Electron spin–lattice relaxation in radicals containing two methyl groups, generated by γ-irradiation of polycrystalline solids

Abstract

The effects of methyl rotation on electron spin–lattice relaxation times were examined by pulsed electron paramagnetic resonance for the major radicals in γ-irradiated polycrystalline α-amino isobutyric acid, dimethyl-malonic acid, and l-valine. The dominant radical is the same in irradiated dimethyl-malonic acid and α-amino isobutyric acid. Continuous wave saturation recovery was measured between 10 and 295 K at S-band and X-band. Inversion recovery, echo-detected saturation recovery, and pulsed electron–electron double resonance (ELDOR) data were obtained between 77 and 295 K. For the radicals in the three solids, recovery time constants measured by the various techniques were not the same, because spectral diffusion processes contribute differently for each measurement. Hyperfine splitting due to the protons of two methyl groups is resolved in the EPR spectra for each of the samples. Pulsed ELDOR data were obtained to characterize the spectral diffusion processes that transfer magnetization between hyperfine lines. Time constants were obtained for electron spin–lattice relaxation ( T 1e), nuclear spin relaxation ( T 1n), cross-relaxation ( T x1), and spin diffusion ( T s). Between 77 and 295 K rapid cross-relaxation (Δ M s=±1, Δ M I=∓1) was observed for each sample, which is attributed to methyl rotation at a rate that is approximately equal to the electron Larmor frequency. The large temperature range over which cross-relaxation was observed suggests that methyl groups in the radical and in the lattice, with different activation energies for rotation, contribute to the rapid cross-relaxation. Activation energies for methyl and amino group rotation between 160 and 1900 K (1.3–16 kJ/mol) were obtained by analysis of the temperature dependence of 1/ T 1e at S-band and X-band in the temperature intervals where the dynamic process dominates T 1e.