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/Tm, 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(trifluoroacetylacetonate), VO(tfac)2 were examined. At low temperatures where 1/T1 << 1/Tm, 1/Tm in 1:1 H2O:glycerol is dominated by solvent protons. At low temperature 1/Tm 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/Tm 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.
Eaton
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Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado, 80208, USA.geaton@du.edu
Eaton
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D002857:Chromium / Q000032:analysis*; D017608:Chromium Compounds / Q000737:chemistry*; D004058:Diffusion; D004578:Electron Spin Resonance Spectroscopy; D005990:Glycerol / Q000737:chemistry; D013113:Spin Labels; D013696:Temperature; D017968:Vanadium Compounds / Q000737:chemistry*
D017608:Chromium Compounds; D013113:Spin Labels; D017968:Vanadium Compounds; D002857:Chromium; C034028:vanadyl sulfate; D005990:Glycerol