ESR linewidths in solution. VII. Nondiffusional anisotropic spin–rotational contributions in hydrogen bonding solvents

Abstract

From variations in the linewidths of the various hyperfine components in the ESR spectra of vanadyl acetylacetonate (VOAA) dissolved in alcohols, the reorientational correlation time τ_ϑ⊥ about an axis perpendicular to the V–O axis can be determined. In order to obtain these times, one needs to know the molecular magnetic parameters; these parameters can be determined in the glassy state, and a detailed analysis is given which indicates that they are effectively independent of temperature and phase. The residual linewidth α^″, that part of the linewidth which cannot be attributed to anisotropies in the molecular magnetic parameters modulated by molecular reorientations, has previously been attributed to spin–rotational interactions. If the reorientational motion is diffusive, one can calculate the theoretical isotropic (spherical) spin–rotational contribution α^″_sph provided τ_ϑ⊥ is known. It is found that α^″/α^″_sph is close to unity at low η/T, where η is the coefficient of shear viscosity, and may in fact be slightly less than 1; at high η/T the ratio α^″/α^″_sph is often much larger than 1, in some cases as large as 30. We have associated this discrepancy between α^″ and α^″_sph with anisotropic spin–rotational relaxation. At low η/T the rotational motion is diffusive and more or less isotropic; at large η/T the angular momentum about the V–O axis relaxes much more slowly than about an axis perpendicular to the V–O axis. Whereas the motion about an axis perpendicular to the V–O axis is diffusive and τ_ϑ⊥ is proportional to η/T, the motion about the V–O axis is nondiffusional, at least at large η/T, and τ_ϑ is not simply proportional to η/T. The nondiffusional motion at high η/T may arise from holes within the alcohol medium in which the VOAA molecule can rotate reasonably freely about the V–O axis; at higher temperatures this structure may become less evident and the rotation less free and more diffusive. It may be that tha analyses of the residual linewidths α^″ in viscous media in terms of spin–rotational relaxation could provide a sensitive test of the anisotropy and nondiffusional nature of molecular rotations; however, it has not yet been proved that α^″ is entirely due to spin–rotational interactions, as we have assumed.