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The proton hyperline coupling constants in three free radicals which are formed during the aerobic oxidation of either chlorogenic acid or of caffeic acid in strongly alkaline solution have been measured using EPR and ENDOR spectroscopy and the three radicals thus characterised. Possible identities for the free radicals are discussed.

The interpretation of EPR and ENDOR measurements on an organic free radical which appears to be a universal concomitant of senescence in plants is discussed. On the basis of EPR spectra obtained at 95 GHz it is speculated that the radical is derived from a quinone.

When germinating Zea mays L. seeds are rapidly desiccated, free radical-mediated lipid peroxidation and phospholipid de-esterification is accompanied by a desiccation-induced buildup of a stable free radical associated with rapid loss of desiccation tolerance. Comparison of the electron paramagnetic resonance and electron nuclear double resonance properties of this radical with those of the radical in dried, desiccation-intolerant moss showed that the two were identical. At the subcellular level, the radical was associated with the hydrophilic fraction resulting from lipid extraction. Isolated mitochondria subjected to drying were also found to accumulate an identical radical in vitro. When increasing concentrations of cyanide were used, a significant positive correlation was shown between rates of respiration and the accumulation of the radical in desiccation-intolerant tissues. Another positive correlation was found when rates of O2 uptake by radicles at different stages of germination were plotted against free radical content following desiccation. This indicates that free radical production is closely linked to respiration in a process likely to involve the desiccation-induced impairment of the mitochondrial electron transport chain to form thermodynamically favorable conditions to induce accumulation of a stable free radical and peroxidized lipids. Modulation of respiration using a range of inhibitors resulted in broadly similar modulation of the buildup of the stable free radical. One site of radical generation was likely to be the NADH dehydrogenase of complex I and probably as a direct consequence of desiccation-impaired electron flow at or close to the ubiquinone pool.

Photolysis of Fe(CO)5 in non-polar solvents containing dissolved NO results in the generation of an EPR spectrum which is assigned to the free radical Fe(NO)3CO.

From a study of the oxidation of bisphenol-A epoxy resin cured with DDM (4,4′-diaminodiphenylmethane), it has been shown that the development of greenish-blue colors is due to an initial reaction between molecular oxygen and secondary amine groups. A full reaction scheme is suggested and supporting evidence, in the form of UV/visible, infrared, and ESR spectra, is presented.

Elelctron spin resonance measurements have been used to follow radical concentrations in the copolymerization of methyl methacrylate and glycol dimethacrylate at 23 and 0°C. and the decay of such radicals. The results have been interpreted to give a qualitative picture of the radical trapping process. It has also been shown that the allyl group in allyl methacrylate is comparatively unreactive and does not take much part in either the polymerization of this monomer or its copolymerization with glycol dimethacrylate. Acrylonitrile and α-methyl acrylonitrile have been shown to give different electron spin resonance spectra illustrating the effect of a chain methyl group. This parallels electron resonance studies of γ-irradiated acrylates and methacrylates.

It is argued that the electron spin resonance spectrum of an ion-pair simultaneously undergoing intramolecular cation exchange between equivalent conformations and intermolecular atom transfer with neutral parent molecules may fortuitously show a binomial intensity distribution even though both exchange processes may be going at rates of the order required to produce linewidth effects. Experiments with potassium-2,5-di-t-butyl p-benzosemiquinone illustrate the phenomenon. At room temperature the rate constant for the reaction TBPBSQ-potassium + TBPBQ ⇌ TBPBQ + potassium-TBPBSQ is estimated to be 6.43 × 107 M–1 s–1.

Complexes of m-dinitrobenzene (MDNB) and 1,3-dinitro-5-t-butyl benzene (TBMDNB) with alkali metals in 1,2-dimethoxyethane have been studied by e.s.r. The behaviour of TBMDNB is analogous to that of MDNB itself, but the spectra are simpler. General observations agree with those of other workers. Several new observations have been made: (i) the rate constant for intramolecular caesium exchange in Cs—TBMDNB is 4 × 1010 exp(–14,300 (J)/RT). (ii) The spin density at the t-butyl protons in Na—TBMDNB is negative. (iii) Na—TBMDNB exists in equilibrium with a different complex, whose e.s.r. spectrum shows no alkali metal hyperfine splitting, at high temperature. At 97°C, K= 0.25, ΔG°370=+ 4.2 kJ M–1, ΔH°=–24.8 kJ M–1, ΔS°370=+ 77.6 J deg.–1 M–1. (iv) The equilibrium Cs—MDNB + NaBPh4= Na—MDNB + CsBPh4 has K= 15 at 23°C. ΔH° and ΔS°296 are positive.

The relative importance of charge transfer states arising by excitation of electrons from π and from nitrogen non-bonding orbitals to the lowest vacant s-orbital of the alkali metal as mechanisms producing metal hyperfine splitting in the electron spin resonance spectra of pyrazine-alkali metal ion pairs is discussed using molecular orbital theory.

Analysis of the electron spin resonance spectra of solutions of m-dinitrobenzene-sodium, and of t-butyl-m-dinitrobenzene-sodium, before and after the addition of sodium-tetraphenyl boron, indicates that in the mixed systems the paramagnetic complex takes part in a cation exchange reaction in which the incoming cation goes to the uncomplexed nitro-group. Approximate rate constant data are presented for the reaction of the t-butyl-substituted compound.

The electron spin resonance spectra of pyrazine-alkali metal systems are interpreted in terms of a model for the structure of the complexes. In the lithium, sodium and potassium systems the cation experiences a potential which has minima near the two nitrogen atoms, whereas for the larger rubidium and caesium ions the effective potential has a single minimum over the centre of the pyrazine ring.