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 H(2)O solution, T(1) is 17+/-1 micros 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 H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros 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.
Yong
L
L
Department of Chemistry, University of Denver, Denver, Colorado 80208-2436, USA.
Harbridge
J
J
Quine
R W
RW
Rinard
G A
GA
Eaton
S S
SS
Eaton
G R
GR
Mailer
C
C
Barth
E
E
Halpern
H J
HJ
D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D012996:Solutions; D013696:Temperature; D014320:Triphenylmethyl Compounds / Q000737:chemistry*
D005609:Free Radicals; D012996:Solutions; D014320:Triphenylmethyl Compounds