen-saturated 0.1 M SDS recorded 70 (O), 148 (O), 232 (m), and 564 (0)ns after the laser puise. Inset shows the the-resolved growth at 390 nrn under nitrogen (O) and oxygen-saturated (0) conditions.
Wavelength / nm
Figure 2-4. Transient absorption spectrum 2chloro- 1-(4-methoxyphenyl)propyl acetate in nitrogen-saturated 0.05 M CïAB recorded 208 ns (@), 552 ns (O), and 1.29 ps (D) after the laser pulse. hset shows the the-resolved growth at 390 nm under nitrogen (0) and oxygen-saturated (0)conditions. 2.4 Laser Photolysis of 2-Bromo-1-(4-methoxypheny1)ethyl acetate
The transient absorption spectnim generated upon irradiation of 2-bromo-1-(4- methoxypheny1)ethyl acetate (lc) in 0.1 M SDS shows absorption at 370 and 600 m, Figure 2-5. In this case, radical cation formation is essentiaiiy complete within the laser pulse, indicating that heterolysis is very rapid with a rate constant exceeding 108 s-1. In addition, only a sdfraction of radical cation formation is quenched by oxygen. This suggests that radical cation (3c) formation is more rapid than trapping of the 2-bromo-b(4- rnethoxypheny1)ethyl radical (2c) by oxygen.
In contrast, the transient absorption spec- when inadiating lc within 0.05 M CIAB dws not represent characteristic radical cation formation. In fact, the spectrum is dominateci by absorption at 350 m. Figure 2-6. The band at 350 nm may be explaineci with the establishment of an equilibrium between the radcal (2c) and the radical cation (3c), equation 2-5. The cationic head groups of CTAB are baianced by bromide anions; therefore, the concentration of bromide within the micelle is greater than in SDS where the only source of bromide is the heterolysis of the radical itself. A fast heterolytic rate constant, as seen in SDS, could be followed by attack of the bromide anion on the radical cation to regenerate the radicai. O
hv / CTAB 266-m -BL An dBrH +Br- Wavelength / nm
Figure 2-5. Transient absorption spectnun of 2-bromo- 1-(4-methoxypheny1)ethyl acetate in nitrogen-saturated 0.1 M SDS recorded 120 (O), 308 (O), and 548 (W) ns after the laser puise. hset shows decay trace at 370 m under N2 (a)and 02(0) conditions.
Wavelength / MI
Figure 2-6. Transient absorption spectmm of 2-bromo-1-(4-methoxypheny1)ethyl acetate in nitrogen-saturated 0.05 M CïAB recorded 130 (a),300 (O),and 670 (H) as &ter the laser pulse. Inset shows decay trace at 350 nm under N2 (e)and 4 (0)conditions. 2.5 Laser Photolysis of 2-Bromo-1-(4-methoxypheny1)propyl acetate
Incorporation of 2-bromo- 1-(4rnethoxyphenyl)propyl acetate (Id) in 0.1 M SDS generates the 1-(4-methoxypheny1)propene radical cation (3d) with absorption ai 390 and 600 nm immediately following the laser pulse, Figure 2-7. However, some quenching of the radical cation monitored at 390 nm occurs with the introduction of oxygen, inset of Figure 2-7, which suggests that heterolytic cleavage of the C-Br bond of the 2-bromo-1-(4- methoxypheny1)propyl radical (2d) is still the major pathway for the formation of the radical cation, equation 2-6. No growth of 3d is detected experimentally indicating that the heterolytic cleavage is occurring rapidly with a rate constant 2 108 s-1, beyond the time resolution of the laser system employed.
II hv / 266- -Br- H3CC0~Br SDS An 0rcTA.B ld 2d
In addition, the spectrum generated upon insidiation of Id in 0.05 M CTAB is also reminiscent of the radical cation (3d) with bands at 390 and 600 nm, Figure 2-8. In CTAB,the formation of the radical cation is prompt as was the case within SDS. In other words, the heterolytic cleavage of the C-Br bond in 2d occurs too fast to be time-resolved using the nanosecond laser system, equation 2-6. The lack of radical absorption (Zd) is the direct result of the fast heterolysis reaction which is complete within the laser pulse. 300 350 400 450 500 550 600 650 700 Wavelength I nm
Figure 2-7. Transient absorption spectmm of 2-brome 1-@-me thoxyp heny1)propyl acetate in nitrogen-saturated 0.1 M SDS recorded 60 (O), 192 (O),408 (a),and 640 (0)ns after the laser pulse. hset shows the decay trace monitored at 390 nrn under nitrogen (0) and oxygen-saturated (O) conditiens.
- Time / ps
300 350 400 450 500 550 600 650 700 Wavelength / nm
Figure 2-8. Transient absorption spectm of 2-bromo- 1-(4-methoxyphenyl)propyl acetate in nitrogen-saturated 0.05 M CTAB recorded 130 (a), 300 (O),and 670 (i)ns after the laser pulse. Inset shows the decay trace monitored at 390 nm under nitrogen (m) and oxygen-saturated (0)conditions. 2.6 Discussion
The observation of radiai cation without simcant radical absorption makes it dificuIt to determine whether the pathway of Scheme 2-2 is occunhg. However, in ali cases where the radical cation was observed, the introduction of oxygen to the sample quenched the formation to some extent. The corresponding alkene radical cations generated through photoionization are known not to be influenced by ~xygen.~~On the other hand, the p-substituted radicaIs are affected by the introduction of oxygem39 Therefore, upon aerating the sample, the radicals are quenched and heterolysis is prevented. This behaviour lends credence to the radical king an intermediate on the pathway to radical cation formation for the precurson employed, Scheme 2-2. The hetedytic rate constants for the ksubstituted radicals 2a-d measured in SDS and CTAB media are tabdated in Table 2- 1.
Table 2-1. The observed ht-order rate constants obtained for the time-resolved growth of the radical cations from P-substituted radicals generated upon 266-nm irradiation of the appropnate precursors in nitrogen-sahirated 0.1 M SDS and 0.05 M CïAB and water.
Radical khn / S-l (SDS) khet/ S-' (CM) kher/ S-l (H70)a 2a (6.6i0.2)~1O6 (0.97io.1 )x106 > 108 2b (10.1H.3)~106 (5.810.1)~106 > 108 2c > 108 Not observed > 108 2d > 108 > 108 > 108 a fiterature values 39
In neat water, the radicals 2a-d have heterolytic rate constants that exceed 108 s- l.39 Within the aqueous micelles of SDS and CîAB, the heterolytic rate constants of 2a and 2 b are measured to be between 106-10' SI, Table 2-1. Micellar inhibition of SN 1 type reactions has been previously reported." In tem of where the heterolytic reaction occurs, escape rate constants for carbon-centered radicais~ssuch as benzyl radicals are on the order of 106- 107 s-1; therefore, the heteroiytic reaction cm occur within the miceliar cavity or immediateIy upon escape into the aqueous phase. The rate constant measured for 2b is faster than 2a by a factor of 1.5; however, the escape rate constant for 2b would be slightly slower or comparable to 2a. This would be attributed to the p-CH3 substituent of 2b as compared to the ~unsubstituted2a radical which enhances the hydrophobicity of the substrate resulting in a slightly slower or at least comparable escape rate for 2b. In fact, methyl substitution of phenyialkyl radicds has been reported to slow the rate constant of escape by a factor of 1.5 in favour of the substituted radid20 If the heterolysis reaction occurred imrnediately upon escape of the radical fkom the micelle then the rate constant for escape would be the rate determinkg step. Therefore, the rate constant for 2b should be smder than that measured for 2a. The fact that the heterolytic rate constant associated with
2b is faster than 2a suggests that the heterolysis reaction is occuming within the micelle. In the case of heterolysis of 2c and 2d, the rate constants are much faster than the expected escape rate constants. If heterolysis occurs in the aqueous phase upon escape of the micelle, the rate constants would be on the order of 106- 107 s- l. Therefore, the measured rate constants of 108 s-1 for 2c and 2d again suggests that the heterolmc reaction takes place within the micelles. In terrns of the influence of substrate structure on the heterolysis reaction, trends sllnilar to those observed in homogeneous solution are found.39 In aqueous SDS, the presence of a CH3group in the P-position for the 2-chloro radical, Zb, accelerated the fmt- order rate constant for the growth of the correspondhg radical cation cornpared to the 2a radical with hydrogen at the P-position. In other words, the heterolytic cleavage of the C-Cl bond is faster for the 2-chioro-1-(4-methoxypheny1)propyl radical (2b). The methyl group through inductive donation stabilizes the positive charge placed on the b-carbon with the cleavage of the C-halide bond. In addition, a 100 mV difference in the oxidation potentialsm of p-methoxystyrene and 1-(4-methoxyphenyl)propene in favour of the latter suggests that the B-methyl substituent stabilizes the radical cation fonned through ionization. The nature of the p-halide also innuences the rate of heterolytic cleavage. In cornparhg chlonde and bromide in te- of leaving group ability, bromide surpasses the ability of chlonde with the $-bromo radicals in SDS micelles having the fastest rate constants for heterolytic cleavage, 2c and 2d in Table 2- 1. Therefore, the well-established p~ciplesof leaving group ability are followed.
ln solution. the ionizing abiiity of the solvent plays an important role in the rate constant measured for heierolytic cleavage? In terms of the polarity of micelles, reports suggest that it is lower than wateP8 and may be comparable to neat rnethanol.lsJ1 The inhiition of the heterolytic rate constants in the micelles as compared to neat water may be related to this medium innuence of micelies within aqueous solutions. By incorporating the precursors in CîAB and SDS, the influence of the stnicturai characteristics of the surfactant on the heterolytic cleavage reaction was investigated. The micelles of CTAB are
Iarger than SDS and cationic in nature. l The 2-chloro- 1-(4-methoxypheny l) propy l radical, 2b. gave a khet of (5.8M.1) 1 o6 s-1 in CTAB which is 1.7 times slower than the value determined in SDS micelles. In the case of the punsubstituted radical, 2a, in CTAB the radical cation is seen to grow-in with a fmt-order rate constant that is approximately 6.8 times slower than that observed in SDS. The decrease in the heterolytic rate constants observed in CTAB as compared to SDS can be related to chah length and micellar charge.
The fact that the CïAE3 surfactant has a longer allcyl chah may explain, in part, the lower rate constants measured in CTAB as compared to SDS. The longer chah may enable the substrate to be in a more nonpolar environment than that obtained within SDS and therefore decrease the rate constant for heterolysis. Furthemore, the charge associated with the micelle may also be related to the inhibition of the reaction. The rate of hydrolysis of several substrates has been shown to be faster in anionic than in cationic mi celles.^ This obsentation was attributed to the electrostatic interaction of the carbocation center with the sulfate head groups of SDS as opposed to the unfavourable interaction with the cationic head groups of CïAB. In comparing SDS and CTAB, a more ciramatic decrease of khet for the P-unsubstituted radical is seen as opposed to the fhethyl radical. Since the pmethoxystyrene radical cation is the ieast stable of the two radicai cations, due to the absence of the P-CH3 group, the cationic nature of the CïB surfactant would be anticipateci to influence the formation of the fbunsubstituted radical cation to a greater extent.
Laser irradiation of 2-bromo- 1-(4-methoxypheny1)ethyl acetate in CïAB resui ted in the formation of the 2-bromo-1-(4-methoxypheny1)ethyi radicai (2c); the radical cation of
Cmethoxystyrene was not observed. Presumably, the radical is produced by rapid heterolysis of the C-Br bond, but the high concentration of bromide present in the aqueous solution of CM3 may result in rapid mpping of the radical cation to regenerate k p-bromo radical. Simila. results have been observed in hornogeneous aqueous solutions w ith added bromide.39 Surprishg1y, the radical cation of Cmethoxystyrene is Observed upon heteroly sis of the 2-chloro- 1-(4-methoxypheny l)ethyl radical (2a) in CTAB.
Addition of the bromide ion present in the solution to the radical cation to generate the 2-bromo-1-(4-methoxyphenyl)ethyl radical should have occurred in the same manner as observed for the radical cation generated fiom the 2-brorno-1-(4-methoxyphenyl)ethyl radical (2c). Currently, no explanation for these results is known.
The radical cation of anethole is observed in CTAB. In this case, the rate constant for heterolysis of bromide is significantly greater than the addition of bromide to the radical cation. Thus, despite the high concentration of bromide present, the radical cation is still the dominant species and is readily observed.
In terms of examining magnetic field effects, the b-substituted radical wül be one member of a triplet radical pair. Such a radical pair confined in the micelle environment has two main reactive pathways, product formation which is dependent on the rate constant for intersystem crossing and escape of either member which is dependent on hydrophilicity.* The rate constant of intersystem crossing slows down with an applied field and therefore plays a key role in the observation of magnetic field effects. The rate at which the tripiet intersystem crosses to the singlet is govemed by spin-orbit coupluig. The spin vector of the electron is flipped and the totai angular rnomentum is conserved with a correlated orbital jump, Scheme 2.3.52
Scheme 2.3
Perturbation of spin-orbital coupling can be accomplished by halogen substitution through the heavy atom effect. Larger nuclear charges, for example with bromine, increase the energy associated with spin-orbital coupling and therefore the probability of intersystem crossing. A study conducted with dibeozylketone noted that the substitution at the 4-position by c~~oM€!was andogous to methyl and did not hinder the observation of magnetic field effects.13 However, the substitution of bromine lead to an elimùiation of the influence of an applied field. This behaviour was attributed to the increase in intersystem crossing induced by bromine substitution. Therefore, the haiide leaving group of brornide will not be suitable for the study of magnetic field effects on the heterolysis reaction of B-substituted radicals. In fact, the rate constant for heterolysis with bromide as the leaving group is 2 108 s-1 which makes the heterolytic reaction too rapid to be resolved. Therefore, in a more general sense, bromide anion or groups of simiiar or pater leaving group abiLity will not be suitable for the magnetic field study. For chloride the situation is completely the reverse. Heterolytic rate constants of 106-107 s-i and the Iack of perturbation of intersystem crossing makes chloride, or leaving groups of equal ability such as fluorinated acetoxy groups, suitable for investigating magnetic field effects on the heteroly sis reac tion. In micellar media the hetitrolysis reaction has ken observed to proceed rapidly with rate constants on the order of 106-108 s-l. With respect to magnetic field effects, intersystem crossing for cahn-centered radi~als~~is known to occur with rate constants in the range of 106- lo7 s- l and benzyl denved radicals have ken determined to escape hm SDS micelles with rate constants15around 106 s-L. The three rate constants of heterolysis, intersystem crossing, and escape are d comparabIe if considering the rate constants measmd for heterolysis with Cl- as the leaving group. Therefore, with appropriate leaving groups, the heterolytic cleavage of p-substinited radicals cm complete with the processes of intersystem crossing and escape which are available to a radical pair confked wiihin micelles. A cornpetitive nature for heterolysis will enable the detemination of magnetic field effects on these reactions. Chapter 3 Reactivitv of Radical Cations with Radicals
3.1 General Introduction
Radical cations of aryl olefins are known to undergo an extensive List of reactions including cis-&ans isomerization and several bimolecdar reacti0ns.~9533~Bimolecular options include reaction with oxygen, neutrai olefh molecules, and nucleophiles. The reactivity of the radical cation wiil depend on steric factors, redox potential, and s~Ivent.~g As weU, in bimofecular reactions, the concentration and structure of the quencher wiil play a role in the rate constant observed for quenching of the radical cation. Quenching rate constants of olefin derived radical cations with numerous nucleophiles, for example, alcohols, amines, and halides have been extensively studied.4sN However, the interaction of radical cations with radical species has received minimal arnount of attention.55-57 Whether radicals can add to radical cations is an important question to address for the examination of magnetic field effects on the yield of p-heterolysis reactions. The system proposed for such a study involves the generation of a radical pair where one rnember is P-substituted. Upon heterolysis of the fi-substituted radical, a radical and radical cation wiU be incorporated within the confines of the micellar interior, Scheme 1-7. The idluence of an appiied rnagnetic field on the proposed system wili be examined in terms of changes in the yield of radical cation as a hction of rnagnetic field strength. However, if the radical cation when fomed reacts signincantly with the radical, magnetic field measurements on radicaI cation formation may be dZ£icult. In the literature, a few snidies report on products that are generated through the interaction of radicals and radical ions.s5-
58 In particular, a "well-laiown mechanism" exists for product formation upon reaction of a radical with a radical anion.S* However, fewer studies report on products observed fiom radical and radical cation interactioos.55-57 The reduction of the bis(4- methoxypheny1)methyl cation to the corresponding radical through electron-transfer quenching of the triplet state of 1-methoxynaphthahe has ken investigated.55 Product formation was observed to occur by coupling of the bis(4-methoxyphenyl)methyl radical and the 1-methoxynaphthalene radical cation, equation 3-1. The isolation of the couphg adduct suggests that couphg cornpetes relatively efficiently with back electron tramfer although the latter was stiU the dominant mode of radicaVradica1 cation decay.
