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

Abstract

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 14N/15N 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 to 2 GHz. The significant 15N/14N 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.

Frequency dependence of electron spin–lattice relaxation for semiquinones in alcohol solutions

“…[7] correspond to c slip ranging from 0.10 to 0.40 with an average of 0.24, including c slip = 0.40 and 0.25 for 25DTBSQ in CH 3 OH and C 2 H 5 OH, respectively, and c slip = 0.36 for TMBSQ in CH 3 OH. These values of c slip are larger than those for nitroxides [23] which are 0.11 for tempone (which has a keto group) in water, 0.15 for mHCTPO (which has a carboxamide group) in water, and 0.21 for proxyl (which has a carboxy group) in water. The larger values of c slip for the anionic semiquinones than for neutral nitroxides are consistent with the expectation of stronger interaction of the semiquinones with the H-bonding polar solvents.…”

“…At lower frequencies it is proposed that the relaxation rates for semiquinones are faster than at X-band because dipolar interactions with solvent protons are modulated by fluctuations in the hydrogen bonding between the solvent and the semiquinone oxygens. For nitroxides the contribution to relaxation from modulation of the large anisotropic nitrogen hyperfine coupling is strongly dependent on frequency and τ R [23]. The contribution from this process is larger at lower frequency and for longer τ R (Eq.…”

“…Spin-lattice relaxation times, T 1 , were measured by inversion recovery at 250 MHz [30], 600 MHz [23], L-band (1.0, 1.5 GHz) [31], S-band (2.57, 3.0 GHz) [31], and X-band (9.5 GHz) [32] on locally designed spectrometers and at Q-band (34 GHz) on a Bruker E580 spectrometer. The experiments were performed at a resonator temperature of ∼293 K. Multifit, a locally written program based on the algorithms of Provencher, was used to determine T 1 from the decay curves [33].…”

Imaging of nitroxides at 250MHz using rapid-scan electron paramagnetic resonance

“…Although 300 MHz pulsed EPR imaging of nitroxides has been accomplished by the NCI group [16], the ~0.5 µs relaxation times [17] are short relative to resonator ring-down even for resonator Q of 10 to 20 at 250 to 300 MHz, which makes pulse detection challenging. We have proven the advantages of rapid scan relative to continuous wave (CW) and pulse EPR for many classes of samples, with improvements in signal-to-noise (S/N) per unit time as high as 250 [18, 19].…”

“…We have proven the advantages of rapid scan relative to continuous wave (CW) and pulse EPR for many classes of samples, with improvements in signal-to-noise (S/N) per unit time as high as 250 [18, 19]. The electron spin relaxation times and linewidths of nitroxides [17, 20] are in the range for which rapid-scan EPR is expected to be a better imaging method than pulse or CW EPR. In this paper we show that rapid-scan imaging of a phantom composed of three different nitroxides provides better S/N per unit time than does CW imaging.…”