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    A Thesis Submitted for the Degree of PhD at the University of Warwick

    http://go.warwick.ac.uk/wrap/71293

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  • ELECTRON SPIN RESONANCE STUDIES OF TRANSIENT RADICAL - ANIONS PRODUCED

    IN LIQUID AMMONIA AT 200 K

    by

    IAN HAMILTON ELSON

    A dissertation submitted in part fulfilment of the requirements for the degree of

    Doctor of Philosophy at the University of Warwick

    September 1972

  • BEST COpy

    AVAILABLE

    Variable print quality

    Page numbers as original

  • ACKNOWLEDGEMENTS

    I would like to thank Dr. T. J. Kemp for not only providing the opportunity to carry out this work but also for his encouragement throughout its course. Thanks are also due to Professor V. M. Clark for providing the facilities of the Department of Molecular Sciences and to the many other members of the academic and technical staff for their assis- tance. The services of the glass-blowing workshop, under the direction of Mr. K. Holden, and the help of Drs. D. Greatorex and D. Benton in operating the flow system is gratefully acknowledged. I would also like to thank Miss Rosemary Went for typing this thesis. I am indebted to I.C.I. Ltd. (Mond Division) who co-operated with this work and provided financial assistance through a C.A.P.S. studentship and to the S.R.C. for the maintenance award.

  • CONTENTS

    Page

    ABSTRACT i

    ABBREVIATIONS AND CONSTANTS iv

    PART A

    INTRODUCTORY SECTION

    CHAPTER 1 : LIQUID AMMONIA 1.1 Liquid ammonia as a solvent 1.2 Metal-ammonia solutions

    1

    1

    1.2.1 Models for metal-ammonia solutions 2

    1.2.2 Reduction by metal-ammonia solutions 4

    1.2.3 Theories of reduction by metal-ammonia systems 4

    1.3 Kinetics of reactions of the solvated electron 7

    CHAPTER 2 RADICAL-ANIONS 2.1 Formation and study of radical-anions

    2.1.1 2.1.2 2.1. 3

    Matrix isolation Steady-state methods Indirect magnetic resonance

    methods

    9

    10 11

    12 2.2 Electron spin resonance in the study

    of radical-anions 13 2.3 Reactions of radical-anions 17

    CHAPTER 3 ELECTRON SPIN RESONANCE 3.1 The resonance condition 21 3.2 The mechanism of hyperfine splitting 23

    3.2.1 Coupling of conformationally fixed protons 25

    3.3 Spin relaxation and line shape 26 3.3.1 General model for chemical

    line-broadening mechanisms 27 3.3.2 Mechanisms contributing to

    line-broadening 29 3.3.3 Alternating linewidths arising

    from conformational intercon- versions 30

  • CHAPTER 4

    PART B

    3.4 Calculations of theoretical spin 31 densities in conjugated IT-radicals

    EXPERII~NTAL TECHNIQUES 4.1 Multicapillary mixer 4.2 .Continuous f Low experiments 4.3 Static reductions 4.4 The esr spectrometer 4.5 Recording spectra 4.6 Spectral analysis 4.7 Activation parameters

    35

    36

    36

    37 38 38

    40

    EXPERIMENTAL RESULTS CHAPTER 5 : AROMATIC AMIDES AND THIOAMIDES

    5.1 Introduction 5.2 Experimental 5.3 Results and analysis of spectra

    CHAPTER 6

    .J

    41 42 42

    5.3.1 Benzamides and ring- substituted benzamides 42

    5.3.2 N-methylbanzamides 44 5.3.3. N,N-dimethylbenzamides 5.3.4 Nicotinamides 5.3.5 Thiobenzamides

    5.4 Discussion

    45 45 46 "46

    5.4.1 MO calculations and assign- ment of coupling constants 46

    5.4.2 Reduction of aromatic amides 51 5.4.3 Halo-benzamides 52 5.4.4 Anomalous results 53

    5.5 Conclusions 54 ACYCLIC AND CYCLIC a,S-UNSATURATED KETONES 6.1 Introduction 6.2 Experimental 6.3 Results

    55 56 57

    6.3.1 Open chain a,S-unsaturated ketones 57

    6.3.2 Cyclic a/S-unsaturated ketones 60

    6.4 Discussion 63 6.4.1 Reduction of a,S-unsaturated

    ketones 63

  • 6.4.2 Assignment of coupling constants and MO calculations 64

    6.4.3 Conformation of cyclic ketyls 70 6.4.4 Spin density distribution 70 6.4.5 Time-dependent effects 71

    6.5 Continuous flow reduction in the presence of a proton source 72

    6.6 Static reduction of a,B-unsaturated 74 ketones

    6.6.1 Open chain a,B-unsaturated ketones 74

    6.6.2 Cyclic a,B-unsaturated ketones 75

    6.7 Conclusions 76

    CHAPTER 7 REDUCTION OF a,B-UNSATURATED CARBOXYLIC ACIDS, ESTERS AND NITRILES

    7.1 7~2 7.3

    Introduction Experimental Results 7.3.1 7.3.2

    a,B-unsaturated acids Esters of a,B-unsaturated

    mono-acids

    77 78 78 78

    80

    7.3.3 Esters of a,B-unsaturated di-acids 81

    7.3.4 a,B-unsaturated nitriles 82 7.4 Discussion 83

    7.4.1 Reduction of a,B-unsaturated acids, esters and nitriles 83

    7.4.2 Dialkyl maleates and fumarates 85

    7.4.3 Molecular orbital calculations 87

    7.5 Conclusions 89

    APPENDIX I PROGRAM HtlCKEL FOR CALCULATION OF HOCKEL AND McLACHLAN SPIN DENSITIES 91

    APPENDIX II PROGRAM ESRTEST FOR SIMULATION OF SINGLE ESR SPECTRA 96

    APPENDIX III : PROGRAM ESRTT2 FOR SIMULATION OF MIXED ESR SPECTRA 99

    APPENDIX IV PROGRAM ACT PAR FOR CALCULATING ARRHENIUS PARAMETERS

    V : PROGRAM ALWETEST FOR SIMULATING ESR SPECTRA SHOWING ALTERNATING LINEWIDTHS DESCRIBED BY THE SLOW LIMIT CONDITION

    104

    APPENDIX

    105

    REFERENCES 109

  • ABSTRACT

  • i

    ABSTRACT

    In this work is described the application of esr spectroscopy to the study of novel transient radical-anions formed by the addition of the an®oniated electron to both anions and neutral molecules at 200 K.

    The apparatus used employs the multicapillary mixing chamber described by earlier workers in this field48 with the addition of a low temperature modification to the main flow system. Prior to mixing, solutions of sodium and the substrate in liquid ammonia are cooled to temperatures in the range = 200 to 240 K by passing them through a cryostat comprising ~. 2m of 6mm i.d. glass tubing immersed in a methanol/C02 bath set slightly below the required temperature. The heat capacity of the bath is sufficient to prevent arty appreciable heating « 1° K) while the esr spectrum is recorded. The temperature of the solutions is maintained during transference to the multicapillary mixer by means of vacuum jacketed tubing. Chapter 5 describes the application of this technique to a series of ring- and N-substituted benzamides and thiobenzamides. In general, ring-substituted benzamides exist in their mono-ionised form in liquid ammonia and electron attachment to these results in the formation of the radical-dianions ArCONH2-. These radical- dianions show restricted rotation about the bond from the ring to the amide group and give rise to esr spectra of low symmetry. N-methyl- and 2-fluoro- benzamides showed anom- alous behaviour in that their esr spectra afford couplings from from a full complement of magnetic nuclei, i.e. electron attachment takes place to the unionised molecules. The

  • ii

    N,N-dimethyl- substituted benzamides, having no acidic amide protons, give rise to their corresponding radical-anions ArCONH(Me)2":'"and these, unlike the ring-substituted compounds, afford highly symmetric spectra. HO calculations are employed to facilitate the assignment of coupling constants to particular nuclei; the assymetry of the spin density distribution being simulated by application of the so-called S effect. A value of Qc~ = -2·75 ± 0·24 mT is derived for the a-IT-interaction parameter for the benzamide ring. Benzamide radical-anions in general have half-lives of the order of ms. N,N-dimethylbenzamide is unique in that it is stable for up to an hour at 200 K and can be observed by a conventional static reduction technique.

    Ethylene itself proves resistant to reduction by solutions of alkali metals in liquid ammonia but substitution in this molecule by groups such as carboxy-, acetyl-, or vinyl- have an activating effect. Chapters 6 and 7 describe the preparation and characterisation by esr spectroscopy of a series of partially reduced a/S-unsaturated carbonyl com- pounds. Chapter 6 is devoted to the ketyls of acyclic and cyclic a/S-unsaturated ketones, and Chapter 7 to the radical- anions of a,a-unsaturated acids, esters and nitriles.

    The open-chain ketyls are found to exist, ~ike the neutral molecules, as ~-cis and ~-trans conformers. A value ,of -2·45 mT has been derived for the a-IT-interaction para- meter for the vinyl protons. Vaules of "experimental" spin densities are compared with those derived from simple MO theory, to facilitate the assignernnt of coupling constants. The simple MO theory is modified to distinguish between ~-cis and s-trans conformations. Ass:ignment of coupling

  • iii

    constants in the series of cyclic ketyls derived from 2-cyclo-

    hexenone f.s straightforward. The couplings show that in most

    cases the ketyls exist in conformations with lifetimes

    generally greater than 10-6s and that the methylene protons

    are either in almost pure axial or equatorial positions.

    The ketyl of 2-cyclopentenone is planar. The esr spectrum

    of the 2-cyclohexenone ketyl shows modulation of

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