A complementay study reported that alky1 radicals derived from diazonium salts were observed to undergo C-S bond formation with the radical cation of tetrathiafulvalene, equation 3-2.56 Recentiy, this study has been extended to include anilide diazonium salts where coupiing with the tetrathiafuivalene radical cation was also observed.s7 The observation of coupling products from radical and radical cation interactions suggests that the coupiing can compte with other reactive pathways.
+e
R' TTF
To investigate the possible coupling of a radical with a radical cation, the following system was designed, Scheme 3- 1. The generation of an aryl olefin radical cation through photoionization is known to occur biphotonic with a 'Yk" electron expelled from the 0lefin.~9 Therefore, molecules which upon scavenging the electron undergo a C-X (X= halide) bond dissociation step generating a radical were chosen to co-incorporate into the rnicelIar media with the olefin. With the generation of both a radical cation and a radical in micellar cavities, a study of the reactivity of these species toward one another cm be conducted.
Scheme 3-1 R"
3.2 Laser Photolysis of Anethole in Acetonitrile
The transient absorption spectrum observed upon irradiation of 1-(4- methoxyphenyl)propene, more commoaly referred to as anethole, in neat acetonitde displayed two bands, Figure 3-1, one in the UV region centered at 390 nrn and one in the visible at 600 nm. The two bands arise fiom a single transient which is assigned to the radical cation of anethole generated through photoionization, equation 3-3.
This assignment is substantiated through literature reporteci spectra which had characteristic absorption at 390 and 600 MI for the anethole radical cation. The introduction of oxygen reduces the radical cation absorption but no alteration in the observed decay rate is detected as clearly hdicated upon cornparhg the decay trace at 600 nm under nitrogen and oxygen conditions, inset of Figure 3- 1. The firstsrder rate constant for the radical cation decay in neat acetonitrile is approximately (6.5M.1)x 105 s-
Wavelength / nm
Figure 3- 1. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile recorded 560 ns (e), 1-88 ps (O), 5.72 ps (i),and 12.6 ps (O) afier the laser puise. Inset shows decay traces monitored at 600 nm under nitrogen (a)and oxygen-saturated (0)conditions.
3.2.1 Laser Photolysis with Benyl bromide
The transient absorption spectnm obtained upon 308-nm irradiation of anethole in nitrogen-saturated acetonitriie in the presence of 12.9 mM of benzyl bromide is shown in Figure 3-2. Immediately after the laser pulse, the spectnim consists of a sharp peak at 3 18 m. In addition, the distinctive absorption at 390 and 600 nm for the radcal cation indicates that the anethole radical cation was generated upon laser photolysis under these conditions. However, the decay of the radical cation is slightly faster in cornparison to tbat observed in the absence of benzyl bromide (vide infa). The band at 3 18 nm, which is not
observed in the absence of benzyl bromide, can be assigned to the benzyl radical which is documented to have a sharp maximum at this w avelength.'539-61 The direct photoly sis of benzyl bromide is prohibited due to the low absorption of benzyl bromide at 308 m. In other words, irradiation with 308-nm enables the selective excitation of anethole. Therefore, the generation of the ben@ radical results from the scavenging of the electmn expekd upon photoionization of anethole facilitaihg a dissociative cleavage of the C-Br bond of benzyl bromide, equation 3-4. The introduction of oxygen quenches the absorption at 3 18 nm contirming the benzyl radical assignment, Figure 3-3.
Decay traces monitored at 318 nrn upon excitation of anethole with various concentrations of benzyl bromide are shown in Figure 3-4. The decay traces show an increase in the absorption immediately following the laser puise as the concentration of benzyl bromide increases. The increase in absorption at 3 18 nm is plotted as a fiinction of benzyl bromide concentration in Figure 3-5. At low concentrations of benzyl bromide, the increase in absorption appears to have a hear relationship to the concentration of benzyl bromide. The increase saturates and then levels off when the concentration of benzyl bromide reaches about 6 mM. This relationship between the absorption at 3 18 nm and the concentration of benzyl bromide supports the conclusion that the reduction of benzyl bromide results in the generation of the benzyl radical, equation 34.
The decay traces at 600 nm represent the change in absorption due to the radical cation as a function of tirne and were rnonitored at various benzyl bromide concentrations, Figure 3-6. Inspection of the decay of the anethole radical cation demonstrates that there are changes associated with the addition of benzyl bromide. The band at 60nm was chosen to represent the anethole radical cation as opposed to the 390 nm band due to complications caused by absorption at 345 nm in the presence of benyl bromide (vide infia). The absorption at 600 nm imniedaîely after the laser pulse is clearly reduced as a fuuction of benzyl bromide concentration, Figure 3-7. This decrease is fairiy drarnaîic with
a reduction of 44 % observed in the presence of 0-013 M benzyl bromide fiom the yield of
radical cation without beoyl bromide added. Furthemore, the decay trace at each benzyl bromide concentration was fiaed using monoexponentiai parameters to obtain the observed
first-orcier rate constants. The decay is found to be slightly faster in the presence of benzyl bromide with a rate constant of (7.7M.1)x 1@ s-1 measured at zero concentration and
(lO.3fl.3)~105 s-1 at 12.9 mM of benzyl bromide.
Along with the formation of the benzyl radical with a band at 3 18 nm, another new transient with absorption centenxi at 345 nm is observed upon radical cation formation in the presence of beqlbromide, Figure 3-2. The yield of the transient absorbing at 345 nrn is dso associated with the amount of benzyl bromide added, Figure 3-8. In a similar fashion to the absorption monitored at 3 18 nm for the benzyl radical, the absorption at 345 nm increases as the concentration of benzyl bromide increases, Figure 3-7. The introduction of oxygen quenches the band at 345 m. inset of Figure 3-3. Wavelength / nm
Figure 3-2. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonihde with benzyl bromide (12.9 mM) recorded 560 ns (a), 1.88 ps (O), 6.12 ps (U), and 12.6 p (O) after the laser pulse. Inset shows decay traces monitored at 3 18 nm under nitrogen (e) and oxygen-saturated (0)conditions.
Wavelength / nm
Figure 3-3. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.42 mM) in oxgyen-saturated acetonitriie with benzyl bromide (12.9 mM) recorded 560 ns (a),1.88 p (O), 6.12 ps (1).and 12.6 ps (O) after the laser pulse. Inset shows decay traces monitored at 345 nrn under nitrogen (a) and oxygen-saturated (0)conditions. Figure 3-4. Decay traces monitored at 3 18 nm upon 308-rn irradiation of anethde (0.42 mM) in nitrogen-saturated acetonitde in the presence of benzyl bromide at concentrations of O (O), 3.2 (O),6.5 (m), 9.7 (D), and 12.9 (+) mM.
[Benzyl bromide] / 10-3M
Figure 3-5. Plot of the optical density monitored at 318 nm upon 308-am irradiation of awthole (0.42 mM) in nitrogen-saturated acetonitde versus the concentration of benzyl bromide. Figure 3-6. Decay traces monitored at 600 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile in the presence of benzyl bromide at concentrations of O (a)and 12.9 (0)mM.
menzyl bromide] / 10-3 M
Figure 3-7. Plot of the optical density monitored at 345 (0)and 600 (@) nm after 308-nm irradiation of anethole (0.42mM) in nitrogen-saturated acetonitde versus the concentration of benzyl bromide. O 5 10 15 20 Time / ps
Figure 3-8. Decay traces rnonitored at 345 MI upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetoniaile in the presence of benzyl bromide at concentrations of O (a). 3.2 (O), 6.5 (i),9.7 (a), and 12.9 (+) mM.
3.2.2 Laser Photolysis with 3-Cyanobenzyl bromide
3-Cyanobenzyl bromide is sûucturally similar to benzyl bromide; however, it is anticipated to be a better electron acceptor by virtue of the cyano group. Therefore. the 3-cyanobenzyl radical should be formed as effectively as the benzyl radical, equation 3-5.
The transient absorption spectrum obtained upon 308-nm irradiation of anethole in nitrogen-saturated acetonitrile with 12.3 mM of 3cyanobenzyl bromide is shown in Figure
3-9. It has characteristic absorption for the anethole radical cation with bands at both 390 and 600 nm. In addition, a transient at 345 nm which is not observed in the absence of 3-cyanobenzyi bromide is seen. The 3-cyanobenyl radical is known to have a weak absorption maximum near 330 nm which is close to that of the benzyl radical.62 However, it is difficult to differentiate the radical centered around 330 nm fiom the absorption at 345 nm. In the presence of oxygen, the transient at 345 nm was quenched and the spectrum more closely resembled that of the anethole radical cation, Figure 3-10.
The yield of the 345 nrn band increases as a function of 3-cyanobenzyl brornide concentration as showa in Figure 3-1 1. In fact, the change in the absorption at 345 nm saturates at 4 mM of 3cyanobenzyl bromide after which the increase levels off, Figure
3-12. From the decay traces monitored at 600 run for the anethole radical cation, Figure 3- 13, both a reduction in the absorption, Figure 3-12, and a small increase in the observed first-order rate constant are seen with iacreased concentrations of 3qanobenzyi bromide. The fmt-order rate constant for the radical cation is (8.6M.2)xf 05 s-l in the absence of
3cyanobenzyl brornide whereas with 12.3 mM present a value of ( 10.810.4)~105 s-1 is measured. Wavelength / m
Figure 3-9. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitde with 3cyanobenzyl bromide (12.3 mM) recorded 560 ns (a), 1.88 ps (O),6.12 ps (u), and 12.6 ps (0)afier the laser pulse. Inset shows decay traces monitored at 330 nm under N2 (a) and O2(0) conditions.
Wavelength / nrn
Figure 3- 10. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.42 mM) in oxygen-saturated acetonitrile with 3cy anobenzyl bromide ( 1 2.3 mM) recorded 560 ns (a),1.88 ps (O),6.1 2 p (i),and 12.6 ps (O) afier the laser puise. Inset shows decay traces monitored at 345 nm under Nt (a) and O2 (0)conditions. Figure 3-1 1. Decay traces monitored at 345 nm upon 308-cm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitriie in the presence of 3cyanobenzyl bromide at concentrations of O (a),2 (O),6 (H), and 12.3 (O) mM.
13-Cyanobenzyl bromide] / 10-3 M
Figure 3-12. Plot of the optical density monitored at 345 (0)and 600 (a) nm after 308-m irradiation of anethole (0.42 mM) in nitrogen-sahmted acetonitde versus the concentration of fcyanobenzyl bromide. Time / ju
Figure 3-13. Decay traces monitored at 600 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-sanirated acetonitde in the presence of 3cyanobenzyl bromide at concentrations of O (a)and 12.3 (0)mM.
333 Laser Photoiysis with Diphenylmethyl chloride
The photoiysis of anethole in the presence of diphenylmethyl chioride generated the anethole radical cation with absorption at 390 and 600 nm, Figure 3-14. In addition, a peak at 330 nm was observed which is assigned to the diphenylmethyl radical.63 Therefore, diphenylmethyl chlonde also scavenges the electron fomed upon photoionization of anethole to generate the anethole radical cation and the diphenylmethyl radical, equation 3-6. The decay traces monitored at 330 nm as a hinction of diphenyhethyl chloride concentration led to an observable increase in absorption immediately following the laser pulse, Figure 3-15. An examination of the increase in absorption as a function of the concentration of diphenyhethyl chloride was accomplished by plotting the optical density monitored at 330 nm against concentration, Figure 3-16. This results in a linear relatiomhip at iow concentrations which levels off &ter the addition of appmximateiy 4 mM diphenyhethyl chloride. The dependency of the increased absorption at 330 nm upon concentration indicates that the diphenyhethyl radical is indeed generated. With respect to the anetfiole radical cation, a reduction in yield is detected by monitoring the absorption at 390 nm immediately following the laser pulse as a function of diphenylmethyl chioride concentration. As well, the fmt-order rate constant for the decay of the radical cation increases from (7.M.l)x105 s-1 at O mM to (10.4+0.1)~105 s-1 with 5.8 rnM of diphenylmethyl chlonde. Figure 3- 14. Transient absorption spectrtm generated upon 308-nm irradiation of anethole in nitrogen-saturated acetonitrile with diphenylmethyl chloride (0-9mM) at 360 ns (a),840 ns (O),7.68 ps ((., and 12.9 ps (O) derthe laser pulse.
Figure 3- 15. Decay traces monitored at 330 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile in the presence of diphenyhethyl chloride at concentrations of O (e),0.8 (O),1.7 (i),2.5 (O), 3.3 (+), and 4.1 (A) mM. - -
0.02 - -
1 1 I 1 I I I O0 0.8 1.6 2.4 3.2 4 4.8 5.6 Piphenylmethyl chloride] 1 10-3 M
Figure 3-16. Plot of the optical density monitored at 330 (0)and 390 (@) nrn upon 308- nm irradiation of anethole (0.42 mM) in nitmgen-saturated acetonitrile versus the concentration of diphenylmethyl chloride
32.4 Identification of the Transient at 345 nm
One possible assignment for the transient at 345 nm observed upon laser irradiation of anethole in the presence of benzyl bromide is the carbocation that couid be generated by a reaction of the benzyl radical with the anethole radical cation, equation 3-7. This carbocation can be estimated to have an absorption maximum near 340 nm based on its similarity to (4-methoxypheny1)ethyl cati0ns.6~
Carbocations are well-known to be insensitive to oxygen. However, the 345 nm transient observed upon irradiation of anethole with beqlbromide present is quenched by oxygen, Figure 3-3. The observed quenching with oxygen could result by two different scenarios. In one case, the unknown transient is quenched in a biiolecular reaction between oxygen and the species itself. Altematively, the fornation route of the transient may be quenched in the presence of oxygen. In other words, if the transient represented by absorption at 345 mn is generated by a reaction between the benzyl radical and the anethole radical cation, quenching of the benzyl radical by oxygen wiIi not ody infiuence the absorption at 318 m but wiil iirnit the reaction with the anethole radical cation as weii. Therefore, the oxygen effkct on the absorption at 345 nm may be an indirect result of the quenching of the benzyl radical by oxygen. In the case of 3-cyanobenzyl bromide. a transient ai 345 nm is aiso generated with the formation of the anethole radical cation. It is anticipated bat if both the benzyl and 3cyanobenzyl radicals interact with the anethole fadical cation, the carbocations formed as shown in equations 3-7 and 3-8, respectively, would absorb at a similar wavelength. The cyano substituted phenyl group in the carbocation is to the cationic center, therefore, the 3-cyano group would not alter the carbocation to a sufficient degree to affect itç absorption maximum in cornparison to the unsubstituted carbocation.
