Kinetic Isotope Effects In

KINETIC ISOTOPE EFFECTS IN UNIMOLECULARDECOMPOSffiONS OF METASTABLE IONS by COLIN EVAN ALLISON, B.Sc. (Hons.) This thesis is submitted in fulfilment of the requirements for the degree of Doctor of Philosophy DEPARTMENT OF PHYSICAL CHEMISTRY SCHOOL OF CHEMISTRY UNIVERSITY OF NEW SOUTH WALES AUGUST 1986 (ii) DECLARATION I, COLIN EVAN ALLISON, hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgement is made in the text. COLIN EVAN ALLISON (iii) ACKNOWLEDGEMENTS I would like to express my gratitude to Prof. Peter Derrick for his supervision and guidance throughout the course of this work. I would also like to thank Dr. Gary Willett for many helpful discussions, and for allowing me access to the molecular orbital programs. I am grateful to Dr. Steen Hammerum, Prof. John Bowie and Prof. Fred McLafferty for their generous donations of chemicals. I would also like to thank Dr. Steen Hammerum for many stimulating discussions. I would like to extend my thanks to Dr. Kevin Donchi and Mr. Peter Cullis for their assistance with the experimentation and for their companionship. I would also like to thank Dr. Anthony Craig and Dr. Bruce Rumpf for their companionship during the course of my studies. (iv) ABSTRACT Isotope effects on ion abundances have been measured and used as a probe of reaction mechanisms for unimolecular ion decompositions. Quasi-equilibrium theory (QET) calculations have been performed to obtain rate coefficients k(E) which have been used within various models for reaction to calculate the isotope effect on ion abundances. For a prototype simple cleavage, elimination of butyl radical from 2H labelled N-methyl dipentylamine molecular ions, the model of the transition state was varied until the calculations reproduced the experimental isotope effects at 10-11 s and 10-5 s. The transition state model indicated relaxation of the pentyl chain accompanies cleavage. A more complex reaction, the McLafferty rearrangement, was studied in 3-ethyl- 2-pentanone using 2H and 13c labelling, in benzyl ethyl ether using 2H labelling, and in cx.,cx.'-diethoxy p-xylene using 2H and 180 labelling. Experimental ion abundances indicate that multi-step processes are occurring in 3-ethyl-2-pentanone, but the experimental ion abundances for benzyl ethyl ether and cx.,cx.'-diethoxy p-xylene do not indicate whether a stepwise or concerted mechanism is operating. QET calculations were performed, and reaction models constructed, to determine whether the McLafferty rearrangement in benzyl ethyl ether and cx.,cx.'-diethoxy p-xylene might best be described as a concerted or stepwise process. Calculations indicated that the model used for the concerted mechanism reproduced accurately the experimental ion abundances, but that the model used for the stepwise mechanism failed to accommodate the experimental results. The McLafferty rearrangement in benzyl ethyl ether and in cx.,cx.'-diethoxy p-xylene is proposed to occur in a concerted rather than a stepwise fashion in apparent disagreement with other systems for which a stepwise mechanism is appropriate. (v) TABLE OF CONTENTS Declaration (ii) Acknowledgements (iii) Abstract (iv) Table of contents (v) Chapter 1. Introduction. 1 1. 1. Stable ion structures 1 1.1.1. Ion thermochemistry 2 1.1.2. Experimental methods of determining stable ion structures 4 1.2. Unimolecular ion decompositions 4 1.2.1. Mechanisms of unimolecular ion decompositions 4 1.2.2. Theory of mass spectra 7 1.2.2.1. Relationship between k(E) and ion abundances 8 1.2.3. Kinetic isotope effects 11 1. 2. 3. 1. Relationship between kinetic isotope effects and isotope effects on ion abundances 12 1. 2. 3. 2. Significance of kinetic isotope effects 14 1.3. Metastable ions 17 1.3.1. Mass-analysed ion kinetic energy (MIKE) spectrometry 17 1. 3. 2. Energy release in metastable ion decompositions 19 1. 3. 3. Field ionisation kinetics 20 Chapter 2. Calculation of ion abundances. 22 2.1. Calculation of k(E) 22 2.1.1. Evaluation of W(E) 23 2.1.2. Choice of energy levels 25 2. 2. Energy deposition function, P(E) 27 2.3. Observation times, t1 and 12 28 2.4. Collection efficiency, G 29 2.5. Correlation of calculated and measured ion abundances 30 2.5.1. Adj_ustment of the transition state 31 2.6. Computer programs 33 2.6.1. Program QET 33 2.6.2. Program CFT3 34 (vi) 2. 6. 3. Program PEAKS 35 2.6.4. Adjustment of vibrational frequencies 36 2.6.5. Othercomputerprograms 36 Chapter 3. Instrumental. 37 3.1. History of the instrument 37 3.2. Vacuum system 37 3.3. Ion beam defining slits 38 3.4. The ion source 39 3.5. Magnetic sector 40 3.6. Electric sector 41 3.7. Field-free regions 42 3.8. Detector system 43 3.9. Data acquisition 43 Chapter 4. Alpha-cleavage of N-methyl-dipentylamine molecular ion. 45 4.1. Introduction 45 4. 