William T. Dixon

Determination of the acidity constants of some phenol radical cations by means of electron spin resonance

Phenoxyl radicals were generated in solutions containing from 0 to 75 % sulphuric acid. The changes observed in the e.s.r. spectra over this range are shown to be due to progressive formation of phenol radical cations. In favourable cases plots of the e.s.r. parameters against acidity enabled us to determine the pKa of the radical cations, which varied from –2.0 (phenol radical cation) to –0.8 (hydroquinone radical cation). Changes in the coupling constants in going from a phenoxyl radical to the corresponding phenol radical cation are usually relatively small, but the g-values change from 2.004 4 ± 0.000 4 to 2.003 2 ± 0.000 4. The free energy changes involved in competing oxidative processes are discussed and the data support the view that the C—O group in phenoxyl resembles that in ketones.

Bond lengths in cyclic polyenes C2nH2n a re-examination from the valence-bond point of view

Abstract The valence-bond resonance method for predicting bond lengths in conjugated hydrocarbon molecules has been reconsidered. New values are obtained for the variation of the fundamental exchange and Coulomb integrals with bond length. Application to the cyclic polyenes C 2n H 2n shows that for large n there will be substantial bond alternation. Inclusion of Dewar structures diminishes this alternation, but does not destroy it.

The molecular orbital theory of some simple radicals

In this paper the molecular orbital theory of hyperconjugation is extended in order to account for the hyperfine coupling constants observed in some simple but interesting free radicals, e.g. cyclopentyl, vinyl. Predictions are made for the phenyl radical.

Substituent effects in the e.s.r. spectra of phenoxyl radicals

Regularities observed in the e.s.r. spectra of many o-, m- and p-substituted phenoxyl radicals allow unambiguous assignments of the coupling constants as well as the determination of their relative signs. It is shown that semiquinone anions can be classed as phenoxyl radicals and similarly for radicals from the oxidation of trihydroxybenzene derivatives.With suitable parameters, the patterns of splittings in these radicals can be rationalised by means of McLachlans SCF theory; a heteroatom model and an inductive model both explain most of the observed trends. INDO calculations gave a rather poor account of the splittings in phenoxyl.

Long-range electron spin resonance coupling constants in radical adducts of maleic acid

The e.s.r. spectra of a number of radical adducts of maleic acid have been observed and hyperfine structure arising from γ-, from δ- and from Iµ-coupling has been revealed. This provides a positive identification of the radicals involved. The origin of the long-range coupling constants is discussed in terms of simple molecular orbital theory and direct magnetic interaction. The difference between spectra observed in acidic and in alkaline conditions, is interpreted in terms of structural changes.

Bond lengths in conjugated polyenes

Abstract Some conjugated polyenes have been examined using the valence-bond approximation including σ-bond compressional energy. The appropriate exchange and coulomb integrals were obtained as functions of bond length, using spectroscopic and thermal data for benzene and ethylene. These semi-empirical functions were then used in investigating linear cyclic polyenes. It was found that the bond lengths predicted for butadiene were in excellent agreement with experiment and that there should be appreciable bond alternation in longer chains. Similarly for n > 3, it was found that bonds should alternate in the cyclic polyenes C 2n H 2n and that the degree of bond alternation tends to increase with n. This agrees qualitatively with the recent application of simple molecular-orbital theory (including σ-bond compression) to long chain and large cyclic polyenes.

Calculations of g-factors of phenoxyl radicals, phenol radical cations and alkyl aryl ether radical cations

Using the semi-empirical method presented by Stone, experimental g-factors of some oxygen-substituted aromatic radicals have been correlated with the energy-level coefficient (λ) of the odd electron Huckel molecular orbitals, and with the total spin density (Σρ0) on the oxygen atoms. Parameters obtained from plots of Δgobserved/Σρ0 against λ were used to directly estimate g-factors of the radicals studied.

An electron spin resonance study of the autoxidation of naphthols in the presence of hydrogen peroxide

A number of radicals have been obtained from polyhydroxylated naphthalenes in alkaline solution by means of autoxidation, and their e.s.r. coupling constants have been measured. A new way of generating radicals derived from naphthalene-1,2-diol is reported in which alkaline hydrogen peroxide is used, and a mechanism for the observed transformation is proposed. Radicals can be obtained from resorcinols in an analogous way.

Secondary radicals in the autoxidation of hydroquinones and quinones

The autoxidation of hydroquinones and quinones in strongly alkaline solution gives rise to radical intermediates other than the expected semiquinones. These ‘secondary’ radicals, which are derived from 1,2,4-trihydroxybenzene, can be characterised by means of e.s.r. spectroscopy. Different isomeric radicals may be formed, depending on the solvent, and apparently these may reduce any excess of quinone present to the corresponding semiquinone.

Electron spin resonance spectra of radical cations related to 1,3,5-trihydroxybenzene(phloroglucinol)

Different e.s.r. parameters of radicals formed by the 1-electron oxidation of phloroglucinol (or of its O-methyl derivatives), vary differently with acid strength. This fact is consistent with there being at least two types of radical cation involved, a possibility which gains support from the observation of line broadening, which increases with acid strength. It is suggested that the line broadening is due to conformational changes of the methoxy groups in the neutral radicals, whereas the apparently anomalous variations of coupling constants arise from solvation effects, which might include changes in the relative concentrations of conformational isomers with acid strength. The magnitudes of the coupling constants are explained in terms of linear combinations of the symmetrical and antisymmetrical orbitals which are degenerate when trigonal symmetry is assumed.