Electron spin-lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Electron spin relaxation times at 295 K were measured at frequencies between 250 MHz and 34 GHz for perdeuterated 2,2,6,6-tetramethy1-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, TR, between 4 and 50 ps and for three N-14/N-15 pairs of nitroxides in water with TR between 9 and 19 ps. To test the impact of structure on relaxation three additional nitroxides with TR between 10 and 26 ps were studied. In this fast tumbling regime T-2(-1) - T-1(-1) at frequencies up to about 9 GHz. At 34 GHz T-2(-1) > T,(-1)(1) due to increased contributions to T-2(-1) from incomplete motional averaging of g-anisotropy, and T-2(-1) - T-1(-1) is proportional to tau(R). The contribution to T-1(-1) from spin rotation is independent of frequency and decreases as tau(R) increases. Spin rotation dominates T-1(-1) at 34 GHz for all tau(R) studied, and at all frequencies studied for TR = 4 ps. The contribution to T-1(-1) from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as tau(R) increases; it dominates at low frequencies for tau(R) > -15 ps. The contribution from modulation of g anisotropy is significant only at 34 GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T-1(-1) was smaller at 250 MHz than at 1-2 GHz. The significant N-15/N-14 isotope effect, the small HID isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values. (C) 2013 Elsevier Inc. All rights reserved.