In the case of diphenyhethyl chioride, the spectnim lacks absorption at 345 nm where the carbocation is anticipated to absorb, equation 3-9. Only the diphenylmethyl radical at 330 nm and the anethole radical cation at both 390 and 600 MI am observed spectraiiy. Therefore, there is no spectrai evidence of a transient formed from the reaction of the diphenylmethyl radical with the anethole radical cation. The Iack of an observable reaction between the diphenyimethyl radical and the radical cation may be due to increased steric hindrance with the second phenyl group as opposed to the hydrogen of the benzyl and 3cyanobenzyl radicîls or to increased radical stability.
To examuie whether the Iack of absorption at 345 nm with diphenyhethyl chloride as opposed to benzyl bromide is due to steric hindrance, anethole was irradiated in the presence of benzyl chloride. Benzyl chloride, like benzyl bromide, will generate the benzyl radical65966 and is similar to diphenyhethyl chloride except for the additional phenyl ring of the latter. Therefore, if the 345 nm band is due to carbocation formation, benzyl chioride should show absorption in that region. As shown in the transient absorption spectrum, Figure 3-17, no band at 345 MI is detected when anethole is irradiated with benzyl chloride instead of benzyl bromide. The presence of a peak at 3 18 nm does hdicate that benzyl chloride does scavenge the electron with a dissociate cleavage of the C-CI bond to form the benzyl radical and chloride anion, equation 3-10.
The fact that there is no band at 345 nm with benzyl chloride raises the possibiiity that the 345 nm transient observed with benzyl bromide and 3-cyanobenzyl bromide is not the corresponding carbocations. For every ben# and f-cyanobenzyl radical formed, there is a bromide anion generated, equation 3-4 and 3-5. Therefore, the similarity of the results obtained with both benzyl bromide and 3cyanobenzyl bromide could be linked to the transient at 345 nm being formed in a reaction of the bromide anion with the anethole radicd cation. The 2-bromo- 1-(4-methoxypheny1)propyl radical generated, equation 3- 11,
is kuown to have an absorption maximum near 345 m.39-45
In the presence of diphenylmethyl chloride and benzyl chioride, the lack of absorption in the 345 nm region may be linked to the fact that it is the chloride anion that is fomed with their respective radicais, equations 3-6 and 3-10. The Z-chloro-1-(4- methoxyphenyl)propyl radical which wodd be fomed with reaction of the chioride anion and the anethole radical cation is known to absorb in the region of 3 10 nm, equation 3- 12.39 With absorption at 3 10 nm, the PthIoro radical is outside the spectral limits of detection with 308-nm irradiation.
To gain merevidence to make a confident assignment to the transient at 345 nm, chioride anion was added to the anetholdtxnzyl bromide system. The chlonde anion should react with the carbocation efficiently but no observable effect on the P-bromo radical is anticipated In fact, the addition of tetraethylammonium chloride (le-10-5 M) did not influence the band at 345 am indicating that the transient at 345 nm cannot be the carbocation. As expected, the rate constant for decay of the anethole radical cation rnonitored at 600 nm did increase as more chloride was added, Figure 3-1 8. To observe the inauence of bromide anion on the anethole radical cation under the conditions employed but without radical formation, anethole was irradiation in the presence of ammonium bromide. The transient absorption spectrum generated has absorption in the region of 345 nm and lshows radid cation formation with characteristic bands at 390 and
600 nm, Figure 3-19. The decay traces monitored at 345 nrn under nitrogen and oxygen- saturated conditions clearly show an inmase rate of decay in the presence of oxygen, inset of Figure 3-19. The quenching of the absorption at 345 nm with oxygen is in accord with the transient at 345 nm king identifid as the 2-bromo- 1-(4-methoxypheny1)propyl radical, equation 3-1 1. Furthemore, an increase in the observed decay rate constant of the anethole radical cation is observed which cleariy shows a lin- relationship to the concentration of bromide anion, Figure 3-20. The quenching rate constant of the anethole radical cation by bromide anion is calculated to be (22û.2)~10'0 M-1 s-1 which is the diftision-contrclled limit in neat acetonitril60 and agrees with previously calctdated val~es.~s From the observations with benzyl chloride and the chioride and bromide anions with respect to absorption at 345 nm, it is concluded that the bromide anion reacts with the anethole radical cation in the presence of benzyl bromide and 3cyanobenzyl bromide to generate the corresponding p-bromo radical. Wavelength I nm
Figure 3- 17. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile with benyl chlonde ( 12.9 mM) recorded 560 as (a),1.88 p (O),6.12 p (a), and 12.6 ps (O) after the laser puise.
Time / ps
Figure 3-18. Decay traces monitomi at 600 nm for the anethole radical cation upon 308- nm irradiation of anethole (0.42 mM) with benzyl bromide (12.9 m.)in nitrogen-saturated acetonitrile with O (e),4 (O),and 12 (m) xlû-5 M of chlonde anion added. Wavelength / nm
Figure 3- 19. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.42 mM) in nitrogen-saturated acetonitrile with ammonium bromide (0.12 mM with 0.2% water) recorded 480 ns (a), 1.80 ps (O), 6.04 ps (i).and 12.6 ps ((O after the laser pulse. Inset shows the decay traces monitored at 345 nm under nitrogen (0)and oxygen- saturated (0)conditions.
1 1 I I I 1 2 2.5 3 3.5 4 4.5 [Bromide anion] / 1O-4 M
Figure 3-20. Plot of observed fit-order decay rate constants (0) for the anethole radical cation monitored at 600 nm upon 308-nm irradiation of anethole (0.42 mM) in nitrogen- saturateci acetonitrile versus the concentration of bromide anion. 3.3 Laser Photolysis of Anethole in SDS
When anethole is photolyzed withh the rnice1la.r medium of SDS, characteristic absorption at 390 and 600 nm indicates that the anethole radicai cation bas been generated, Figure 3-21. There is also broad absorption that extends beyond 700 nrn which is quenched by oxygen. The broad absorption in the visible region of the specttum cm be identifid as the hydrated electron67 whch is confirmed by oxygen quenching, equation 3-13.
O+ hv/SD + e- (aq) (3-13) An
The electron was not visible in the acetonitrile spectrum, Figure 3-1, as the solvated electron in acetonitrile has no detectable absorption band~.~5The appearance of the hydrated electron in SDS reinforces that the anethole radical cation is formed through photoionization since electron formation accompanies this process. The fmt-order rate constant for decay of the anethole radical cation in SDS was determined to be (3.at0.02)xl@ s-l at 390 m.The anethole radical cation is considerably longer lived in the SDS media compared to neat acetooitrîle where the decay rate constant was (6.5S.1) xi05 s-1. Figure 3-2 1. Transient absorption spectmn generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS recorded 560 ns (a),2.76 ju ((0, 7.60 ps (W), and 13.4 ps (O) after the laser pulse.
33.1 Laser Photoiysis with Benyl bromide
The photolysis of anethole with CO-incorporatedbenzyl bromide in O. 1 M SDS resuits in a transient absorption spectrum that is still predominantly due to absorption from the anethole radical cation, Figure 3-22. However, in the region of 600-700 nm the broad and characteristic absorption for the electron is reduced substantially. Furthemore, a srnail but distinct peak at 318 nxn characteristic for the generation of the benzyl radical is observed. The increase in absorption at 3 18 nm cm be more readily seen when the decay traces are compared in the absence and presence of benzyl bromide, inset of Figure 3-22.
As mentioned, the absorption due to the electron is quenched by the addition of benzyl bromide. This is anticipated shce the generation of the benzyl radical necessitates the scavenging of the electron by benzyl bromide, equation 3- 14. The effects of benzyl bromide on the electmn were monitored at the wavelength of 700 nm where the anethole radical cation has no distinctive absorption. For similar
reasoning, the decay traces obtained at 390 nm were taken as more characteristic of the
radical cation than at 600 nm which is complicated by absorption due to the electron in that
region. There is a dramatic quenching of the absorption at 700 nm as the concentration of benzyl bromide is increased, Figure 3-23. Both the initial absorption and the observed fmt-order rate constant monitored at 700 nm are aff'ted by increased concentrations of beql bromide. Both pieces of data confirm that benzyl bromide does scavenge the electron to generate the benzyl radical within SDS micelles. In terms of the initiai absorption, there is a steady decline in its value as the concentration of benzyl bromide is
increased, Figure 3-24. In contrast, a hear increase in the fmt-order rate constant is observed when plotted as a function of benzyl bromide concentration, Figure 3-25. Linear Ieast squares fitting gives a rate constant for quenchùig of the electron by benzyl bromide of
(4.4w.2)xlOS M-1 s-1.
The dynamics (Le. rate constant) and yield of radical cation at 390 nm seem to be Iargely unafkcted by the addition of benzyl bromide. The observed f~st-orderrate constant at 390 nm had no significant alteration as the concentration of benzyi bromide was increased and only smd changes in the optical density at 390 nrn are detected, Figure 3-24 Figure 3-22. Transient absorption spectrum generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with benzyl bromide (7.94 mM) recorded 560 ns (O), 2.76 ps (O), 7.60 ps (H), and 13.4 ps (O) afkr the laser pulse. Inset shows the decay traces rnonitored at 318 nm in the presence of bentyl bromide a. concentrations of O (O) and 7.94 (a)mM.
Figure 3-23. Decay traces rnonitored at 700 nm upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with O (a), 1.98 (O),and 7.94 (m) mM of benzyl bromide added. penzyl bromide] 1 10-3 M
Figure 3-24. Plot of the optical density monitored at 390 (0)and 700 (m) nm upon 308- nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide.
[Benzyl bromide] / 10-3 M
Figure 3-25. Plot of observed fmt-order decay rate constants monitored at 700 nm (0)for the electron upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl brornide. 33.2 Laser Photolysis with 3-Cyanobenyl bromide
The photolysis of anethole with CO-incorporated 3-cyanobenzyl bromide in 0.1 M
SDS led to the generation of the transient absorption spectnrm shown in Figure 3-26 which is characteristic for the anethole radid cation with bands at 390 and 600 m. The absorption due to the hydrated electron in the visible region of the spectnun is quenched in the presence of f-cyanobenzyl bromide in the same rnanner as was seen with benzyl bromide. No clear absorption at 330 nm for the 3cyanobenzyl radical is measurable due to the strong absorption hmthe anethole radical cation in this region.
As the concentration of 3-cyanobenzyl bromide increased, the absorption and observed decay rate constant for the electron are aitered dramaticaily, Figure 3-27. With respect to the optical density, the value decreased as the concentration of 3cyanobenzyl bromide increased, Figure 3-28. The rate constants for the decay of the electron were obtained by fitting decay traces monitored at 700 nm using a monoexponential expression. The fmt-order rate constants were iinearly related to the 3-cyanobenzyl bromide concentration, Figure 3-29, giving a rate constant for quenchhg of the electron by
3cyanobenzyl brornide of (1.1M.03)~log M-l s- 1. The behaviour of the electmn with 3-cyanobenzyl bromide present confinns that the electron is scavenged to generate the correspondhg 3-cyanobenzyl radical, equation 3- 15.
+CH3 h~ / SDS [ +~"3]*' -~r-)vCH? + es+ 1 (3- 15) An 308-nm * A, = 390,600 nm CN CN = 330 nm
In terms of the observed fmt-order rate constant for decay of the anethole radical cation, no appreciable change was detected in the presence of 3-cyanobenzyl bromide.
However, a slight decrease in the optical density monitored at 390 nm is seen as the concentration of 3cyanobenzyl bromide is increased, Figure 3-28. Wavelength / nm
Figure 3-26. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with 3-cyanobe~qlbromide (7.76 mM) recorded 440 os (a),2.32 ps (O), 6.96 p (i).and 12.8 ps (O)after the laser pulse.
Figure 3-27. Decay traces monitored at 700 nm upon 308-nrn irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS in the presence of 3-cyanobenzyl bromide at concentrations of O (a), 1.94 (O),and 7.76 (i)mM. 0.2 rl I I 1 I I I I
0.05 - -
O' O' I 1 I 1 1 i I O 1 2 3 4 5 6 7 [3-Cyanobenzyl bromide] 1 1û-3 M
Figure 3-28. Plot of the optical density monitored at 390 (0)and 700 (O) nm upon 308- LUXI irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3cyanobenzyl bromide.
[3-Cyanobenzyl bromide] 1 M
Figure 3-29. Plot of observed fmt-order decay rate constants monitored at 700 nrn (0)for the electron upon 308-mn irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3cyanobenzyl bromide. 333 Laser Photolysis with Toluene and 3-Cyanotoluene
Small changes were observed in the initial optical density of the electron monitored at 700 nm upon irradiation of anethole in O. 1 M SDS in the presence of benzyl bromide and f-cyanobenzyl bromide. In addition, a sddecrease in the yield of radical cation in the presence of 3cyanobenzyl bromide was observed. To examine whether these changes were due to the presence of the organic compounds within the micelle or by the scavenging of the electron by ben@ bromide and 3cyanobenyl bromide, experiments were carrieci out in the presence of toluene. Toluene has similar stmctural features to beql bromide and 3cyanobezlzyl bromide, but is not expected to trap the electron. The transient absorption spectnim obtained upon 308-nm irradiation of anethole with toluene in nitrogen-saturated 0.1 M SDS is shown in Figure 3-30. In the presence of toluene, absorption in the visible region of the spectrum is broad and extends to 700 nm which is characteristic of the electron. With the introduction of oxygen, the absorption monitored at 700 nm is effectively quenched and codm that the electron is indeed present. The absorption monitored at 700 and 390 nm for the electron and radical cation, respectively. shows no changes with increasing concentration of toluene as shown in Figure 3-3 1. Furthemore, the observed first-order rate constant for the electron decay was found to be unaffected by increased concentrations of toluene, Figure 3-32. Therefore, photoionization of anethole within SDS micelles is unperturbed by the CO-incorporationof toluene, equation 3- 16.
The effect of incorporating an organic molecuie into the micelles with anethole that can scavenge the electmn formed with the radical cation was investigated using 3cyanotoluene. 3-Cyanotoluene wiü scavenge the expe1Ied electron to generate the correspondhg 3-cyanotoluene radical anion but lacks the C-Br bond to form the f-cyanobenzyl radical. Irradiation of anethole in the presence of 3cyanotoluene in 0.1 M SDS generated the traosient absorption spechum shown in Figure 3-33. The anethole radical cation is clearly detected with absorption at the characteristic wavelengths of 390 and 600 nm. However, no absorption at long wavelengths due to the electron is detected which is not the case when anethole is Wated in SDS alone, Figure 3-21. This indicates that 3cyanotoluene scavenges the electron. Decay traces monitored at 700 nm for the electron were fitted using a monoexponential equation to give the observed fmt-order decay rate constants. The decay rate constant for the electron increased as a function of increasing concentration of 3cyauotoluene, Figure 3-34, giving a second-order rate constant of (1.8~.6)x109 M-1 s-1. Furthemore, the spectrum shows a peak at 320 mn immediately following the laser pulse which is not present in the absence of 3-cyanotoluene and is quenched with oxygen, inset of Figure 3-33. This is associated with the
3cyanotoIuene radcd anion68 coofîg that the electron is scavenged by
3-cyanotoluene, equation 3-17. The anethole radical cation monitored at 390 MI showed a slight decrease in the intensity of absorption, Figure 3-35. However, no measurable change in the observed first-order decay rate constant for the anethole radical cation is detected- Figure 3-30. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with toluene (8.8 mM) recorded 440 us (a), 2.32 ps (O), 6.96 ps ((i),and 12.8 ~LS(u) after the laser pulse.
Figure 3-3 1. Plot of the optical density monitored at 390 (0)and 700 (@) nm upon 308- nm hdiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of toluene. Figure 3-32. Plot of observed first-order decay rate constants monitored at 700 MI (0) upon 308-nm irradiation of anethole (0.4 mM)in nitrogen-saturated- 0.1 M SDS versus the concentration of toluene.
