Electron Spin Resonance and Molecular Motion of the RCH2ĊHCOOR′ Radicals in X-Irradiated Ester—Urea Inclusion Compounds

Abstract

Single crystals of the inclusion compounds formed between long-chain alkyl esters and urea were x irradiated at room temperature, and the free radicals produced were investigated by electron spin resonance. The 11 esters studied were diethyl adipate and the methyl, ethyl, hexyl, and octyl esters of monocarboxylic acids. The long-lived free radicals observed in all of these compounds are of the type RCH2CHCOOR′. These are π-electron radicals with the unpaired electron largely localized on one carbon 2p orbital. The coupling constants of the alcohol protons (R′) were resolved in the spectra of several radicals and the values ranged from 3 to 6 Mc/sec. The ester radicals undergo motion in the tubular cavities formed by the urea molecules of the crystal, and this affects the magnitudes of the proton coupling constants. The dependence of the α-and β-proton coupling constants on this motion is briefly considered. From the ethyl heptanoate radical ESR data, recorded over the range of 352° to 7°±3°K, and from the room-temperature ESR data of all eleven ester radicals, information is obtained regarding the motions and orientations of the ester radicals. In addition, two carboxylic-acid—urea inclusion compounds were investigated, and the orientations and motions of the well-known radicals produced in these systems (RCH2ĊHCOOH) are compared to those of the ester radicals. Approximate equations are given which relate the observed α-proton coupling constants and spectroscopic splitting factors to the diagonal elements of the α-proton tensors and g tensors of all radicals investigated.