2. Experimental results 45 4.3. Calculation of ion abundances 46 4.3.1. The kinetic model 46 4.3.2. Reactant ion vibrational frequencies 47 4.3.3. Transition state vibrational frequencies 47 4. 3 .4. Critical energy 48 4.3.5. Optimisation of the transition state 49 4.4. Results of the calculations 50 4.5. Discussion of the transition state model 53 Chapter 5. Investigations of the McLafferty rearrangement. 55 5 .1. Introduction 55 5.2. Evidence for the stepwise nature of the McLafferty rearrangement 56 5. 3. 3-Ethyl-2-pentanone 58 5.4. The McLafferty rearrangement in some substituted benzenes 62 5.5. Benzyl ethyl ether and a.,a.'-diethoxy p-xylene 65 (vii) 5.5.1 Benzyl ethyl ether 66 5.5.2 cx,cx'-Diethoxy p-xylene 68 5.6. Energetics 71 5. 7. Concerted elimination of acetaldehyde from cx,cx'-diethoxy p-xylene 72 5. 7. 1. Modelling of the transition state 73 5.7.2. Results 74 5.8. Hydrogen exchange in the.benzyl 1,1-Drethyl ether radical catio 77 5. 8. 1. The kinetic model for hydrogen exchange 79 5.8.2. Ion structures and energetics 81 5.8.3. Vibrational frequencies and critical energies 83 5. 8. 4. Details of the calculations 86 5.8.5. Results obtained using the thermochemical energetics 87 5.8.6. Results obtained using the MOPAC energetics 88 5. 8. 7. Discussion 90 5. 9. Stepwise elimination of acetaldehyde from cx,cx'-diethoxy p-xylene 91 5. 9. 1. The kinetic model 91 5. 9. 2. Ion structures and energetics 93 5.9.3. Vibrational frequencies and critical energies 94 5. 9 .4. Details of the calculations 97 5.9.5. Results of the calculations based on the thermochemical energetics 99 5.9.6. Results of the calculations based on the MOPAC energetics 101 5. 9. 7. Discussion of the stepwise models 103 5 .10. Conclusion 104 REFERENCES 106 Appendix 1 Computer program QET Al.I Appendix 2 Calculation of metastable ion abundances A2.l Appendix 3 Adjustment of the vibrational frequencies for the N-methyl-dipentylamine molecular ion A3.1 Appendix 4 Estimation of heats of formation for neutral species using Bensons' rules A4.1 Appendix 5 Optimised ion geometries A5.l Appendix 6 Solution of kinetic schemes using Laplace transformation A6.1 Appendix 7 Computer programs CALKT and NKT A7.l Appendix 8 Estimation of heats of formation for distonic ions A8.l 1 Chapter 1. Introduction. The dynamics of unimolecular decompositions constitute one of the most fascinating aspects of mass spectrometry. Reliable structures have been determined for many gaseous ions, and provide starting points for discussions of reaction dynamics. Mechanisms describing which atoms are involved have been postulated for many reactions. To elucidate further the mechanisms with regard to the sequence and timing of the atomic movements is more challenging. No reaction mechanism can be proven to be correct (1), but a proposed mechanism can be demonstrated to be consistent with all available data. In the work described here, kinetic isotope effects, or more specifically isotope effects on ion abundances, have been studied with a view to exploring their value for distinguishing concerted from non-concerted, or step-wise, processes. Particular attention has been paid to the McLafferty rearrangement in benzyl ethyl ether. 1.1. Stable ion structures. The term "ion structure" as used in this thesis refers to the question of which atoms are directly connected to which others, rather than to the geometrical relationships among atoms. Geometric data from experiment are becoming available for small polyatomic cations [2,3), but most geometric data have been obtained from molecular orbital calculations [4]. The elucidation of ion structures is important as many molecular ions isomerise prior to reaction. The [C4H8]+. species exemplify the problem [5-13). The [C4H8]+. ions formed from 1-butene, cis- and trans- 2-butene, methyl propene, methyl cyclopropane and cyclobutane all produce similar electron impact (EI) mass spectra [5-10). Presumably the initially generated molecular ions are quite distinct. Decomposition of the [C4H8]+. ions has been proposed to involve common species (or 2 mixture of species). At short times (10-12 s) decomposition is believed to occur from the molecular ions [11-13], but at longer times decomposition is believed to occur from the common species formed by isomerisation of the molecular ions. The possibility of isomerisation of molecular ions indicates that it is essential that the structures of ions be known, and hence methods of determining stable ion structures are required.

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