300 350 400 450 500 550 600 650 700 Wavelength / nm
Figure 3-33. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.4 mM) in nitrogen-saturated 0.1 M SDS with 3cyanotoluene (5.6 rnM) recorded 560 ns (O), 2.76 ~LS(O), 7.60 ps (i),and 13.4 ps ((O) after the laser pulse. Inset shows decay traces monitored at 320 nm under nitrogen (a) and oxygen-saturated (0)conditions. Figure 3-34. Plot of observed first-order decay rate consuints for the electron monitored at 700 nm (O) upon 308-nrn irradiation of anethole (0.4 mM) in nitrogen-sahirated 0.1 M SDS versus the concentration of 3-cyanotoluene.
Figure 3-35. Plot of the optical density monitored at 320 (0)and 390 m (O) upon 308- nm irradiation of anethole (0.4 mM) in nitmgen-saturateci 0.1 M SDS versus the concentration of 3cyanotoluene. 3.4 Laser Photolysis of Anethole in CTAB
The photolysis of anethoie within CïAE% micelles results in absorption at 390 and
600 nm which is suggestive of the generation of the radical cation, Figure 3-36. As was the case with SDS micelles, the hydrated electron is deteaable in the visible region of the speceum. The presence of absorption at 700 nm gives evidence for the fact that the photoionization process occurs within CTAB miceiles, equation 3-18.
The obsemed decay rate constant for the anethole radical cation was measured to be (2.3HI.03)xlfl s-1 when generated in CTBmicellar solutions which is a hirther increase in lifetime as compared to SDS and to neat acetonitde. ln the presence of oxygen, the rate constant for the decay of the anethole radid cation increases to (8.9M.2)~104 s- l. Since the radical cation of anethole is largely insensitive to oxygen, the observed effect of oxygen is presumably caused by the addition of brornide, which is present as the countenon, to the radcal cation which rapidly generates an equili'brium between the anethole radical cation and the 2-bromo- 1-(4-methoxyphenyl)propyl radical hed. At 0.05 M of bromide anion, the radical cation is the favoured spe~ies3~so that the radical is difficult to detect. Under these circumstances, the anethole radical cation decay increases in the presence of oxygen due to the trapphg of the radical. Similar results are observed in aqueous homogeneous solution.@ Wavelength 1 nm
Figure 3-36. Transient absorption spectmm generated upon 308-nm irradiation of anethole (0.27 mM) in nitrogen-saturated 0.05 M CïAB recorded 400 ns (@), 1.60 ps (O),4.48 ps (U), and 10.8 ps (O) after the laser pulse.
3.4.1 Laser Photolysis with Beoyl bromide
The transient spectnm generated with 308-nm irradiation of anethole in the presence of benzyl bromide in 0.05 M CïAB is still predominantly characteristic of the anethole radical cation with absorption at 390 and 6Oû nm, Figure 3-37. The generation of the benzyl radical is indicated by the distinctive absorption at 3 18 nm, inset of Figure 3-37, and the quenching of the absorption above 600 nm due to the hydrated electron. The decrease in electmn absorption monitored at 700 nm as a function of the concentration of benzyl bromide is linear and almost completely returns to zero, Figure 3-38. This is consistent with scavenging of the electron by benzyl bromide and subsequent generation of the ben@ radical, equation 3-19. CH,. ]:.-+(-p-Br-mf-J (3-19)
In terms of the observed fmt-order rate constant for the decay of the electron, a ciramatic inmase is found with the rate constant increasing fiom (2.9M.02)~106s-1 at
O mM to (3 lf 1)x106 s-1 with 2 mM of benzyl bromide.
With respect to the radical cation monitored at 390 nm, no signifiant change in either the maximum absorption, Figure 3-38, or the observed ht-order decay rate constant is measurable 6thincreasing concentration of benzyl bromide.
Wavelength I nm
Figure 3-37. Transient absorption spectnim generated upon 308-nm irradiation of anethole (0.27 mM) in nitrogen-saturated 0.05 M CïAB with benzyl bromide (7.94 mM) recordai 400 ns (a),1.60 ps (O),4.48 ps ((i,and 10.8 ps (O)after the laser pulse. Inset shows decay traces monitored at 3 18 nm in the presence of benzyl bromide at concentrations of O (0)and 7.94 (e)mM. penzyl bromide] 1 10-3 M
Figure 3-38. Plot of the optical density monitoring at 390 (a) and 700 (0)nm upon 308- nm irradiation of anethole (0.27 mM) in nitrogen-saturated 0.05 M CïAE3 versus the concentration of benzyl bromide.
3.5 Laser Photolysis of p-Methoxystyrene in Acetonitrile
The transient absorption spectnim generated upon irradiation of p-methoxystyrene in acetonit.de exhibited bands at 370 and 600 nm which are characteristic for the p-me thoxy styrene radical cation, Figure 3-3 9.46 The observed fm t-order rate constant for the decay of the p-methoxystyrene radical cation in neat acetonitriie generated through photoionization is ( 1.=.02)x 106 s- l, equation 3-20. In addition, the introduction of oxygen does not influence the observed decay rate constant which is typical for the p-methoxystyrene radical cation. 73
The pmethoxystyrene fadicai cation, as opposed to anethole, has been documented to undergo a dimerization reaction with neutral p-methoxystyrene molecules. It was reported that upon 355-nm irradiation of chloranil in the presence of p-methoxystyrene, an electron transfer occdto yield the conesponding radical ions. As weil, a band at 500 nrn was observed which was assigned to the substituted hexatriene radiai cation fomed upon the interaction of the olefh denved radical cation with a neutral olefïn molecule, equation 3-2 1.
However, the absorption of the monomer radicai cation is distinctly visible at the concentration employed in this sîudy and the dirnerization route for radical cation decay should pose no hindrance to the investigation of whether the benzyl radicals will react with the pmethoxystyrene radical cation. Wavelength I nm
Figure 3-39. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated acetonitrile recorded 320 ns (a), 1.30 ps (O),3.80 ps (W), and 10.1 ps (O) after the laser pulse.
3.5.1 Laser Photoiysis with Benzyl bromide
The transient absorption spectnim generated upon 308-nm irradiation of p-methoxystyrene in the presence of 12.9 mM benzyl bromide in acetonitrile stiU exhibits radical cation formation with absorption at 600 nm, Figure 3-40. However, a band at 330 nm is present which was not observed in the absence of benzyl bromide. In addition, characteristic absorption at 3 18 nm confkms that the benzyl radical is generated through scavenging of the electron which promotes the dissociative cleavage of the C-Br bond, equation 3-22, as was seen with anethole and benzyl bromide. The increase in absorption at 318 nm appears linear at concentrations of benzyi bromide below 6 mM and then levels off at higher concentrations. A dependence on benzyl bromide concentration is in agreement with the benzyl radical assignment, Figure 3-42. As well, a dependence was observed with the absorption monitored at 330 am on the benzyl bromide concentration, Figure 3-43. In addition to the increases in absorption monitored at 3 18 and 330 nm, a decrease is observed at 600 nm, Figure 3-43. This rnay correspond to a reaction between the bromide anion generated in addition to the beuzyl radical with the pmethoxy styrene radical cation to form the 2-bromo- 1-(4-methoxyphenyl)ethyl radical,45 equation 3-23.
The assignment of the 330 nm band to the Zbromo- 1-(4-methoxyphenyl)ethyl radical agrees with the resuits obtained with anethole and benzyl bromide (vide s~ipra). Further evidence is provided with the introduction of oxygen. The spectnun generated in the presence of oxygen is more characteristic of the p-methoxystyrene fadical cation with the dominant absorption at 370 and 600 m. Figure 3-4 1. The radical cation is unaffected by the presence of oxygen; however. the absorption at 3 18 and 330 nrn is effectively quenched in the presence of oxygen, which agrees with the assignment to the benzyl and 2-brome 1- (4-methoxypheny1)ethyl fadicals respectively. In terms of the observed fht-order rate constant for decay of the p-methoxystyrene radical cation. no effet is measured with increasing concentration of benzyl bromide. Since the changes associatecl with radical cation absorption are detected in the intensity of the absorption immediately foilowing the laser pulse and not in the decay rate constant, the partial quenching of the p-methoxystyrene radical cation by the bromide anion is a static process, Figure 343, and not a dynamic one. Wavelength / nm
Figure 3-40. Transient absorption spectrum genemed upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with benzyl bromide (12.9 mM) in nitrogen-saturated acetonitde recorded 320 ns (a), 1.36 ps (O)),3.80 ps ((., and 10.1 ps (0)after the laser pulse. Inset shows 3 18 nm traces under N2 (a) and 02(0) conditions.
Wavelength / nm
Figure 3-41. Transient absorption spectm generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with benzyl bromide (12.9 mM) in oxygen-saturated acetonitrile recorded 320 ns (O), 1.36 ps (O), 3.80 ps (i),aud 10.1 ps (O) after the laser pulse. Inset shows 330 nrn traces under N;?(a) and 02(0) conditions. 0.07 l 1 1 l 1 1 0.06 - -C - - - - 0.01 - -
O I 1 I I 1 I O 2 4 6 8 IO 12 Benzyl bromide] / IV3 M
Figure 342. Plot of the optical density monitored at 3 18 nm upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated acetonitriie versus the concentration of benzyl bromide.
penzyl bromide] 1 IO-3M
Figure 3-43. Plot of the optical density monitored at 330 (0)and 600 (a) nm upon 308- nm irradiation of p-methoxystyrene (0.75 mM) in nitrogen-saturated acetonitrile versus the concentration of beqlbromide. 35.2 Laser Photoiysis with 3-Cyanobeilyl bromide
The transient absorption spectnun generated upon 308-nm irradiation of pmethoxystyrene in acetonitrile with 3-cyanobenzyl bromide showed a sharp band at 330 nrn, Figure 3-44. The radical cation was essentidy quenched with only minor absorption near 370 and 600 m. The peak at 330 nm is characteristic for the 3-cyanobenyl fadical and in the presence of oxygen the absorption at 330 nm is quenched, inset of Figure 3-44. Therefore, irradiation of p-methoxystyrene in the presence of 3cyanobenzyl bromide generates the 3cyanobenzyI radical, equation 3-24.
However, the absorption at 330 am is probably a combination of the 3-cyanobenzyl radical and the 2-bromo- 1-(4-methoxypheny1)ethyl radical w hich is known to have a maximum in this region.45 The p-bromo radical is generated upon a reaction between the bromide anion, fonned with the 3cyanobenzyl radical, and the p-methoxystyrene radical cation, equation 3-23. The effxts on the radical cation are static and not dynamic in nanire as indicated by the decrease in the absorption due to the radical cation, Figure 3-46, upon addition of 3-cyanobenzyl bromide but no change in the observed fmt-order decay rate constant monitored at 600 m. The quenching of the absorption due the radical cation is by the bromide anion to generate the P-bromo radical as was discussed with benzyl bromide (vide supra). Wavelength 1 nm
Figure 344. Transient absorption spectnun generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with 3-cyanobenzy1 bromide (12.3 mM) in nitrogen-saturated acetonitrile recorded 320 ns (a), 1.36 ps (O), 3.64 ~LS(i), and 9.68 ps (a)after the laser pulse. Inset shows 330 nm traces under N2 (a)and 02(0) conditions.
300 350 400 450 500 550 600 656 700 Wavelength / nm
Figure 3-45. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with 3cyanobenzyl bromide (12.3 mM) in oxygen-saturated acetonitrile recorded 320 ns (a),1.36 ps (O), 3.64 ps (W), and 9.68 ps (O) after the laser puise. inset shows 370 nm traces under N2 (a)and 02(0) conditions. 0.025 - 8 0.02 - - 4 0.015 - - 0.01 - - 0.005 - -
0.- I I I I I 1 1 1 O 0.8 1.6 2.4 3.2 4 4.8 5.6 6.4 [3-Cyanobenzyl bromide] / 10.3 M
Figure 3-46. Plot of the optical density monitored at 600 nm upon 308-nrn irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated acetonitrile versus the concentration of 3- cyanobeazyl bromide.
3.53 2-Bromo-1-(4-methoxyphenyi)ethyl Radical Formation
As was the case with anethole and the radical precmors, a new transient is observed with hdiation of pmethoxystyrene in the presence of benzyl bromide and 3cyanobenzyl bromide with an absorption maximum at 330 am. In agreement with the identifcation of the transient observed with anethole to the 2-bromo-1-(4-methoxyphenyl)propyl radical, the transient at 330 nm was assigned to the correspondhg 2-bromo-1-(4- methoxypheny1)ethyl radical. The other possibility would be carimcation formation. To confimi the radical assignment, chloride anion was added to the pmethoxystyrenehenzyI bromide system. The addition of the nucleophilic chioride anion to a p-methoxystyrenelbenzyl bromide sample created no change in the transient at 330 nm although an increase in the observed rate constant for the radical cation at 600 nm was detected, Figure 3-47. This agrees with the assignment of the 330 nm band to the radical and not the cartmcation which would be expected to react efficientiy with the cidonde anion. In addition, Uradiation of p-methoxystyrene in the presence of bromide anion was cmied out to determine the absorption position of the pbromo radical under similar
conditions employed with the radical precursors. The hansient absorption spectnun generated upon irradiation of prnethoxystyrene in the presence of ammonium bromide,
Figure 3-48, is dominated with absorption centered at 330 nm which is quenched by oxygen, inset of Figure 3-48. Srnail absorption due to the radical cation is detected at 600 nm but the conesponding band at 370 nm is not dis~guishablenom the intense 330 tun band. The absorption at 330 nrn is assigned to the 2-bromo-1-(4-methoxypheny1)ethyl radical, equation 3-23. The increase in the observed first-order decay rate constant for the p-methoxystyrene radicai cation shows a linear relationship to the concentration of bromide anion, Figure 3-49. The quenching rate constant for bromide anion of the p-methoxystyrene radid cation is calcdated to be (2.7&0.2)~10~~M-l s-1 which corresponds to the diffusion-controlled limit in neat acetonitrile50 and a previously calcdated value.45 Time / ps
Figure 3-47. Decay traces monitored at 600 nm for the radical cation upon 308-nm irradiation of pmethoxystyrene (0.75 mM) and benzyl bromide ( 12.9 mM) in nitrogen- sanirated acetonitrile with O (O), 6 (O), and 12 (i)x 10-5 M of chloride anion.
O I I I I 5 10 1s 20 Time / ~ls
Wavelength I nm
Figure 348. Transient absorption spectnim generated upon 308-MI irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated acetonitrile with ammonium bromide (0.47 mM) recorded 320 ns (@), 1.36 ps (O),3.80 ps ((i),and 10.1 ps (O) akr the laser pulse. Inset shows 330 nm traces under N2 (0)and 02(0) conditions. promide anion] / 10-4 M
Figure 3-49. Plot of observed first-order decay rate constants monitored at 600 um (0)for the radical cation upon 308-nm irradiation of p-methoxystyrene (0.75 mM) ui nitrogen- saturated acetonitrile versus the concentration of bromide anion.
3.6 Irradiation of p-Methoxystyrene in SDS
Upon irradiating p-methoxystyrene in the anionic miceiles of SDS, the spectnim showed absorption at 370 and 700 nm which represents the radicai cation and hydrated electron. respectively, Figure 3-50. Therefore, the spectrum is characteristic of photoionization of p-methoxystyrene, equation 3-25. The first-order decay rate constant for the p-methoxystyrene radical caîion was measured to be (4.8~.03)xl05s-1 in 0.1 M SDS, - 300 350 400 450 500 550 600 650 700 Wavelength / nm
Figure 3-50. Transient absorption spectnun generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS zecorded 320 ns (a), 1.12 y s (O), 2.88 p (H),and 6.92 ps (O) after the laser puise.
