EPR and ELDOR Studies on Spin Relaxation in Perdeuterated 2,2,6,6-Tetramethyl-4-piperidone-N-oxyl in Liquid Solutions. The Slowly Relaxing Local Structure Mechanism

Article abstract of DOI:10.1021/j150655a019

The spin relaxation of perdeuterated 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (PDT) in various solvents was studied via EPR line width and saturation and ELDOR.Saturation and ELDOR results are incompatible with the line-width result when the latter are analyzed in the standard manner: rotational diffusion (RD) with temperature invariance of the anisotropy factor N and of the non-Debye parameter ε in the nonsecular spectral densities j(ω₀).In a more general approach, first all relevant intramolecular relaxation terms are obtained independently from the combined T₂, T₁, and ELDOR data.Further analysis of these terms suggests that, in general, the rotational relaxation of PDT may arise from rotational diffusion (RD) and a slowly relaxing local structure mechanism (SRLS).Separation of RD and SRLS contributions seems possible via the electron spin relaxation rate, more particularly via the spin-rotational interaction part W_e^SR.This is most clearly shown by the results on PDT in methylcyclohexane (MCH) and in the isotropic phase of the liquid crystalline solvent 4'-n-octyl-4-cyanobiphenyl (8CB).Particularly, by virtue of the results on 8CB the combined T₂-T₁-ELDOR method seems promising in spin label/spin probe research on ordering media by its option to yield directly the ordering dynamics of the nearby environment.