Solvent and temperature dependence of spin echo dephasing for chromium(V) and vanadyl complexes in glassy solution

The solvent and temperature dependence of the rate constant for spin echo dephasing, 1/T-m, for 0.2 to 1.2 mM glassy solutions of chromyl bis(1-hydroxy-cyclohexanecarboxylic acid), CrO-(HCA)(2)(-); aquo vanadyl ion, VO2+ (aq), and vanadyl bis(trifluoro-acetylacetonate), VO(tfac)(2) were examined. At low temperatures where 1/T-1 much less than 1/T-m, 1/T-m in 1:1 H2O:glycerol is dominated by solvent protons. At low temperature 1/T-m increases in the order 1:1 H2O:glycerol or 9:1 CF3CH2OH:ethyleneglycol (no methyl groups) < 9:1 i-PrOH:MeOH (hindered methyl groups) < 9:1 n-PrOH: MeOH (less hindered methyl groups). This solvent dependence of 1/T-m, is similar to that observed for nitroxyl radicals, which indicates that the effect of solvent methyl groups on spin-echo dephasing at low temperature is quite general. At higher temperatures the echo dephasing is dominated by spin-lattice relaxation and is concentration dependent. As the glass softens, echo dephasing is dominated by the onset of molecular tumbling. (C) 1999 Academic Press.