Electron-electron double resonance of irradiated dimethylmalonic acid, α-aminoisobutyric acid, and L-alanine single crystals: The role of methyl substituents

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Abstract
Electron-electron double resonance (ELDOR) spectra of the (CH3)2ĊCOOH radical in irradiated single crystals of dimethylmalonic acid and α-aminoisobutyric acid, the (CH3)2ĊCOOD radical in irradiated N,N-dideutero-α-aminoisobutyric acid single crystals, and the CH3ĊHCOOD radical in irradiated single crystals of deuterium substituted L-alanine (CH3CHND2COOD) were measured over a temperature range from 170 to 370°K. Two competing relaxation mechanisms, an electron-nuclear dipole relaxation (END) and a cross relaxation mechanism, resulted in an observed reduction in the allowed ESR lines above 283°K with ELDOR lines observed at aH,2aH, 3aH, and 4aH. Below 283°K an enhancement of the ESR line occurred at aH, a reduction at 2aH and negligible ELDOR intensity at higher multiples of aH. The qualitative aspects of the temperature dependence of the ELDOR spectrum of (CH3)2ĊCOOH can be explained in terms of the correlation times of the intramolecular dynamic processes. Approximate calculations based on these mechanisms give reasonable agreement with experiment.