Electron spin relaxation of triarylmethyl radicals in fluid solution

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1 : 1 water: glycerol, and 1 : 9 water: glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H2O solution, T-1 is 17 +/- 1 mus at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T-1 in 1 : 1 water: glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H2O solution, T-2 is 11 +/- 1 mus at X-band, increasing to 13 +/- 1 mus at L-band. For more viscous solvent mixtures, T-2 is much shorter than T-1 and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T-2. In water and 1 : 1 water: glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1 : 9 water: glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T-2-determined contributions to the linewidths at higher frequencies. (C) 2001 Academic Press.