Molecular motions in liquidlike pockets of frozen solutions: Electron spin relaxation study of semiquinones in DMSO and DMSO–ethanol mixtures

Abstract

The ESR spectra of semiquinones in frozen DMSO show motional averaging of anisotropic ESR parameters. The following observations support the liquidlike pocket model for the environment of semiquinones giving such spectra in these systems even up to 50° below the freezing points of the solutions: (i) The temperature dependence of linewidths and spin lattice relaxation times T1 of semiquinones in frozen DMSO are characterized by an activation energy ΔE?3.2 kcal mol−1. This value is close to the ΔE associated with the viscosity η[ = η0 exp (ΔE/RT)] of DMSO in the liquid state. (ii) The dependences of linewidths and T1 on the sizes of semiquinones are significantly different. Also, T1 calculated from linewidth data using the Hubbard relation do not agree with the experimentally observed values of T1. These behaviors are similar to that seen for semiquinones in liquid alcohols. (iii) There is good agreement between the η0 estimated in this model from the linewidth data or T1 data of semiquinones of different sizes in frozen DMSO. Similar conclusions can be drawn about semiquinones in frozen DMSO–ethanol mixtures. The dominant contributions to T1 comes from spin rotation interaction, and that to linewidths comes from the modulation of anisotropic ESR parameters, in this temperature range.