ELECTRON-SPIN-LATTICE RELAXATION IN NATURAL-ABUNDANCE AND ISOTOPICALLY ENRICHED OXO-CHROMIUM(V)BIS (2-HYDROXY-2-ETHYLBUTYRATE)

Electron-spin-lattice relaxation times, T-1, were measured by saturation recovery for natural abundance and isotopically enriched (Cr-53) oxo-chromium(V) bis(2-hydroxy-2-ethylbutyrate), (CrO)-Cr-53(HEBA)(2)(-) in 1:1 water:glycerol between 50 and 150 K. The relaxation times are similar for natural isotope abundance and isotopically enriched samples and do not depend on nuclear spin m(1). These observations are not consistent with a model of relaxation due to modulation of anisotropic hyperfine interaction. The temperature dependence of T-1 is similar to that observed previously for perpendicular orientations of chromium(V) nitrido tetratolylporphyrin, CrNTTP. For these Cr(V) complexes, modulation of spin-orbit coupling makes a larger contribution to spin-lattice relaxation than modulation of anisotropic hyperfine interaction. For both natural abundance and isotopically enriched samples, the variation in T-1 with position in the spectrum is less than 30%. The smaller orientation dependence of T-1 for (CrO)-Cr-53(HEBA)(2)(-) than for CrNTTP is attributed to the lower symmetry of CrO(HEBA)(2)(-) than of CrNTTP. (C) 1995 Academic Press, Inc.