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Title: A study of the electron spin resonance spectra of radicals generated from phenols and from related compounds
Author: Kok, Pat Moi
Awarding Body: University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1977
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The work reported in this thesis started as an investigation of the autoxidation of naphthols, phenols and dihydroaromatics in hexamethylphosphoramide (HMPA). Attention was then focussed particularly on the calculation of ring proton coupling constants and those of the substituents using McLachlan's method and including the structures of the substituents in the theory. A theoretical interpretation of the g-factors of radicals examined has also been made. The autoxidation is base-catalysed and HMPA was chosen as a solvent because the bases used such as sodium methoxide were of low solubility so that the basic strength of the medium could be kept at a minimum. Also secondary radical formation was minimised. The radical intermediates of the autoxidation were identified from their electron spin resonance spectra. Primary radicals were observed from the 1,2-; 1,4-; 1,5-; 1,7-; and 2,6-naphthalenediols, the latter three radicals being of transient existence only. The electron spin resonance spectrum from 1,7-naphthal-enediol shows a large hyperfine splitting of 0.930 mT which is the largest ever observed for an aromatic proton in a static system. At later stages of the autoxidation, beta-naphthols gave 1,2-naphthosemi-quinones as intermediates, while in the case of alpha-naphthols, though 1,2-naphthosemiquinones were generally observed first, these soon decayed leaving weaker spectra of 1,4-naphthosemiquinones. The structures of the secondary radicals were deduced from sets of identical spectra obtained from different starting materials. The possible mechanism for the formation of secondary radicals was suggested. Where no primary radicals were observed, there was an induction period of an hour or even more. A characteristic of such cases was that the sudden rise of radical concentration was accompanied by a fluorescence; the colour of which was characteristic of the starting materials. The complex (I), between the naphthoxide ion and oxygen was suggested as the species responsible for the fluorescence phenomenon. Phenol, monosubstituted phenols, resorcinols and dihydroaromatics were also found to be successfully autoxidised in this system, giving rise to 1,2- and 1,4-semiquinones. To have theoretical interpretation of the esr spectra, the McLachlan method was used to calculate the spin densities in some phenoxyl radicals, phenol radical cations and some alkyl aryl ether radical cations using a full set of basis pi atomic orbitals forthe substituents concerned. The methyl- and methoxy-substituents were treated in terms of the hyperconjugation model so that the aliphatic hydrogen coupling constants can be calculated directly. An unusual parameter was chosen for the methyl group in order that the odd electron in the toluene anion might go to the antisymmetrical orbital with respect to the plane passing through the substituent and the aromatic ring. In the calculations of spin density distributions of carbonyl-substituted radicals, the carbonyl-group was described in terms of two basis atomic orbitals. The theoretical values were in excellent agreement with experiment. The methylene proton splittings in the 1,3-benzodioxole radical cation and its derivatives were well accounted for by the hyperconjugation theory. The larger values of g-factors of phenoxyl radicals compared with those of hydrocarbon radical anions and cations arise from the larger spin-orbit coupling constant of the oxygen atom and the smaller excitation energy from the non-bonding orbital of the oxygen to the odd electron orbital. The g-factors and the energy level coefficients of the Huckel molecular orbitals occupied by the unpaired electron in m-substituted phenoxyl radicals are more or less parallel to the electron donating effects of the substituents. The correlation of g-factors of this group of radicals with the energy-level coefficients of the odd electron HUckel molecular orbitals and with the total spin densities on the oxygen atoms are excellent, while fair correlations were obtained for the ether radicals examined. The calculated g-factors using Stone's semiempirical equation agreed well with experimental results.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physical Chemistry