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Title: Electron spin resonance studies of transient radical-anions produced in liquid ammonia at 200 K
Author: Elson, Ian Hamilton
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1972
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In this work is described the application of esr spectroscopy to the study of novel transient radical-anions formed by the addition of the ammoniated 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 ca. 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 any 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 anomalous behaviour in that their esr spectra afford couplings from a full complement of magnetic nuclei, i.e. electron attachment takes place to the unionised molecules. The 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 compounds. Chapter 6 is devoted to the ketyls of acyclic and cyclic a/S-unsaturated ketones, and Chapter 7 to the radical-anions of a,ß-unsaturated acids, esters and nitriles. The open-chain ketyls are found to exist, like the neutral molecules, as s-cis and s-trans conformers. A value ,of -2·45 mT has been derived for the õ‖-interaction parameter for the vinyl protons. Vaules of "experimental" spin densities are compared with those derived from simple MO theory, to facilitate the assignrnent of coupling constants. The simple MO theory is modified to distinguish between s-cis and s-trans conformations. Assignment of coupling constants in the series of cyclic ketyls derived from 2-cyclo- hexenone 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 certain hyper fine couplings due to ring inversion occuring during the time of the esr experiment with an activation energy of 23-59 kJ mol-I. Inclusion of·a proton source in the reduction medium of 3,5,5-trimethyl-2-cyclohexenone enables the neutral radical to be observed. Static reduction tends to lead to the formation of semidiones arising through oxidation of the radical-anions. Ionisation of the a,ß-unsaturated acids in liquid ammonia results in the s-cis and s-trans isomers becoming equivalent and single species corresponding to [ R1R2C = CR3C02 ]2~ are observed in the flow system. These radical-anions are short lived and give rise to esr spectra of low intensity. The radical-anion of fumaric acid was observed by reduction of acetylenedicarboxylic acid. Partial. reduction of a,ß-unsaturated esters yields somewhat more stable radical-anions which show s-cis and s-trans conformations. Radical-anions of maleic and fumaric esters are ) found to isomerise and give rise to several isomeric and conformeric species. Reduction of ethyl propiolate gives rise not only to its own radical-anion but also to that of ethyl acrylate as a secondary species. The high-field spectral lines of the former compound are of enhanced intensity, probably through the agency of Chemically Induced Dynamic Electron Polarisation (CIDEP).
Supervisor: Not available Sponsor: Imperial Chemical Industries Ltd ; Science Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry