Effects of Hydrostatic Pressure on Linewidths of Free Radicals in Solution. I. Anisotropic Region

Abstract

We have measured the hyperfine linewidths of solutions of di‐t‐butyl nitroxide in methylcyclohexane vs hydrostatic pressure in the range 6500 to 10 300 kg/cm². In this pressure range, the linewidths vs viscosity are compared with the theory developed by Kivelson. The pressure dependence of the solvent viscosity was obtained from the data of Bridgman. The anisotropic parameters of the spin Hamiltonian of the radical were obtained from measurements in a frozen glass. Apparent deviations from Kivelson's theory at high viscosities are readily accounted for by assuming an explicit pressure dependence of the anisotropic part of the ¹⁴N hyperfine interaction, which is approximately linear with pressure in the region above 8500 kg/cm². Deviations at high radical concentration are shown to arise from intermolecular dipole—dipole interactions.