Frequency (250MHz to 9.2GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin–lattice relaxation of nitroxyl radicals in temperature ranges that span glassy solutions to low-viscosity liquids

“…The present study found no evidence of an intramolecular H/D isotope effect, which indicates that a generalized spin diffusion contribution is not significant at X-band for the range of tumbling rates examined. By contrast a substantial H/D isotope effect was observed for spin-lattice relaxation of trityl-CD 3 vs. trityl-CH 3 and for solvent deuteration at 250 MHz [48]. The relaxation processes for the trityl radicals differ from that for the nitroxyls in two key respects: the modulation of a large anisotropic hyperfine interaction is not present and g values are closer to 2.0023 so spin-orbit coupling is smaller.…”

“…Thus, the tumbling-dependent processes that dominate for the nitroxyls are not present for trityl. In addition, the H/D isotope effects on the trityl relaxation were much larger at 250 MHz than at X-band [48]. Further studies of nitroxyl relaxation rates will be needed to determine whether H/D isotope effects for nitroxyls are larger at lower frequencies.…”

Pulsed saturation recovery 250 MHz electron paramagnetic resonance spectrometer

“…During the design stage of this spectrometer it was decided not to include pulsed saturation recovery as a performance feature, because saturation recovery is a less sensitive technique than are FID-and spin-echobased time domain measurements, and it was expected that sensitivity would be so low at 250 MHz that SR would not be feasible. However, it became evident that extending relaxation time measurements to the VHF region is important for determining relaxation mechanisms (18,19). Saturation recovery is a particularly useful method for species with T 2 short enough that the echo or FID signal decays below noise level in a time that is shorter than the dead time of the spectrometer.…”

“…As has been shown for some radicals, some relaxation mechanisms cause the relaxation times to be frequency dependent (17)(18)(19). Until now, low-frequency electron spin relaxation measurements have been performed with FID or electron spin echo (ESE) detection.…”

“…X-band saturation recovery is now commercially available from Bruker BioSpin. We previously described SR measurements at L-band (1.9 GHz) (19), but to our knowledge, this is the first SR measurement in the VHF frequency range.…”