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Title: Electron spin resonance studies of transient radical-ions in liquid ammonia
Author: Neal, Graham Trevor
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1969
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This work describes the use of electron spin resonance spectroscopy in investigations of the reduction of a number of organic compounds by solutions of sodium in liquid ammonia. Solutions of alkali metals in ammonia contain the ammoniated electron, in all its possible forms, and essentially this species is the effective reducing agent. The simple reduction step is the one-electron addition to a suitable substrate molecule, which may then undergo a number of successive reactions before a stable product is formed. The nature of the metal-ammonia solutions, and the paths the reductions may take are discussed in Chapter 2. The simple one-electron reduction product, a radical-anion, is often too short-lived to allow observation by normal static methods, and therefore a relatively high concentration of radicals must be artificially maintained to permit their detection. Basically two different approaches have been utilised: i. Stabilising the radicals either by direct production or trapping in solid matrix, therefore preventing the radicals from reacting, and ii. Production of a high steady state concentration of radicals in solution by either continuous electrolysis or buy use of a flow system, A brief discussion of the methods used in the study of transient radicals is given in Chapter 1. Generally the e.s.r spectra are less well-resolved in solid matrices than in solutions, and for species showing a large amount of hyperfine structure, production of the radicals in solution is preferred. A rapid mixing device has been developed to allow observation of transient intermediates by e.s.r. spectroscopy. The mixer was designed particularly for the type of system and experimental technique under consideration here, and is based on a design used in biological kinetic studies. Observations 2-5 msec. after mixing are possible, and this represents a considerable advantage over the widely used aqueous solution mixing device, where observations are made on a 10-2 sec. time scale. The mixing chamber and experimental technique are presented in Chapter 3. Analysis of the e.s.r. spectrum of a compound allows the calculation of its unpaired electron distribution, the coupling constants being related to the unpaired electron spin density. Spin densities have been calculated theoretically, and it has been found that simple Hückel calculations of pi-electron spin densities show good agreement with “experimentally” determined values. Accordingly, both Hückel and McLachlan spin densities have been calculated for most of the substrates used, and in general it is found that the McLachlan treatment gives better agreement with experiment than the simple Hückel model. A brief discussion of the relevant molecular orbital theory is presented in Chapter 1, and the computer programme used to perform the theoretical calculations is given in Appendix 3. Analysis of the e.s.r. spectra I sometimes very difficult if a large number of lines are present, and two computer programmes used for the simulation of single and mixed e.s.r. spectra are given in Appendices 2 and 3 respectively. In Chapter 4 is described the reduction of a number of aryl halides. With the exception of fluoro-substituted compounds, the halo-pyridines, pyrimidines, benzenes, biphenyls, and naphthalene, all give the e.s.r. spectrum of the radical-anions of the parent compounds on reduction. On the other hand, fluorine is retained as is evident from the spectra, for a much longer period, as shown by observation 0.1 and 1.0 sec. after mixing, while static experiments show some products to be stable for longer periods of time in this system. Mechanisms have been proposed to account for these reductions. Halobenzonitriles have also been studies and the results are in agreement with the proposed reduction mechanisms. Reduction of pyridine, pyrimidine and some simple ring-substituted compounds has been investigated, and their e.s.r. spectra characterised in most cases. Previous attempts to observe the pyridine radical-anion had failed, the spectrum of the 4,4’-bipyridyl being obtained instead. Observations 0.1, 1.0 sec. and 1 min. after mixing allow the reduction path of these nitrogen heterocyclics to be followed, and it is shown that pyridine, pyrimidine and simple alkyl- and alkoxy-substituted pyridines undergo dimerization to give exclusively the respective 4,4’-dimers, unless the 4-position is blocked. Pyridine-N-oxide undergoes a more complex reduction, giving pyridine-N-oxide, pyridine and finally 2,2’-bipridyl radical-anions. Pyridine-2-carboxylic acid gives a spectrum suggesting some form of nitrogen-hydrogen bonded species. Pyridine dicarboxylic acids all have on feature in common, a splitting from an extra proton which arises through a protonated nitrogen atom. Reduction of pyridine in the presence of excess ethanol also shows this feature, and its spectrum is due to C5H5NH. Calculation of the nitrogen sigma-pi interaction parameters gives values of QNN=+28.5 oe. And QCNN=-O.5 oe., in good agreement with results obtained previously. Theoretical calculations have been performed, and comparison with experiment has enabled the assignment of coupling constants to particular positions, and also gives the best set of parameter values required for the calculations, providing a check with values used for comparison with other measurements. The results are presented in Chapter 5. In Chapter 6 is described the reduction of benzoic acid and some of its simple ring-substituted derivate. Ionisation occurs initially, followed by reduction to give the corresponding radical-anions. These species are short-lived, as no spectra are obtained when observations are performed 0.1 sec. after mixing. No further paramagnetic products are found. Molecular orbital calculations have been performed for each compound, and excellent agreement with experiment obtained using the parameter values kC’-C=1, 2, kC-O-1, 6, hO-2.0 for the ionised carboxyl group. Reduction of nitro-substituted isophthalic and terephthalic acids shows the presence of two distinct species, one being unstable while the other is stable for more than one hour, in both cases. The unstable species show a large splitting from an extra proton which is thought to be attached to the nitrogen atom. This proton is lost to give the stable radicals, experimental evidence and comparison with computer calculations suggesting them to be the nitro-substituted radical-anions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry