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-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50 ps and for three (14)N/(15)N pairs of nitroxides in water with τR between 9 and 19 ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26 ps were studied. In this fast tumbling regime T2(-1)~T1(-1) at frequencies up to about 9 GHz. At 34 GHz T2(-1)>T1(-1) due to increased contributions to T2(-1) from incomplete motional averaging of g-anisotropy, and T2(-1)-T1(-1) is proportional to τR. The contribution to T1(-1) from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1(-1) at 34 GHz for all τR studied, and at all frequencies studied for τR=4 ps. The contribution to T1(-1) from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>~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, T1(-1) was smaller at 250 MHz than at 1-2 GHz. The significant (15)N/(14)N isotope effect, the small H/D 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.
Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, United States; Center for EPR Imaging In Vivo Physiology, University of Denver, Denver, CO 80208, United States.
Joshua R
JR
Biller
Hanan
H
Elajaili
Virginia
V
Meyer
Gerald M
GM
Rosen
Sandra S
SS
Eaton
Gareth R
GR
Eaton
D000465:Algorithms; D000818:Animals; D016880:Anisotropy; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005609:Free Radicals; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D015233:Models, Statistical; D009587:Nitrogen Isotopes; D009589:Nitrogen Oxides / Q000737:chemistry*; D009590:Nitrogen Radioisotopes; D012997:Solvents; D014783:Viscosity
D005609:Free Radicals; D007202:Indicators and Reagents; D009587:Nitrogen Isotopes; D009589:Nitrogen Oxides; D009590:Nitrogen Radioisotopes; D012997:Solvents
Modulation of anisotropy; Spin lattice relaxation; Spin rotation; Spin–spin relaxation; Thermally-activated process