3.6.1 Laser Photolysis with Benyl bromide
The transient absorption spectnun generated upon irradiation of p-methoxystyrene in 0.1 M SDS with CO-incorporated benzyl bromide is shown in Figure 3-5 1. Characteristic absorption for the p-rnethoxystyrene radical cation is still prevalent at 370 and 600 nm. In addition. a sharp band at 3 18 nm is present immediately following the laser pulse which is consistent with the formation of the benzyl radical. Further evidence for the generation of the benzyl radical is the effective quenching of the absorption due to the electron. Decay traces monitored at 700 nm reveal that in the presence of benzyl bromide both the absorption and observed decay rate constant are altered fiom that observed in its absence, Figure 3-52. The optical density showed a marked reduction as the concentration of benzyl bromide was increased, Figure 3-53. In terms of the observed fmt-order decay rate constant, the decay traces monitored at 700 nm were fit using a monoexponential equation. the introduction of benzyl bromide was observed to aaelerate the decay. Furthemore, the increase is iinearly related to the concentration of benzyl bromide added, Figure 3-54. Linear Ieast squares fitting gives a quenching rate constant of (1.0f0.7)~109M-1 s-1 for the electron by benzyl bromide. The influence of benzyl bromide on the electmn supports the generation of the ber@ radical upon photoionization of p-methoxystyrene with benzyl bromide in SDS micelles, equation 3-26.
h, = 370,600 nrn h, = 318 nrn
The influence of added benzyl brom..deon the pmethoxystyrene radical cation is not dramatic. No measurable change in the observed fus-order decay rate constant is found dthough a smd decrease in the opticai dcnsity monitored at 370 nm was observed at increased concentration of benzyl bromide, Figure 3-53. Wavelength / MI
Figure 3-51. Transient absorption spectmm generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0. l M SDS with benzyl bromide (7.94 mM) recorded 320 ns (a),1.12 ps (O), 2.88 ps (i),and 6.92 p (O)after the laser pulse.
Time / ps
Figure 3-52. Decay traces monitored at 700 nm upon 308-nrn irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS in the presence of benzyl brornide at concentrations of O (a), 2 (O), and 7.9 (B) mM. [Benzyl bromide] / 1C3M
Figure 3-53. Plot of the optical density monitored at 700 (a) and 370 (0)nm upon 308- nm irradiation of p-methoxystyrene (0.75 mM) in nitrogen-sahuated 0.1 M SDS versus the concentration of benzyl bromide.
O 1 2 3 4 5 6 7 8 penzyl bromide] / 10-3 M
Figure 3-54. Plot of observed fit-order decay rate constants monitored at 700 nm (0)for the electron upon 308-nm irradiation of p-rnethoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of benzyl bromide. 3.6.2 Laser Photoiysis with 3-Cyanobenyi bromide
The transient absorption spectnnn generated upon 308-nm irmdiation of pmethoxystyrene in nitmgen-saturated 0.1 M SDS in the presence of 3-cyanobenzyl bromide represents absorption specific for the p-methoxystyrene radicai cation at both 370 and 600 nm, Figure 3-55. In addition, there appears a distinct transient band at 330 nrn characteristic of the f-cyanobenzyl radical. The broad absorption characteristic for the hydrated electron between 600-700 nm is altered significantly in the presence of 3cyanobenzyl bromide, Figure 3-56. Both the observed ht-order decay rate constant and the absorption monitored at 700 nm are dfkcted by the introduction of 3cyanobenzyl bromide, as was seen with benzyl bromide. The absorption showed a reduction with increasing amounts of 3cyanobeuzyl bromide, Figure 3-57. The observed decay rate constants obtained from a monoexponential expression increased iinearly as a function of 3cyanobenzyl bromide concentration, Figure 3-58. Least squares fitting gives the rate constant of (9.2H.1)x108 M-1 s-1 for quenching of the electron with 3syanobeoyl bromide. The influence on the electron of CO-incorporationof 3cyanobenz-1bromide with p-methoxystyrene in SDS is consistent with the scavenging of the electron expeiled upon photoionization to generate the corresponding radical and radical cation, equation 3-27.
The presence of the 3-cyanobenyl radical, like the unsubstituted ben@ radical, has only a small effect on the p-rnethoxystyrene radical cation. The observed fit-order decay rate constant monitored at 370 nm for the radical cation is unchanged as the concentration of 3-cyanobeql bromide is increased; however, the intensity of the optical density monitored at 370 nm is reduced, Figure 3-57. Figure 3-55. Transient absorption spectrtm generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with 3cyanobenyl bromide (10.9 mM) in nitrogen-saturated 0.1 M SDS recorded 320 ns (a),1.12 ps (O),2.88 ps (a),and 6.92 ps (0)after the laser pulse.
Time / ps
Figure 3-56. Decay traces monitored at 700 MI upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-saturated 0. l M SDS in the presence of 3- cyanobenzyl bromide at concentrations of O (@), 2.72 (O),and 10.9 (H)mM. Figure 3-57. Plot of the optical density monitored at 700 (a)and 370 (0)nm upon 308- nm irradiation of pmethoxystyrene (0.75 mM) in nitrogen-saturated 0.1 M SDS versus the concentration of 3-cyanobenzyl bromide.
[3-Cyanobenyl bromide] / 10-3 M
Figure 3-58. Plot of the observed fmt-order decay rate constants monitored at 700 nm (O) for the electron upon 308-nm irradiation of pmethoxystyrene (0.75 mM) in nitrogen- saturated O. 1 M SDS versus the concentration of 3cyanobenzy1 bromide. 3-63 Laser Photoiysis with ToInene and 3-Cyanotoluene
As mentioned with respect to anethole, the CO-incorporationof 3cyanotoluene with pmethoxystynne in SDS will aUow an investigation of the effects on the radicd cation of an electron scavenger whereas toluene allows an examination of the effects on the electron with the addition of another organic substrate to the micellar media The transient absorption spectmm generakd upon 308-nm irradiation of pmethoxystyrene with CO-incorporatedtoluene in 0.1 M SDS is characteristic for radical cation formation, Figure 3-59. In fact, absorption at 370 nm and in the visible region of the spectnm is rypical of the radical cation and hydrated electron formed through photoionization of p-methoxystyrene, equation 3-28. No change in the absorption or observed kt-order rate constants for decay of the radical cation at 370 nm or the electmn at 700 nm are measured as a function of toluene as was the case with anethole and toluene.
The transient absorption spectrum generated upon irradiating p-mthoxystyrene in the presence of 3-cyanotoluene is show in Figure 3-60. The radical cation of pmethoxystyrene is generated with absorption at 370 and 600 nm. In addition, a band at 320 nm is present which is not observed in the absence of 3-cyanotoluene. in fact, the increase in absorption at 320 nm is dependent upon the concentration of 3-cyanotoluene, Figure 3-61, and therefore assigned to the radical anion of 3-cyanotoluene.68 The appearance of the 3cyanotoluene radical anion confirms that 3cyanotoluene scavenges the electron generated upon photoionization of p-methoxystyrene, equation 3-29. hv / SDS
The observed rate constants for electmn decay were obtained by fitting the decay traces monitored at 700 nm with an monoexponential expression. Plotting the fmt-order rate constants as a function of the concentration of 3cyanotoluene resuited in a linear correlation, Figure 3-62, which gave a value of (1.3fl.1)x Hl9 M-l s-i for the quenching of the electron by 3-cyanotoluene. In tenns of the pmethoxystyrene radical cation, no measurable change in the kt-order rate constant monitored a-370 nm was detected with increasing concentration of 3cyanotoluene. However, a substantial increase in the absorption at 370 nm was observed, Figure 3-60.
Figure 3-59. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) with toluene (8.8 mM) in nitrogen-saturated 0.1 M SDS recorded 320 ns (a),1.12 ps (O),2.88 ps (i).and 6.92 ps (O)after the laser pulse. Wavelength I nm
Figure 3-60. Transient absorption spectrum generated upon 308-nm hdiation of p- methoxystyrene (0.75 mM) in nitmgen-saturated 0.1 M SDS with 3-cyanotoluene (5.63 mM) recorded 320 ns (e),1.12 ps (O),2.88 p (R), and 6.92 p (O) after the laser pulse. Inset shows decay traces monitored at 320 nm under N2 (O) and 02 (0)conditions.
Figure 3-6 1. Plot of the optical density monitored at 320 (@) and 370 (0)nrn upon 308- nrn irradiation of p-methoxystyrene (0.75 mM) in nitrogen-sahuated 0.1 M SDS versus the concentration of 3-cyanotoluene. Figure 3-62. Plot of obsenred fmt-order decay rate constants monitored at 700 nm (0)for the electron upon 308-nm irradiation of p-methoxystyrene (0.75 mM) in ni trogen-saturated 0.1 M SDS versus the concentration of f-cyanotoluene.
3.7 Irradiation of p-Methoxystyrene in CTAB
When p-methoxystyrene was incorporated within the cationic micelles of cetyltiimethylammonium bromide (CTAB), the appearance of absorption at 600 nrn due to the radical cation and extending beyond 700 nm due to the electron indicated photoionization to generate the radical cation had occurred, Figure 3-63 and equation 3-30.
However, the peak in the UV region which had previously been centered at 370 nm is now centered at 350 m. In this case, the radical cation is rapidly trapped by bromide to generate an equilibrium between the radical cation and the 2-bromo-1(4- methoxypheny1)ethyl radical that in the presence of 0.05 M bromide lies essentiaily to the side of the radical,'g equation 3-3 1. In the presence of oxygen, the W .band is centered nearer 370 nm, Figure 3-64, and furthermore the UV band is observed to have an acceierated decay rate constant, inset of Figure 3-64, which is consistent with radical formation. Since the dcaI cation itseif is not influenceci by oxygen, the introduction of oxygen indirectiy influences the radical cation by quenching the radical component of the equilibrium, equation 3-3 1.
Br- micelle (3-31) An &= 370,600 nm &=330m
Due to the substantial quenchiog of the p-methoxystyrene radical cation by the countenon of cTAB,no further experiments were cmied out with this system. Figure 3-63. Transient absorption spectrum generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in nitrogen-sahuated 0.05 M CïAB recorded 320 ns (O), 1-36 ps (O), 4.08 ps (M), and 10.9 ps (O)after the laser pulse. Inset shows the decay trace monitored at 600 nm.
Wavelength / nm
Figure 3-64. Transient absorption spectnim generated upon 308-nm irradiation of p- methoxystyrene (0.75 mM) in oxygen-saturateci 0.05 M CTAB recorded 320 ns (O), l .36 ps (O),4.08 ps (i),and 10.9 ps (Ci) after the laser pulse. Inset shows the decay trace monitored at 370 nrn under nitrogen (a) and oxygensaturated (0)conditions. 3.8 Discussion
3.8.1 Homogeneous media: Acetonitde
Laser photolysis of olefins with benzyl bromide and 3-cyanobenzyl bromide in neat acetonitrile generated the corresponding olefin radical cation. In addition, the benzyI radicals themselves were observed which CO- that the electron expelleci upon photoionization of the olek can be scavenged by the radical precunors employed, Scheme
3-1. The radicals, and not the radical anions, are observed as the product of the reduction of both benzyl bromide and 3cyanobenzyl bromide due to a concerted dissociative bond cleavage that generates the corresponding radical and bromide anion? There is no spectral evidence for a reaction between the radicak and radical cations to fom the corresponding carbocations. In fact, a reaction involving the anethole or the prnethoxystyrene radical cation and the bromide anion takes place instead to generate the corresponding Pbromo radicals with absorption maxima at 345 and 330 nrn. respectively. Identification of the 345 and 330 nrn bands as the radicals is based on the observation that the same p-bromo radicals are produced upon quenching of the anethole and p-methoxystyrene radical cations by bromide anion with absorption maxima in the 330-350 nm region.39.45
The carbocations that would be formed by addition of the benzyl radicals to the radical cations also would have absorption maxima in the 340 nm region? However, the fact that absorption at -345 nm was observed with benzyl bromide but not benzyl chloride argues against carbocation formation. Since the benzyl radicai is generated fiom both benzyl chloride and benzyl bromide, the lack of a 345 nm band with benzyl chloride indicates that the reaction involving the radical cation is attack of the halide and not the benzyl radical. Furthemore, the 345 run band produced upon irradiation of anethole with benzyl bromide was not quenched by chloride, as would be expected for the carbocation. In ternis of the appearance of the &bromo radicals, the surprishg result is that no resolved growth is deiected. In other words, the transient is seen immediateiy following the laser pulse which has a time duration of 10 ns. Withui this time fiane, two processes must occur in order for the p-bromo radicals to be formed. First, the electron generated hm the olefin must be scavenged by the radical precursor, either beql bromide or
î-cyanobenzyl bromide, so that the dissociative cleavage of the C-Br bond can occur to generate the bromide anion. Second, the formation of the kbromo radical requires reaction between the bromide anion generated and the radical cation. These processes would have to occur in less than 10 ns for the Fbromo radical to be observed at 345 nm irnmediately following the laser pulse. One way by which this cmbe accomplished is for direct electron transfer from the olefk to the aryimethyl bromide to generate the bromide anion in close proximity to the olem radical cation. Addition of the bromide anion to the radical cation cm then take place before the reactants diffuse apart, equation 3-32. Therefore, the P-bromo radical would be formed promptly. .+ Br'
AcN 1'
In order for the electron to be tmsfer directly to the radical precursors, the process mut be thermodynamicaüy favourable. The Weller eq~ation~o-~'can be used to calculate the thermodynamics of the photoinduced electron msfer from the olefm to the arylmethyl brornides. Application of this equation requires the redox potentials of the donor and acceptor as well as the singlet energy of the olefm, equation 3-33.
AGm =~~.o~[E,(D/D+*)-E~(A''/A)-~~/~E]-E,(3-33)
For example, the oxidation potential of anethole in acetonitrile versus ferrocene is 0.915 Vso whereas the reduction potential of benzyl bromide in acetonitde versus ferrocene is eshated at -2.05 1 V.66 In addition, the anethole singiet energy is 93.1 kcaUrn01.7~ Therefore, assuming the solvent correction factor, e*/~,is isegiigible, the electron transfer is determined to be favourable by 25 kcaihol. In terms of 3cyanobeflzyl bromide, the reduction potential reported is -1.788 Va6 while the oxidation potential of p-methoxystyrene is 1.015 V50 with a singlet energy of 94 kcaVmol.73 As a result, electron tramfer between each donor and acceptor pair will be themodynamically favourable, Table 3- 1.
Table 3- 1. Free energy change associated with electron rransfer (A*) from the olefm to benzyl bromide and 3cyanobenzyl bromide calculated using the WeiIer equation. 1 Benzvl bromide 1 3-Cvanobenzvl bromide 1
In addition, the singlet lifetime of the particuiar alkene must be sufficiently long in order for photoinduced electron trarisfer to compete with other modes of decay available to the singlet. The singlets of anethole and p-methoxystyrene are reasonably long-lived with Metimes of 8.5 and 6 respectively. Therefore, the singlets have a sufficient lifetime to be quenched by the "inxage" electron transfer.
The observation that the bromide anion rather than the arylmethyl radicals attacks the olef5.n radical cations indicates that the bromide anion has the highest reactivity of the following species toward the radical cations in acetonitxiie.
