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 saturati on recovery for natural abundance and isotopically enriched (Cr-53) ox o-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 s imilar for natural isotope abundance and isotopically enriched samples and do not depend on nuclear spin m(1). These observations are not co nsistent with a model of relaxation due to modulation of anisotropic h yperfine 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, modul ation of spin-orbit coupling makes a larger contribution to spin-latti ce relaxation than modulation of anisotropic hyperfine interaction. Fo r both natural abundance and isotopically enriched samples, the variat ion 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 Cr NTTP is attributed to the lower symmetry of CrO(HEBA)(2)(-) than of Cr NTTP. (C) 1995 Academic Press, Inc.