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, T1 is 17±1 μs at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T1 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, T2 is 11±1 μs at X-band, increasing to 13±1 μs at L-band. For more viscous solvent mixtures, T2 is much shorter than T1 and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T2. 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 T2-determined contributions to the linewidths at higher frequencies.