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Rearrangement and Fragmentation Processes in the Methanethiol And a1? A J. Am. Chem.Soc. 1984,106,2774-2781 smallereffect than E and C predicts. Therevery likely is a an E and C typeof modelthe conditionis that both the enthalpy -window-of bondstrengths in whichthe logof thestretching force and entropyfit an equationof the f and C form. It is not constantchanges in the adductcan be fit to E andC. For very necessarythat AS be a linearfunction of Afl, for linearitywould weak adducts,E and C probablyoverestimates the covalent be require that AII and A-Shave the sameC^/81 ratio. We perturbationto the existingbonds, causing these systems to miss emphasizethat AG correlationsshould only be carried out with the predictionsof the correlations.The calculatedand experi- data from a givcnsolvent that doesnot interactwith the reagent mentalresults in CCl. for the Et2Sadduct of 4-fluorophenolare beingvaried (donors in the abovecase). Evenunder these con- alsoin conflictas the calculatedentropy is - l0 eu too large. This ditionsthe discussiongiven indicates why exceptionsmay still occur may be due to complexationof the solventwith this donol as well in AG fits. as the effect describedabove. Thc changeswe haveobserved in the E and C fits of cntropies Acknowledgment.We acknowledgethe partialsupport of this and free energiesare se€nto be reasonablewhen compared to researchby the National ScienceFoundation. physicalmodels for theseeffects. Furthermore,we havedem- onstratedthat in ordcr toobtain a free energycorrelation with RegistryNo. Iodine,7553-5G2; 4-fluorophenol, 371-41-5. Rearrangementand FragmentationProcesses in the Methanethioland Dimethyl Sulfide Radical Cations Ross H. Nobes,t Willem J. Bouma, and Leo Radom* Contributionfrom the Research School ol Chemistry, Australian National Uniuersity, G.P.O. Box 4, Canberra. A.C.T. 2601, Australia. Receiued September 26, 1983 Abstrsct: Ab initio molecular orbital calculationswith large, polarization basissets and incorporating electroncorrelation havebeen uscd to examineasp€cts of the CH3S+,CH.S+-, and C2H5S+.potential-energy surfaccs. Detailed comparisonsare drawn with correspondingCH3O+, CH.O+., and C2H5O+.systems. The most stableCH3S+ ion is clearly the mercaptomethyl cation,CH2SH+ (l). Triplet thiomethoxycation, CH35+ (2), liessignificantly higher in energy,while singletthiomethoxy cation (3) is found ro rearrangewithout activationenergy to CH2SH". For the CH4S*- system,the calculationsreveal two stable isomeric ions, namely, the well-known methanethiolradical cation, CH3SH+. (5), and the recently discoveredme- thylenesulfoniumradical cation, CH2SH2+.(6). The latter is catculatedto lie 76 kJ mol-r higher in energy than 5, with a barrier to rearrangementto 5 of I l4 kJ mol-r. Both 5 and 6 will form CH2SH+ upon lossof H-- Examination of the relevant part of the C2H65+.potential-energy surface establishes a rearrangement-dlssociationmechanism for the productionof CH35+ ions of structure CH2SH+ from ionized dimethyl sulfide, CH3SCHT+.(10). This involvesinitial formation of a sulfonium ion intermediate,CH2SHCHt*. (ll), which subsequentlyundergoes simple bond cleavage. The CH2SH+ ion producedvia this processhas little excessenergy. The intermediatesulfonium ion (ll) Iies 82 kJ mol-l abovc CH3SCHT+.