This may not be surprishg since addition of the bromide to the radical cation produces a neutral product which should be favoured in acetonitrile. With diphenylmethyl chloride, the generation of the diphenylmethyl radical was the only spectral observation besides radical cation absorption. The lack of carbocation
formation is again presumably due to a reaction between the chloride anion and the radical
cation to form the 2chloro- 1-(4-methoxypheny1)propyl radical. However, absorption of
the khloro radical at 3 10 nm)9 is outside the spectm limits for measurement with 308-nm laser irradiation. As show in Figures 37-3-12, 3-43, 3-46, a significant decrease is observed in the absorption due to the radical cation of anethole or pmethoxystyrene immediately after the
Laser pulse upon the addition of increasing amounts of the arylmethyl bromides. This
decrease is presumably caused by quenching of the singlet by the aryhethyl bromides to generate the radical catiodbromide ion pair that collapses rapidly to give the bbromo
radicals. Quenching of the oleh singlets reduces the amount of '%et=" radical cation produced by direct photoionization resdting in the reduced yield of observed radical cation, equaîion 3-34. The observation that the yield of the P-bromo adducts increases, Figures 3-7, 3-12, 3-43, as the àetected amount of radical cation decreases is consistent with equation 3-34.
l* *+ hv An 308-nm AcN 3.83 Heterogeneous media
The CO-incorporationof the radical cation and radical precursors in aqueous miceflar solution provides two distinct pathways for the fonnation of the radical cation, Scheme 3-
2. The generation of the olefh radical cation initidy involves excitation of the olefin to the singiet state. The singlet state of the olefin may then undergo photoionization upon the absorption of a second photon to generate the radical cation and electron, pathway 1 of Scheme 3-2. Pathway 1 functions when the olefins are photolyzed alone in the micellar media The presence of absorption in the visible region of the spechum due to the electron(e is indicative of photoionization.67
However, in the presence of the radical precursors another route for radical cation formation is feasible which is a photoinduced dissociative electron msfer, pathway 2 of
Scheme 3-2. The direct transfer of the electron to the radical precursor would result in the cleavage of the carbon-halide bond to generate the corresponding radical and halicie anion. The radical cation, radical and hafide anion intermediates would be generated in close spatial contact to each other within the same micelle. A siightly altered system would be generated if the electron formed through photoionization is trapped within the same micellar cavity in which it is generated, pathway
1 of Scheme 3-2. In this case, the radical cation and radicaknion would exist as a "separated" system within the micelle. The electron generated by the photoionization pathway may also escape the micelle. Trapping of the electron can then occur by a remote benzyl bromide which will fom the radical isolated fkom the radical cation. Formation of the radical by electron trapping should accelerate the decay of the electron. As well, increased absorption due to the radical at characteristic wavelengths would be expected.
The CO-incorporation of the fadical precursors with the olefins in SDS did have a sigolncant influence on the electron with observable quenching by both ben# bromide and Scheme 3-2
rapid dissociative e-eSCapJ eIemm tmp
Escape Coupling Product
3-cyanobeazyl bromide. In each case, the absorption at 700 nm immediately following the laser pulse decreased as the concentration of the aryimethyl bromide increased, Figures
3-24, 3-28, 3-53, 3-57. In addition, the kt-order rate constant for the electron decay increased as a function of the concentration of the arylrnethyl bromides. Therefore, photoionization to generate the hydrated electron saoccurs in the presence of acceptor, with the eîectron king trapped by the acceptor to generate the radical. The decrease in the yield of the hydrated electron indicates that trapping of the olefin sioglet by the aryhethyl bromide is occurring, pathway 2 of Scheme 3-2; or, that upon photoionization the electrm is trapped before escape into the aqueous phase. Intramiceilar electron tmnsfer seems Wrely if one considers the occupancy levels in O. 1 M SDS of the aryimethyl bromides compared to the olefins. The molat concentration of micelles for a 0.1 M solution of SDS is 1.5~10-3
M.15 The molar concentration of anethole and pmethoxystyrene in SDS were 0.4~10-3 M and 0.75~10-3M and the maximum concentrations of benzyl bromide and 3cyanobenzyl bromide were 8x 1 M and 7- 11 x le3M. The occupancy of each molecule per micelle are 114 for anethole with beozyl bromide and 3-cyanobenzyl bromide distributed in a 511 ratio whereas the molecule/miceIle ratio with pmethoxystyrene was IL2 with benzyl bromide at 5/1 and for î-cyanobenzyl bromide 711. Therefore, the probability of having a radical precursor next to a singiet oleh is high, which will enable the "direct" dissociative electron tramfer to occur. The reduced yield of the hydrated electron indicates that the radical cation and radical are king generated within the same micelle at least to some extent. The observation that toluene had no influence on the electron yield indicates that decreasing the polarity of the micellar interior has no impact on the degree of photoionization, Figure 3-3 1. If the radical cationkadical pair generated within the micelles rapidly coiiapsed to give a carbocation product, one would expect to see a significant decrease in the yield of radical cation. DecRases in the absorption due to radical cation were observed in some cases, Figures 3-24, 3-28, 3-53, 3-57. For example, the yield of the anethole radical cation decreased by 30% upon going fonn O rnM to -7.5 mM of 3cyanobenzyl bromide. Similar decreases of 30% were observed for the yield of the p-methoxystyrene radical cation in the presence of both benzyl bromide and 3cyanobeozyl bromide. On the other hand, no significant alteration in the absorption due to the anethole radical cation was detected in the presence of benzyl bromide. One possible cause for the decrease in radical cation formation may be the decrease in the polarity of the micelle intenor caused by the introduction of the nonpolar acceptor molecules. A decrease in the polarity codd in principle inhibit photoionization of the olefin and therefore decrease radical cation formation. However, the radical cation yieId remained unchanged in the presence of toluene which indicates that photoionization is unaffecteci upon the CO-incorporation of nonpolar molecules within the miceiles. In addition, the reaction of the radical cations with bromide as observed in acetonitrüe should not play a sipnincant role in aqueous micelle solutions. As descni in Chapter 2, the &bromo radicals that wouid be produced are short-lived in miceliar media and wodd rapidly undergo heterolysis to regenerate the bromide anion and radical cation. Thus, the obsewation that the yield of detected fadical
cation immediately after the laser pulse decreased in some cases suggests thai radical cation/radical coupling may occur to some extent in these systems. The results also show that absorption due to the radcals in SDS and CïAB is very
weak compared to that observed in acetonieile. The observation that the electron is
quenched by the addition of the benzyl bromide and 3-cyanobenzyl bromide indicates that radical formation by dissociative reduction must be taking place. In addition, the formation of the radical anion of 3-cyanotoluene upon photolysis of both anethole and p-methoxystyrene, Figures 3-33 and 3-60. confîms that the electron is indeed king
transferred to, or scavenged by, the acceptor. Therefore. weak absorption by the radicals
does not appear to be caused by inefficient trapping of the electron by the acceptor molecules. Ow possible explanation for the Iow radical absorption in the micelles is that the mdicals are reacting with the radical cation to give the carkation. This would be consistent with the conclusion made above that such coupling is responsible for the decreased yield of the radical cations in the presence of benzyl bromide and 3-cyanobenzyl bromide. In tem of the magnetic field study on the heterolysis reaction, the presence of the nucleophilic bromide as the counterion of CTAB complicates the system so that =AT3 is not believed to be a good miceiiar media for the magnetic field study. In addition, the basic conclusion fiom this chapter is that a radical-radical cation couplhg reaction may be taking place in SDS but that the reaction does not appear to be dominant over competing processes such as separation or escape hto the aqueous phase. Cha~ter4 Preliminarv Work in Radical Pair Generation
4.1 Generai Introduction
The methodology for generating the tripIet radical pair in order to snidy magnetic field effects on the p-heterolysis reaction is the photochemicaiiy induced decarbonylation of substituted diaryIketones, equation 4- 1.
The synthetic preparation of molecuIes with a ledg group in the P-position to the carbonyl group wili aUow a heterolytic cleavage of the C-X bond in the initial radical pair to generate a radical cation. If the heterolysis pathway for the radical pair cornpetes with intersystem crossing, then the efficiency of the heterolytic process can be monitored in the absence and presence of magnetic fields. Based on the results from the heterolysis of
2-halo-1 -(4-methoxypheny1)alkyl radicals described in Chapter 2, a dibenzy lketone denvative where R = 0CH3 and X = CI (or a leaving group of similar abiiity such as fluorinated acetoxy groups) is an ideal candidate for the study of magnetic field effects. This tirst requires preparing 1,3-bis(4-methoxypheny1)-2-propanone and then adding an appropnate CHZX group to the 1 position. Since 1.3-bis(4-methoxypheny1)-2-propanone is not commerciaily avaiiable, initial synthetic studies involved establishing the best method for adding the CH2X group using the readily available dibenzylketone. 4.2 Addition of -CH2& and -CH20COPh to Dibenzylketone
Atternpts were made to add the methylene bromide group (-CH2Br) and the methylene benzoate (-CH20COPh) group to dibeozylketone. The addition of methylene br0mide7~was attempted in the hopes that the 1-brome-2,4-diphenyl-3-proponane product codd be isolated, equation 4-2, and hirther substituted with more apropriate leaving groups. The bromide anion itself, according to the results of Chapter 2, results in a heterolysis reaction that is too fast to be resolved on the nanosecond laser system. However, the results from this reaction were not as gwd as anticipated, the desired product was not isolated. Resumably, the dibromomethane is not reactive enough to add to the enolate ion.
The addition of the methylene benzoate moiety was attempted using the reaction sequence showed in equation 4-3 and 4-4. Chlorornethyl beozoate was preparedv from benzoyl chloride and paraformaldehyde, equation 4-3, and then added to a tetrahydrofuran (THF) solution of the enolate of dibenzylketone prepared using sodium hydnde (Nd), equation 4-4. The two main products nom the reaction could be 1-(2,4-diphenyl-3- oxobutany1)benzoate and 1,3,4triphenyl- 1,3-butandione which would be fonned fiom attack at the methylene and carbonyl carbon of the chloromethyl benzoate reagent, respectively. Column chromatography ( 10/ 1, hexandethyl acetate) seemed successful in separating the desired product of 1-(2,4-diphenyl-3-oxobutanyl)benzoate; however, the recovery yield was very low at 2.5% and further attempts at utilizing this procedure were not carrïed out. O
@omq P~T Ph - Ph dry THF
4.3 Addition of Formaldehyde to Dibenzylketone
An obvious method for generating the desind dibenzyketone derivaiive is to carry out a base-catalyzed condensation78 with dibenzylketone and formaldehyde. This method appears routine where forrning the enolate ion and subsequent addition of formaldehyde would yield the substituted dibenzylketone product, equation 4-5.
Initial attempts involved generating the enolate ion in anhydrous conditions with tetrahydrofuran as solvent and a strong base such as potassium hydride (KH), and then introducing gaseous formaldehyde by thermal decomposition of paraformaldehyde.
Unfominately, ail attempts at this procedure failed with unreactive starting material king recovered in each case. The major snimbling block to the reaction is introducing the formaldehyde gas to the reaction vessel. If the transfer of the formaldehyde gas is not successful then the reaction is unsuccessful. The paraformaldehyde did in fact disappear but the efficiency of the method by which the gas was placed into the reaction vessel is unknown. As weii, it is possible that the product was fomed but underwent a retro condensation under the conditions employed. In the literature, an alternative route was diçcovered which avoided the complication of introducing gaseous foddehyde.79 The procedure involved adding 40% fomaldehyde with water to 1-phenylpropan-2-one in water and, over a period of thhours, adding small amounts of calcium hydroxide to maintain allcaline conditions. The same reaction was carrieci out using dibenzyiketone which required the modification of using a waterlmethanol solvent in order to dissolved the dibeqlketone. One &action was isolated by column chromatography and idenfieci by NMR as 1,5-dihydroxy-2,4-diphenyl-3-pentanone,equation 4-6. The formation of the condensation product indicated that the milder conditions using calcium hydroxide as the base is supenor to the conditions requiring a stronger base such as KH.
4.4 Preparation of 1,3-bis(4-methoxypheny1)-%propanone
A procedure describeci by Tmwas used to prepare 1,3-bis(4-methoxypheny1)-2- propanone.13 The method involved heating, under nitrogen, a dry mixture of 4-methoxyphenylacetic acid and reduced iron.
Fe A H3C OCH, H3C nitrogen ' atmosphere
Product was isolated using this method, but yields were very low. ui addition, the reaction was extmnely sensitive to the source of iron. In fact, when the need to replenish the supply of iron was reached, al further attempts at product formation failed.*O No clear reasoning for this anomaly has been found. 4.4.1 Addition of -%Br and -C&OCOPh
The sample preparative procedures described in section 4.2 were used in attempts with 1,3-bis(4-methoxyphenyl)-2-propanone. Preliminary triais did not yield the desired products.
4.4.2 Addition of Formatdehyde
The CaOH&CO reaction described with dibeqlketone was performed with 1.3- bis(4-rnethoxyphenyl)-2-propanone as the starthg material with the modification of using a water/tetrahydrofuran (50/50) solvent to dissolve the substrate. The proposed products of this reaction were the mono and disubstituted versions, equation 4-8
Separation was performed using a 50/50 ethyl acetatemexane eluting solvent. One hction was coilected and determined to be 1-hydroxy-2f (1-rnethoxypheny1)-3-butano through characteristic 13~NMR dara; however, a srnaIl amount of an alkene byproduct was detected. Further pdkation was not performed due to the small amount recovered. Attempts to increase yield to enable improved separation were postponed due to problems in obtaioing 1,3-bis(4-methoxyphenyi)-2-propanone (vide supra). However, the success of the CaOH/H2C0 reaction is that it provides a route to generate substituted diarylketones with relative ease. This type of precursor cm then easily be derivatized as needed to observe heterolysis of the Psubstituted radicals generated upon laser irradiation. 45 Laser Experiments
Preliminary laser studies were conducted on 1,5-dihydroxy-2,4-&phenyl-3- pentanone. The radical produced upon a-cleavage of the substrate will not undergo kheterolysis to give the radical cation and this substrate is therefore not suitable for shidies of magnetic field effects on the &heterolysis reaction. However, laser studies were Carned out to determine the effects of the addition of the -CH20H group on the photochemistry of the substrate. With 266-nm irradiation, the transient spectnim generated is dominated by a band at 320 nm immuliately foliow the laser pSe as is shown in Figure 4-1. This band is characteristic for the benzyl radical observed with irradiation of dibenzylketone.1524 The inset shows queochg of the absorption with the introduction of oxygen confirming the radical assignment. Therefore, laser excitation of 1,5-dihydroxy-2,4-diphenyl-3-pentanone leads to a-cleavage foilowed by the loss of -CO to generate a pair of benzyi radicals with a -CH20H substituent, equation 4-9.
The -1 moiety is not observed to have an infiuence on the characteristic absorption position of 320 nrn for the benzyl radical. 280 320 360400 480 520560- Wavelength / nrn
Figure 4-1. Transient absorption spectrum generated with 266-run irradiation of 1.5- dihydroxy-2,4-diphenyl-3-pentanone(0.85 mM) in nitrogen-saturated O. 1 M SDS recorded 4.40 (e), 17.6 (O),47.6 (a),and 123 (Ci) ps after the laser pulse. Inset shows decay traces monitored at 320 nm under nitrogen (O) and oxygen-saturated (0)conditions.
4.5.1 Mapetic Field Trial
Laser excitation in the presence and absence of a rnagnetic field was perfonned on 1,5-dihydroxy-2,4diphenyl-3-pentanone as weii as dibenzylketone for cornparison.
Dibenzyketone has been weildocurnented to be influence by an applied îieId'~Jj~24with an increased number of ben@ radicals escaping the micelle as the field strength is increased. In the laser experiments preformed with dibenzylketone, the trailing absorption 0.5 ps after the laser pulse increased in the presence of a magnetic field, Figure 4-2, which represents an increased number of benzyl radicals which have escaped the micelle. Therefore, the observations with dibenzyketone are in agreement with the iiterature. The increased number of radicals which escape is a direct resuit of slowing dom intersystem crossing by the introduction of the magnetic field. As discussed with respect to the radical pair model, the radical pair cohed within micelles can undergo intersystem crossing or escape, Scheme 4-1. Intersystem crossing is decreased in the presence of a magnetic field by splitting of the triplet subIevels fiorn the singlet energy; therefore, the probability of escape is increased and the number of "freeYTradicals increases.
