Impact of molecular size on electron spin relaxation rates of nitroxyl radicals in glassy solvents between 100 and 300 K

Electron spin lattice relaxation rates were measured for 12 nitroxyls with molecular weights between 144 and 438, and for galvinoxyl, 1,3-bisdiphenylene-2-phenylallyl (BDPA), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) in glassy sucrose octaacetate. Relaxation rates for polar nitroxyls also were measured in glassy sorbitol. Dependence on TMPH_A_272409_O_XML_IMAGES\TMPH_A_272409_O_ILM0001.gif T where Veff is effective molecular volume and γ is a material-specific parameter, was used to compare processes. Values of Veff were determined based on molecular libration in glassy sucrose octaacetate (γ = 3.5), tumbling in viscous decalin at 233 K (γ = 4.7), or tumbling in heavy mineral oil (γ = 6.0). For nitroxyl relaxation there is a master curve: log(1/T1) vs log( TMPH_A_272409_O_XML_IMAGES\TMPH_A_272409_O_ILM0002.gifT) (γ = 0.89). The similarity of the values of γ for the Raman process and for the additional process that contributes at higher temperatures, and the absence of frequency dependence between X- and Q-band, support assignment of this additional process as a local mode. For these radicals the contributions from the local mode and the Raman process are correlated and follow trends in spin-orbit coupling. The temperature dependence of spin echo dephasing in sucrose octaacetate is dominated by rotation of methyl groups and by a motional process analogous to the Raman process