(10), with a barrier to rearrangementto l0 of 120 kJ mol-r, and representsa new, stable C2HES+.isomcr. Introduction CH3S+,thought to be generated2-4'6-8by ionizationand frag- mentationof alkyl methylsulfides (CH3S-R), and CHrSH+, years, beenconsiderable in the For many there has interest thought to be generateds by ionizationand fragmentationof com- ionizationand subsequentfragmentation of organosulfur alkanethiols (R{H2SH). ln this way,CD3SH was considered6,? which probably mostattention is pounds.The ion has attracted to produc€Uoitr ;ons, viz.,CD3S+ and bDrSH*. by H. and D'loss, consideredtwo structuralisomers: CHr5+.2-ts Earlier studies respectively.The heas of formation (AI{"2eE)of CH,S+ and CH2SH+were reportcd6to be 895 and 920 kJ mol-r, respectively. However,recent studiesllFls have cast doubton the existence (l) Presentaddress: University Chemical Laboratory, Lcnsfield Rd., of an ion with the CH3S+structure of energysimilar to that of CambridgeCB2 lEw, United Kingdom. key observationwas made (2) Franklin, J. L.: Lumpkin. H. E. J. Am Chem.Soc. 1952, 74, 1023. CH2SH+. The by Mclafferty and (3) Palmer,T. F.; Lossing,F. P. J. Am. Chem. Soc. 1962,84,4661. co-workers,r0'16who combinedthe techniquesof collisional-ac- (4) (a) Hobrock,B G.; Kiser, R' W. J. Phys.Chem. 196t,67,1283. (b) tivationmass spectrometry and ab initio molecularorbital tbeory GowenlockB. G.; Kay, J.; Mayer, J. R. Iraas. Faraday Scr.1953, 59,2463. to investigatethe CH3S+and CH30+ systems.Contrary to ex- (5) Taft, R. W.; Manin, R. H.; l:mpe, F. W. J. Am. Chem.Soc. 1965, pectations, found on fragmentation low 87, 2490. tbey that, at ionizing- (5) Kcyes,B. G.: Harrison,A. G. J. Am. Chem.Soc. 1968,90,5671. electronenergies, the dimethylsulfide and dimethylether radical (7) Amos, D.; Gillis, R. G.: Occolowitz,J. L.; Pisani,J. F. Org. Mass cations(CH3XCHI*-, X = S, O) yield the mercaptomethyland Spectrom. 1969, 2,209. (8) Jonsson,B.-O.; Lind, J. J Chem. Soc.,Faraday Trans. 2 1974,70, I 399. (13) Kutina, R- E-: Edwards, A. K.; Goodman, G. L.; Berkowita J. J. (9) Williams,D. H.; HvistendahlrG. J. Am. Chem.Soc. 1974,96,6753. Chem. Ph;s. 19E2,77, 5508- ( I 0) (a) Dill, J. D.; Mclaffeny. F. W. .L A m Chem So{. 197E,I 00, 2907. (la) Roy, M.: McMahon, T- B. Org. Mass Spectrom. 19E2, 17,392. (b) Difl, J. D.; Mclaffeny, F. w. Ibid.1979, 101,6526. (15) Butler, J. J.: Baer, T-; Evans. S A. J. Am. Chem.Soc. 1983, tOJ, (l l) Harrison,A. G. J. Am. Chem.Soc. 1978,IM,4911. 345l. (12) Gilbcrt,J. R.; von Kopprn, P. A M.; Huntress,W. T.; Bouers,M (16) Difl, J. D-; Fischer,C. L-; Mclafferty. F. W. J. Am Chem. Soc T. Chem. Ph-,-sbtt. t981. 8?. 455. t979.t0t,6531. 0002-?863/84I I sO6-277 150r.50/0,e 19E.1American Chemical Societv Vol. 106. 1984 2775 Mqthanethiol and Dinethyl Sulfide Radical Cotions J. Am. Chem. Soc.. No. 10, Tablcl. CalcularcdTorall:ncrgics(hartrccs)andZcro-l'ointVibrationall:nergics{ZPVt.,kJmot-r)forContponcntsoflhcCH.Saand ('? I 1..S'' 51'stcnrso t 'I-I 2A, cH3* A' HS' CH2'' H.SIA, cH3.2Ar" li. (D,11 (Cr) (c'?u) ic,")' lD16) -39.342 -0.49620 -39.009l3 -396.il 846 -38.34747 -396.70467 6l 3-2tCll3-2tG -398.204l2 -39.504 4-31Gll4-3tG -0.49823 -39.175l3 -39'1.61',18'l -38.51257 97 -398.627 -39.54666 6-3tG//4.3IG -0.49823 -39.2162l -398.04025 -38.55389 s4 -298.67 -39.56444 6-31G1'll4-3lc -0.49823 -39.23627 -398.06792 -38.5?0s2 4 24 -398.689 -39.6217 MP2l6-3tGll4-3tG -0.49823 -39.28076 -398.0815? -38.s9804 0? | -398.699 -39.63446 