Scheme 4- 1
The results with 1,5-dihydroxy-2,4diphenyC3-pentanone are very similar to that obtained with dibenzyketone. However, the fast decay of the radical is aitered which may reflect a faster rate of decarbonylation andor exit rate from the micelles for the substituted benzyl radical. Nevertheless, the trailing absorption representing escaped radicals is increased in the presence of an applied field fiom that observed in the absence, Figure 4-3. The increase in the number of escaped radicals rtflects the increased probability of escape facilitated by the decrease in htersystem crossing caused by the introduction of a magnetic fieId, as was seen with dibenzyiketone. O 0.5 1 1.5 2 Time / ps
Figure 4-2. Decay traces monitored at 3 18 nm upon 308-nm irradiation of dibenzyketone (3.6 mM) in nitrogen-saturated 0.1 M SDS in the absence (0) and presence (a) of a magnetic field
Figure 4-3. Decay traces monitored at 320 nm upon 308-nm irradiation of 1,5-dihydroxy- 2,4-diphenyl-3-pentanone (0.85 mM) in nitrogen-saturated 0.1 M SDS in the absence (O) and presence (a)of a magnetic field. 4.6 Future Work
Future work will focus on preparing more suitable precursors for the study of magnetic field effects on the 8-heterolysis reaction. This wiU require a more reliable and efficient method for the preparation of 1,3-bis(4-methoxyphenyl)-2-propanone. One method8l which recently has shown good success involves fomiing the magnesium salt of
4-methoxyphenylacetic acid which is heated to 200 OC for three hours using a distillation apparatus, equation 4- 10. The product distills and solidifies.
With a reliab1e mode of producing the 1,3-bis(4-methoxypheny1)-2-propanone, the CaOH/H2C0 reaction cm be used to generate hydroxy substituted diarylketones. Determinkg the best substituents to replace the hydroxy group in order to observe magnetic field effects on the heterolytic process will involve work using the laser flash photolysis equipment on several precurson with various leaving groups, each requing synthetic preparation. Chapter 5 Ex~erimental
5.1 Materiais
Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CïAB) were received fiom Aldrich and BDH, respectively. The 0.0 1 M SDS and 0.05 M =AB were prepared using distiUed water. Spectroscopie grade acetonitrile (Omnisolve. BDH) was used as received The 2-halide-1-(4-methoxypheny1)aUcyi acetate denvatives (la-d) were prepared by bromination or chlorination of the appropriate styrene in glacial acetic acid in our lab0ratory.8~ Anethole, p-methoxystyrene, benyl bromide, beql chloride,
3-cyanobenzyl bromide, toluene, and 3-cyanotoluene were received îÏom Alcirich and used as received. Diphenylmethyl chloride was synthesized fkom benzohydrol (5.4 mM) using thionyl chloride ( 16 mM). The mixture was dissolved in dichloromethane and refluxed for two hours and then distilled.83 The product (-2.5 mM, 46%) was cbaracterixd using 1H and 13~NMR. 'H NMR (Bruker 250 MHz, CDC13) bS: 6.2 ( 1H, s) and 7.5 ( 10H, m) ppm; 13~NMR (Bniker 250 MHz, -3) hs: 64.3, 127.8, 128.1. 128.6, and 141.1 ppm.
5.2 General Instrutmention
AU NMR data was obtained on a Bruker 250 MIIz spectrometer. Absorbante measurements were performed using a Hewlett Packard 8452A Diode-Amy UV-Vis spectrophotorneter. Gas chromatography analysis was performed on a Perkin Elmer Autosystem GC using a (5% pheny1)methylpolysiloxane column of 15 meter length and 1.5 p film thickness. 5.3 Laser Flash Photobsis
53.1 Equipment
Sample irradiation was performed using nanosecond laser flash techniques.84
Samples were prepared in 7x7 mm2 laser cells comprïsed of Suprasil qum tubing.
Preparative procedures are to be discussed (vide infa). The standard design of the laser flash photolysis apparatus used is shown in Figure 5-1. The two major components are excitation and detection. The excitation source employed varieci depending on the system under study. The fourth harmonic from a Continuum Nd:YAG NY-61 laser (266 nm; 5 8
&puise; I10 mJ/pulse) or a Lambda-Physik excimer laser containing a Xe/HCl/He ggas mixture (308 m; 5 10 ns/pdse; 5 100 mTlpuise) were used. The time-resolved detection component was uniform for ail system studied. The monitoring lamp was a puised 150 mJ xenon arc lamp. The transient signds after passing through the combination of a monochromator and photomultiplier tube were captured by a Tektronix 620A digital osciiioscope. The digitized data was then transferred to a Power Macintosh cornputer for storage and analysis. The laser flash photolysis system is cornputer-interfaced which aliowed control of experimental conditions such as wavelength and time scale settings and firing of the laser From the Power Macintosh. The system software was designed using LabVIEWTM fkom National Instnunents and allowed for both kinetic and spectroscopie measurements of the reactive intemediates.
Figure 5-1. The general layout of a typical nanosecond laser flash photolysis apparatus. 5.32 Data Processing
The data accumulateci in the laser flash experiments is in the form of the change in optical demity, at a particdar wavelength, as a function of time. Decay and growth traces for transient species generated upon laser irradiation had the form shown in Figure 5-2.
Time / ~LS Time / ps
Figure 5-2. Transient traces obtained with laser flash photolysis where the transient is shown to decay (a) or grow-in (b) after the laser pulse.
In the case of spectra, the decay traces monitored at individual wavelengths are arranged in series typicdy ranghg from 320-700 nm. Four time windows are chosen from a typical kinetic trace so that the absorbame at each wavelength is monitored at various times after the laser pulse providing a spectra with includes information on the evolution of the species over the. AU data was analyzed using Abelbeck KaleidagraphTM software version 3.0.4. for Power Macintosh.
5.4 Sample Preparation
5.4.1 B-Heterolysis studies
For samples prepared for the shidy of Pheterolytic rate constants (Chapter 2) in the heterogeneous solutions of 0.1 M SDS and 0.05 M CTAB, stock solutions of the precursors were prepared in acetonitrile. The concentration of the stock solution was in the range of ~xl@M so that the injection of 25-30 pL of the stocks into 3 mL of the irradiahg media (miceIlar solutions) would redt in an absorbance in the range of 0.3-0.4 at the excitation wavelength of 266 m.ls Solution samples of the typM 3 3 voolume were transferred into laser ceus, sealed with a rubber septa, and deaierateci with a flow of nitrogen for a duration of 30 minutes prior to laser irradiation. To aerate the samples, the cell was exposed to a flow of oxygen for a similar duration. Each experiment was carried out in an individual static cell which was exposed to two-three laser pulses after which the celi was shook.
5.4.2 Photoionization studies
Homogeneous solution: In the case of the olefin radical cation precursors (Chapter 3), stocks in the range of 7-8x 1@ M were prepared in acetonitrile. The injection of 15-30 pL into 3 mL of acetonitrile resulted in absorbance of 0.3-0.4 at the irradiatïng wavelength of 308 nm. Samples of 3 mL volumes were prepared in laser cek which were sealed with rubber septa and deaerated for 15-20 minutes with nitrogen pnor to laser irradiation. h the case of aerated measurements, the ce11 was exposed to a flow of oxygen for a similar time
We. Each static ceii sample was exposed to 2-3 laser shots after which the celI contents were shook. The concentration of anethole and pmethoxystyrene in the laser ceil was 4x 10-4 and 8x10-4 M, respectively. Stocks of the radical precmors were prepared in acetoniaile immediately pnor to laser irradiation. Soiutions with concentrations in the range of 2 M benzyl bromide, 1 M 3-cyanobenzyl bromide, 7x10-2 M ammonium bromide, and 3x10-1 M diphenylmethyl chlonde were prepared. The addition of these precursors was perfomed by injecting the stocks directly into laser cells of the olefh samples using a microliter syringe to volumes not exceeding 50 pL. The injections were in 5-10 Cu, intervals to allow a concentration dependent analysis of the kinetic decay traces generated at appropnate wavelengths. Upon each addition and after the exposure of the sample to 2-3 laser shots, the ceIl was shook. Heterogeneons solutions: The incorporation of the olek of anethole and pmethoxystyrene into 0.1 M SDS was accomplished by neat injection of the molecules into the miceIIar solution. Stocks in the range of 2x10-3 M anethole and 4x1~3M pmethoxystyrene were prepared in this manner. Into 10 mL volumetric flasks, 2 mL of either stock sample was pipetted and the Bask filled to the mark with 0.1 M SDS. The diluted solutions of anethole orpmethoxystyreme gave absorbante at 308 nm between 0.3- 0.4. Samples of the diluted solutions were transferred to laser cells in 3 mL aüquots which were seaied with rubber septa and a flow of nitrogen to deaerate or oxygen to aerate the sample was used for a duration of approximately 30 minutes each. Each sample was exposed to 2-3 laser shots before the cell was shook.
The same procedure was utilized for 0.05 M CTAB using 3x1~3M anethole and 4x 10" M p-methoxystyrene stock solutions.
Co-incorporation: A measured amount of benzyl bromide, 3-cyanobeoyl bromide and toluene was mixed with dry SDS surfactant powder, typicaily 1.44 g, thoroughly before dissolving in 50 mL of distilled water- The 3-cyanobenzyl bromide micellar solutions were saturated so the stock solution was filtered before use. Solutions in the range of 1û-2 M ben@ bromide, 3-cyanobenzyl bromide, and toluene were obtained in O. 1 M SDS. Into a series of five 10 mL volumetric flasks, a 2 rnL volume of the olefm stock solution was pipetted Volumes nom O to 8 mL of the aryhethyl molecules were also added. The flasks were fillecl to the mark using 0.1 M SDS, Table 5-1. This resulted in various solutions of identical concentration of radical cation precursor but with increasing concentration of the radical precursofs or toluene. Each individuai solution was transferred to laser cells in 3 mL volumes, sealed with rubber septa, and then deaerated with a flow of nitrogen for 30 minutes prior to laser irradiation. In the case of aerated measurements, the sample cells were exposed to a flow of oxygen for approximately 30 minutes. The laser ce11 sample was shook between each sequence of 2-3 laser shots at a particular wavelength. The same procedure was preformed in the case of 0.05 M CTAB. The benzyl bromide stock was prepared by adding a measured amount of benzyl bromide, 59 pL, to a weighed amount of dry CTAB surfactant, typically 0.92 g. which resulted in a 50 mL aqueous solution of approximately 1@ M benzyl bromide in 0.05 M CTAB.
Table 5-1. Typical volumes of stock solutions used in experiments for radical cation generation from photoionization of oleh derivatives in the presence of radical precursors and toluene in miceliar media 1 VolumeImL I VolumeImL I Miceliar I Total 1 Oiefin stocka Aryl stockb solution / mLC volume / mL 2 O 8 10 2 2 6 10
2 6 2 10 2 8 O 10 a Olefin stock = anethole or pmethoxystyrene Aryl molecule stock = benzyl bromide. 3cyanobenzyl bromide or toluene. Micellar solution = 0.01 M SDS or 0.05 M CTAB
In the case of 3cyanotoluene, the heterogeneous solutions of the radical cation precursors were prepared as discussed and the 3-cyanotoluene was then injected, in increments to a concentration not exceeding 12x10-3 M, using a microLiter sy~gedirectly into the laser ceil sample which had an initial volume of 3 mL. Mer each addition and proceeding exposure of the sample to 2-3 laser shots, the cell sample was shook.
5.5 Laser Photolysis with an Applied Magnetic Field
5.5.1 Sample Preparation
Stocks of the precursors were prepared in acetonitrile so that 30-40 uL injected into 3 mL of 0.1 M SDS gave an absorbance of 0.3-0.4 at the laser excitation wavelength.15 The samples were then placed into the laser ceus, sealed with nibber septa, and deaerated with nitrogen for approximately 20-30 minutes or oxygen for aerated measurements. The magnet was designed by Brian Millier of the Electrooics Shop at Dalhousie University. The magnet provided a static field kom a DC power supply and gave a field strength of approximately 1ûûû gauss.
5.6 Synthetic Procedures
Dibenzylketone (DBK):Dibenzyketone was obtained from Aldrich and used as received.
THF drying technique: Tetrahydrofuran (THF) was distilled over phosphorus pentoxide (Aldrich) and was further dried by refiuxing the recovered THF over sodium metal
(Aldrich). Benzophenone (Aldrich) was added at the 1st stage so that the blue coloration would indicate that the THF was sufficiently dry.
Dibromomethane with DBK: KH (0.01 mole, rinsed several times with dry THF to remove mineral oil) and DBK (0.01 mole) were mixed with dry THF in a three-neck, round-bottom flask under a nitrogen atmosphere. The solution was then cooled with an iœ bath and allowed to stir for -30 minutes. The ice bath was removed and dibromomethane
(0.01 mole, Aldrich) was added dropwise tbrough a dropper funnel. After the addition was complete, the flask was equipped with a reflux condenser, closed with a dryhg tube, and the solution was refluxed for approximately two hours. Water was added and diethyl ether was used for extraction. The ether layer was washed with water, dried with magnesiun sulfate, filtered, and rotovapped. No product was isolated.
ChZorumethyl bmoare: Benzoyl chloride (0.21 mole, Aldrich) and paraformaldehyde (0.071 mole, Aldrich) were added to a round-bottom flask equipped with a condenser. To the flask, a very sdquantity of Mcchloride (Aldrich) was added and the contents then shd and heateà using a wam water bath. Heating was continueci untii the paraformaldehyde disappeared (-1-2 hour). The flask was then equipped with a distillation apparatus and the cmde product distilled under vacuum conditions. Chloromethyl benzoate
@.p. 80 oC at 5 mm) was obtained with a yield of -5 96. NMR (250 MHz, CDC13) bs:6.0 (ZH, s), 7.5 (3H, m), and 8.1 (2H, m) ppm; 1% NMR (250 MHz, ClX13) bS:69.3, 128, 130. 133.9, 135.4, and 164.6 ppm.
1-(2.4-diphenyl-3-oxob~yl)benzoa1e:To a suspension of sodium hydnde (2.4 mmole) in dry THF (-2 mL) placed within a three-necked round-bottom flask, DBK (2.4 mmole) dissolved in dry THF (-2 mL) was added dropwise using a dropper hel. Under a nitrogen atmosphere and with an ice bath applhd, the mixture was stirred for about 1 hour after which the chloromethyl benzoate (14.7 mmole) reagent was added. The reaction mixture was ailowed to stir for 30 minutes before water was added (-15 mL). Diethyl ether was used for extraction and the organic layer was washed with water. The ether Iayer was then dried with magnesiun sulfate, fdtered, and rotovapped. Mication was performed using column chromatography with a 10: 1 eluting solvent of hexane:ethyl acetate with 0.06 mmole of 1-(2,4-diphenyl-3-oxobutany1)benzoate (2.5 %) king recovered. IH NMR (250 MHz, CDC13) bS:3.7 (2H, s), 4.3 (1H, dd), 4.6 ( 1H, dd),
4.85 ( LH, dd), and 7.5 (15H, in); 13~NMR (250 MHz, CDC13) b:49.2, 56.0, 65.0, 127.1, 128.3, 128.5, 128.7, 129.3, 129.5, 129.6, and 133 ppm
DBK with fonnaidehyde derstrongly basic conditiom: In a three-neck, round-bottom flask equipped with a dropper bel, a stirred suspension of potassium hydnde (0.02 mole, Aldrich) in dry THF was placed under a nitrogen atmosphere. To the flask, dibenzylketone (0.02 mole) dissolved in tetrabydrofuran was added dropwise. The addition of formaldehyde was accomplished by heating an excess (0.04 mole in two portions) of parafonnaldehyde in a separate flask which had been initiaüy COM~C~~to the reaction vesse1 through a piece of tubing. During this step, the flow of nitrogen was turned off. The work-up procedure involved the addition of water which was aciw to just above neutral. Extraction was then perfomed ushg dichloromethane which was washed with an aqueous solution of NaCl. The organic layer was then dned with magnesiimi sulfate, nItered, and rotovapped. Only dibenzylketone was recovered.
