I. Electron magnetic resonance studies on irradiated single crystals of sebacic acid. II. Rate of electron exchange between di-para-anisyl nitrogen-oxide molecules by electron paramagnetic resonance

Abstract

Part I The electron magnetic resonance spectrum of freshly x - ray damaged single crystals of sebacic acid shows that two radical species, I and II, are produced, the spectrum of I about twice as intense as that of II. Upon illumination of a damaged crystal with ultraviolet light, radical II is destroyed, leaving I unaffected. Analysis of the magnetic hyper fine spectrum of such a bleached crystal identifies I as HOOCĊH(CH_2)_7COOH and shows that I is oriented within experimental error in the same way in the crystal lattice as the undamaged molecule. The σ proton and π proton hyperfine interactions with the unpaired spin are measured for I and are found to be consistent with those values observed for similar radicals formed in other irradiated dicarboxylic acids . Radical II is not identified but is judged to be structurally similar to I, primarily because of the observed and deduced similarities between the spectra of I and II. A damaged crystal which has been heat treated has a spectrum interpretable in terms of three radicals, I, III, and IV. The evidence is that radical II is the precursor of III and IV. Several models for III and IV are offered, which invoke fractional spin densities to account for the observed small hyperfine splittings. Arguments are advanced that favor IV being (•OOC)(CH_2)_8COOH. Radical III may be HOOCCHCH=CH(CH_2)_6COOH or an ionized species as (O=CHCHC+/H)(CH_2)_5COOH. Part II The line shape of the EPR spectrum of di-p-anisyl nitrogen oxide (DANO) in CHC1_3 was studied over a range of concentrations (0.006-0.F) so chosen to show the transition from the hyperfine triplet spectrum (arising from the hyperfine contact interaction of the electron spin with the nitrogen nucleus) to the exchange narrowed single line spectrum as the concentration of free radical increases. The observed change of line shape with concentration agreed well with that predicted by the modified Bloch equations which allow for spin magnetization exchange. The rate of electron exchange between molecules of DANO was calculated to be 7.5 x 10^9 liter mole ^(-1) sec ^(-1) at room temperature. The lifetimes of the electron spin state with respect to exchange ranged from 3 x 10 ^(-8) sec to 1 x 10 ^(-9) sec.