MP3/6-3rG//4-3r G -0.49823 -39.29380 -398.08924 -38.60880 85 ZPVI: 0.0 8?.0 I 5.5 45.0 39.6 7 8-l = o 4-3lG gcontctricalparameters are: CHr*,r(C-H) = 1.0?6 ; HS'.r(H-S) = 1.364; CHr', r(C-H) = 1.0?8,IHCH 141.7";H.S,r(H-S1= 1.353,rHSH =95.j": CHr.,rrC-H)=I.070. (All1"s, kJ rnol-r hydroxymethylcations (CH2XH+), respectively,rather than tbe Table It. ExperimentalHeats of Forml1lsn ) CrHoS" lonsand Component Systems thiomeitroxybr methoxycaiions (CH3X*). Thcy proposedro'r5 for the CHrS;,CHnS'', and that such a fragmentationoccurs via an intramoleculardis' species Afilgoo species Nt1',, placementreaction: cHrsH' 8704 H. 216b cH3sH1. 899b CHr' I 398b cH2sH2*. 92Sc H,S -l8b -*t' cHsscHs' 8l? b CHr' 1095b o._r_."r. a'r-r,' (l) :_ CHr=5 g9d HS. M2b H. 1528b CHt' 146b ' c i: J Rrf"r.n.. 15. b Rcference51. Basedon N!1o7r"= 916 kJ that, in In fact, rec€ntab initio calculationslThave indicated mot (ref20c), correctgd to 0 K by usingcalculaed vibrarional fre' the caseof the dimethylether radical cation, the fragmentation quencies(see te)it). d Basedon a protonaffinity for CHr=S of, is likely to proceedby way of a clcely relat€d twostep mechanism i5? kJ mof t (refl4), andall3oo values for CH'SH'andH' (ref (e42,X=O): 5l ). havebeen restricted to the Hartree-Fock levelwitb small,non' polarizationbasis sets. To our knowledge,no theoreticalstudies bf the CH.S*. and C2H.S+.systems have yet beenreported- [n the calculationspresented here, fully optimizedgeometries are et al.l6 The oxvsenanalogue of methanethiolis methanol,and recent ab initio cilculationstthavc shown that for the ionizedCH4O+' CH3SCH3+.and for the observedthermochemistrYz'rs'1g of tntt also indicatedthe existenceof the methylenesulfoniumion of (CH2SH2+.)as a stableisomer of the methanethiolradical cation reaction? Metbod and Results Standardab initio molecularorbital calculationshave teen carriedout usinga modihed25version of the Gaussian80 system of programs.26Geometries of equilibriumstructures and transition structureshave been determined at the Hartree-Fock.(HF) level usinganalytical gradient procedures and the split'valence4-3lG basisset.2? In orderto characterizestationary points as minima (equilibriumstructures) or saddlepoint (transitionstructures), and to allow for inclusionof tbe effectsof zero'pointvibrations in estimatingrelative energies, harmonic vibrational frequ^encies havebeen calculateda at the HF levelwith the 3-2lG basiszeusing (23) Yamabe,T.; Yamashita,K.: Fukui, K.; Morokuma, K. Chem. Phys- Izil. 1919, 63, 4Jt. (24) Bernardi, F.; Bottoni, A.; Epiotis, N. A. J' Am. Chem. Sor. 1978, 100' 7205. (28) Using a program written by Dr. L. Farnell. (29) (a) Binkley,J. S.; Popls,J. A.; Hehre, W. J. J. Am. Chem.Soc. t980' I0?,939. (b) Gordon,M. S.; Binkley,J. S.',Pople, J. A.; Pietro,W. J.; Hehre, w. J. Ibid.1982, 104,2797. 27.76 J. Am. Chem.Soc., Vol. 106,No. 10, 1984 Nobes, Bouma, and Radom geometriesoptimized with thisbasis set. More reliablerelative energieshave been obtained from higherievel calculations using H thesplit-valence G3lGl0'3r and split-valence plus dp-polarization \<os a .-q 6-3lGr13r'32basis sets, and incorporating valence-electron cor- ..)- ' 91? rclationvia Moller-Plessetperturbation theory terminated at third o,, order(MP3).33 All calculationson odd-electronspecies utilize "."'fH the spin-unrestrictedformalism.
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