I,5-Dihydroxy-2,4-diphenyl-3-pentanone:Dibenzylketone (4.8 mmole) was mixed with formaldehyde (4.8 mmole, 37% solution with water) in a water/methanol solvent (40 mL each). The soIution was cooled using an ice bath and stirred for approximately 3 hours during which thesrnall amounts of calcium hydroxide were added perbdically to kept the solution just slightly basic. The extraction was performed with dichloromethane which was dried with magnesiun sulfate, nItered, and rotovapped. Purification was preformed using column chrornatography with a 50/50 hexandethyl acetate eluting solvent and 0.042 mmole was recovered (-1 8).lH NMR (250 MHz, CDC13) bs:2.1 (2H, m), 3.6 (2H,in), 3.8 (2H, m), 4.1 (2H, m), and 7.2 (8H.m) ppm; I3C NMR (250 MHz, CDC13) hs:59.0, 63.8, 128.1, 128.8, 129.4, 135.0, and 210.8 ppm.
I,3-Bis(4-methoxyphenyl)-2-proparzone: In a three-necked, round-bottom fiask 4-methoxypheny lacetic acid (0.047 mole, Aldrich) was rnixed wi th reduced iron (0.025 mole, Merck & Co.).*O The fiask was then closed using rubber septa, placed under a nitrogen atmosphere, and heated for approximately 3-4 hours. The crude product was dissolved in àieùiyl ether and washed with a saturateci sodium bicarbonate solution and distilled water. The ether layer was dried with magnesium sulfate, fdtered, and product was recovered upon removal of solvent Typical yields were 10- 15%. NMR was used to characterize the product. NMR (250 MHz, CDC13) bs:3.6 (4H, s), 3.8 (6H,s), 6.8 (4H, d), and 7.2 (4H,d) ppm; 13C NMR (250 MHz. CDC13) bS:48,55, 114, 126, 130, 158, and 206 ppm. Dibrornornethane with 1,3-bis(4-methoxyphenyi)-2-propmone:A suspension of NaH (3-7 mmole) in dried THF (20 mL) was cooled with an ice bath and stured in a three-necked, round bottom flask equipped with a dropper funne1 in which was placed a solution of 1,3- bis(4-methoxypheny1)-2-propanone (1.9 mmole) in dried THF (20 mL). The 1.3-bis(4- methoxypheny1)-2-propanme solution was added dropwise to the flask with stimng. The solution was then dowed to stir for 1 hour. In the dropper hinnel, dibromornethane (20 mmole, Aldrich) in dried tetrabydrofuran (15 mL) was placed and subsequently added to the reaction fiask. The work-up procedure was to add water and extract with âiethyl ether.
The organic layer was washed with distilleci water and a solution of water and sodium chloride. The organic layer was then dried, fdtered, and rotovapped. No product was isolated.
Chloromethyl bmzoate with 1.3-bis(4-methoxyphenyl)-2-propanone: NaH (3-7 mmole) was added to dried THF (-2-3 mL) in a round-bottom flask equipped with a dropper bel. This suspension was stirred and chilled with an ice bath. With stimng, a solution of 1,3-bis(4-methoxypheny1)-3-propanone ( 1-9 mmole) in dried THF (2-3 mL) was added dropwise to the suspension. Chloromethyl benzoate (19 mmole) was then added after which the reaction was allowed to stir for approximately four hours. At that time, water was added and diethyl ether was used for extraction. The ether layer was washed with water and a water/NaCl solution. The ether Iayer was then collecteci, dried with magnesium sulfate, filtered, and rotovapped. Only starting material was detected using gas chrornatography .
FomIdehyde substitution of 1,3-bis(4-methoxyphenyl)-2-propanone: 1.3-bis(4- methoxypheny1)-2-propanone (2.3 mmole) w as mixed w ith formaidehyde (2.34 mmole, 37% solution in water) in a 50/50 water/tetrahydrofuran solvent (50 mL). The solution was cooled using an ice bath and stirred for a duration of 4-5 hom during which time the solution was maintained slightiy basic with the addition of calcium hydroxide in small portions, as required. The work-up involved extraction with dichloromethane. The dichloromethane layer was then dried with magnesium sulfate, filtered, and rotovapped.
Punncation was performed by column chromatography using 50150 ethyl acetate/hexane eluting solvent with 0.05 mmole recovered (6.5 96). IH NMR (250 MHz, CDC13) &: 2.3 (lH, m), 3.6 (2H, s), 3.65 (IH,m), 3.8 (3H,s), 3.9 (3H,s), 4.0 (2H,m), and 7.0 (8H,m) ppm; 13C NMR (250 MHz, CDC13) hs:47.9, 55.2, 55.3, 58.9, 64.2, 114. 114.6, 125.8, 127.2, 129.9, 130.5, 158.6, 159.3, and 21 1 ppm. References
Taubes, G. Science, 1993,262,649. Hileman, B. C & E Naos, 1993, November 8, 15-29. McLaucMau, K. Physics Worlà, 1992, Janunry, 4135. Scaiano, J. C.; Cozens, F. L.; McLean, J. Photochem Photobio., 1994, 59, 585- 589.
Scaiano, J. C. In neSpectnmt; Bowling Green State University Publication, 1995; pp 1,440. Turro, N. J.; Kraeutler, B. Acc. Chem. Res., 1980,13, 369-377. Bittl, R.; Schulten, K.; Turro, N. J. J. Chem. Phys., 1990,93, 8260-8269. Hayashi, H.; Nagakura, S. Bull. Chem. Soc. Jpn., 1984.57, 322-328. Gould, 1. R.; Turro, N. J.; Zimmt, M. B. Adv. Phys. Org. Chem., 1984, 20, 1- 53. Turro, N. J. Pure and Appl. Chem., 1981.53, 259-286. Steiner, U. E.; Ulrich, T. Chem. Rev., 1989,89, 51-147. Grissom, C. B. Chem. Rev., 1995.95, 3-24. Turro, N. J.; Weed, G. C. J. Am. Chem. Soc., 1983,105, 1861-1868.
Korolenko, E. C.; Cozens, F. C.; Scaiano, J. C. J. Phys. Chem., 1995, 99, 14123-14128.
Cozens, F. L.; Scaiano, J. C. J. Am. Chem. Soc., 1993,115, 5204-52 1 1. Bunton, C. A.; Saveili, G. Adv. Phys. Org. Chern., 1986,22, 213-309. Thomas, J. K. Chem. Rev., 1980,80, 283-299. Scaiano, J. C.; Abuin, E. B. Chem. Phys. Letr., 1981.81, 209-213. Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J. Am. Chem. Soc., 1982, 104, (20) Gould, 1. R.; Zimmt, M. B.; Turro, N. J.; Baretz, B. H.; Lehr, G. F. J. Am. Chem. SOC., 1985,107, 4607-46 12. (21) Tm,N. J-; Zimrnt, M. B.; Gould, 1. R. J. Phys. Chem., 1988,92, 433437. (22) Sakaguchi, Y.; Nagakura, S.; Hayashi, H. Chem- Phys. Lett., 1980,72,420-423. (23) Sakaguchi, Y.; Hayashi, H. Chem. Phys. Lett., 1982,87, 539-543. (24) Turro, N. J.; Chow, M. F.; Chung, C. J.; Tanimoto, Y.; Weed, G. C. J. Am. Chem. Soc., 1981,103, 4574-4576.
(25) Evans, C.; Ingold, K. U.; Scaiano, J. C. J. Phys. Chem., 1988,92, 1257-1262. (26) Scaiano, J. C. J. Pineal Res., 1995,19, 189-195. (27) Scaiano, J. C.; Jovanovic, S. V.; Morris, D. G. J. Photochem Photobio. A: Chemistry, 1998,113, 197-201. (28) Scaiano, J. C.; Cozens, F. L.; Mohtat, N. Photochem Photobio., 1995, 62, 8 18- 829. (29) Chagovertz, M. A.; Grissom, C. B. J. Am. Chem. Soc., 1993, II.5, 12152- 12157. (30) Grissom, C. B.; Natarajan, E. Methods of Enrymology, 1997,281, 235-247. (31) Harkins, T. T.; Grissom, C. B. Science, 1994,263, 958-960. (32) Harkins, T. T.; Grissom, C. B. J. Am. Chem. Soc., 1995,117, 566-567. (33) Taraban, M. B.; Leshian, T. V.; Anderson, M. A.; Grissom, C. B. J. Am. Chem. SOC., 1997,119, 5768-5769. (34) Frey, P. A. Chem. Rev., 1990.90, 1343-1357. (35) Stubbe, J. Biochemistry, 1988,27, 3893-3900. (36) Stubbe, J. J. Biol. Chem., 1990,265, 5329-5332. (37) Giese, B.; Beyrich-Graf, X.; Burger, J.; Kesselheim, C.; Sem, M.; Schafer, T. Angew. Chem Int. Ed. Engl., 1993,32, 1742-1743. (38) Stubbe, J.; Kozarich, J. W.; Wu, W.; Vanderwall, D. E. Acc. Chem. Res., 1996, 29, 322-330. (39) Cozens, F. C.; ONeili, M. A.; Bogdonova, R.; Schepp, N. J. Am. Chem. Soc., 1997,119, 10652-10659.
(40) Scaiano, l. C.; Barra, M.; Krzywinski, M.; Sinta, R.; Calabrese, G. J. Am. Chem. Soc., 1993, f 15, 8340-8344.
(4 1) Barclay, L. R. C.; Lusztyk, J.; hgold, K. CI. J. Am. Chem Soc., 1984, 106, 1793- 1796.
(42) Mülier, S. N.; Batra, R.; Sem, M.; Giese, B.; Kisel, M.; Shadyro, O. J. Am. Chem. Soc., 1997,119, 2795-2803. (43) Koltzenburg, G.; Behrens, G.; Schulte-Frohhde, D. J. Am Chem. Soc., 1982, 104, 73 11-73 12. (44) Koltzenburg, G.; Bastian, E.; Steenken, S. Angew. Chem. Int. Ed. Engl., 1988, 27, 1066-1067.
(45) Johnston, L. P.; Schepp, N. P. L Am. Chem Soc, 1993,115, 6564-6571. (46) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc., 1994,116, 6895-6903. (47) Cozens, F. C.; Schepp, N. C. unpublished results. (48) Bunton, C. A.; Ljunggren, S. J. Chem. Soc. Perkin Trans. 11, 1984, 355-361. (49) Johnston, L. J.; Schepp, N. P. Pure and Appl. Chem., 1995,67, 71-78. (50) Workentin, M. S.; Schepp, N. P.; Johnston, L. J.; Wayner, D. D. M. J. Am. Chem. SOC.,1994,116, 1141-1 142. (51) Scaiano, J. C.; Selwyn, J. C. Can. J. Chem., 1981,59, 2368-2372. (52) Turro, N. J. Modem Molecular Photochemistry; University Science Books: Susaiito, Cal., 1991.
(53) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Phys. Chem., 1996, 100, 13615-13623.
(54) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Org. Chern., 1996, 61, 7793- 7800.
(55) Johnston, L. I.; Kanigan, T. J. Am. Chem. Soc., 1990,112, 1271-1273. (56) Lampard, C.;Murphy, J. A.; Norman, L. J. Chem. Soc.. Ch. Cumm., 1993, 295-297.
(57) Murphy, J. A.; Romme, S. J. J. Chem. Soc. Perh Trans 1, 1995, 1349- 1358. (58) Arnold, D. R.; Borg, R. M.; Cameron, T. S. Can. J. Chem., 1984, 62, 1785- 1802.
(59) Hodgkins, J. E.; Megarity, E. D. J. Am Chem Soc., 1965.87, 5322-5326.
(60) Meiggs, T. O.; Grossweiner, L. 1.; Müler, S. 1. J. Am Chem. Soc., 1972, 94, 798 1-7986. (61) Tokumura, K.; Ozaki, T.; Nosakam, H.; Saigusa, Y.; Itoh, M. J. Am Chem. Soc., 1991, 113, 4974-4980. (62) Neta, P.; Behar, D. J. Am. Chem. Soc., 1981,103, 103-106.
(63) Barlt, J.; Steeken, S.; Mayr, K.; McCleUand, R. A. J. Am. Chem Soc., 1990, 112, 6918-6928.
(64) McClelland, R. A.; Kanagasabapathy, V. M.; Steeken, S. J. J. Am. Chern. Soc.,
1988,110, 69 13-69 14.
(65) Daasbjerg, K. J. Chem. Soc. Perkin Trans. 2, 1994, 1275-1277. (66) Zhang, X. M. J. Chem Soc. Perkin Trans. 2. 1993, 2275-2279. (67) Boch, R.; Whittlesey, M. K.; Scaiano, J. C. J. Phys. Chem., 1994, 98, 7854- 7857.
(68) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (69) Schepp, N. P.; Johnston, L. J. unpublished results.
(70) Weller, A.; Rehm, D. lsr. J. Chem., 1970,8, 259-271. (71) Wayner, D. D. M. In Handbook of Orge Photochernktry; J. C. Scaiano, Ed.; CRC Press, Inc.: Boca Raton, Florida, 1989; Vol. II; pp 363-368.
(72) Caldwell, R. A.; Ghali, N. 1.; Chien, C. K.; DeMarco, D.; Smith, L. J. Am. Chem. SOC-, 1978,100, 2857-2863. (73) Kojima, M.; SaEcuragi, H.; Tokumaru, K Bull. Chem. Soc. Jpn., 1989, 62, 3863- 3868.
(74) Lewis, F. D.; Kojima, M. J. Am. Chem. Soc., 1988.110, 8660-8664. (75) Brede, O.; David, F.; Steenken, S. J. Photochem Photobio. A: Chemistïy, 1996, 97, 127-131. (76) Koppes, M. J. C. M.; Beentjes, P. C. J.; Cerfontain, H. Recveil des Travaux Chimiques des Pays Bas, 1988,107,313-324. (77) ULnch, L. M.; Adams, R. Am Chem. Soc., 1921,43, 660-667. (78) Ege, S. Organic Chemistry; 2nd ed.; D. C. Heath and Company: Lexington, Mass., 1989, pp 656-660.
(79) Marshall, P. A.; Prager, R. H. Aust. J. Chem., 1977,30, 141- 150. (80) The inital iron was reduced through an eletdytic process and had been obtained fkorn Merck & Co. Limited. Attempts to replenish this brand of reduced iron were unsuccessful. The reduced iron obtained from Aldrich did not yield product. (8 1) Schepp, N. P. unpublished results.
(82) Ortiz-Ojeda, W.; Cozens, F. C.;Schepp, N. P. unpublished results. (83) Furniss, B. S.; Hannaf'ord, A. J.; Smith, P. W. G.; TatcheiI, A. R. Vogel's Téxtbook of Practical Organic Chemistry; Fiifth ed.; John Wiley and Sons, Inc.: New York, 1989. (84) HadeI, L. M. In Handbook of Orgmtic Photochemistry; J. C. Scaiano, Ed.; CRC Press: Boca Raton, Fi., 1989; Vol. 1; pp 279-292.
/ IMAGE EVALUATION TEST TARGET (QA-3)
APPLIED IMAGE.lnc 1653 East Main Street --.- , Rochester. NY 14609 USA ----2 Phone: 71 61482-0300 ------Fcuc 716/28&.5989
O 1993. Applied Image. Inr. AU FIQhts Reacmed