Anomalous Relaxation of Hyperfine Components in Electron Spin Resonance. II

Abstract

A detailed study has been made of the linewidth and peak height of each of the five components in the electron spin resonance spectrum of the p‐benzosemiquinone ion as a function of microwave power at X‐band frequencies. Studies were also made of the spectrum of the peroxylamine disulfonate ion as a function of power. A 20% variation in the linewidths from one hyperfine component to another was found in the p‐benzosemiquinone ion spectrum, and a 13% variation in the saturation parameters (T₁). The average value for the linewidth parameter at low power for the p‐benzosemiquinone ion was found to be (1/T₂) = 1.11×10⁶ sec^—1, and the reciprocal of the average value of the saturation parameter was found to be about (1/T₁) = 0.74×10⁶ sec^—1. No variation in linewidths or saturation parameters among the three hyperfine components in the peroxylamine disulfonate ion spectrum was found, and the results were T₂ = 2.5×10^—7 sec, and T₁ = 3.3×10^—7 sec. The variation in the linewidth of the p‐benzosemiquinone ion can be accurately represented by a quadratic equation in m_J, the field‐direction component of the total nuclear spin angular momentum, and the form of this equation is in excellent agreement with the linewidth predicted for a relaxation mechanism arising from an anisotropic intramolecular dipolar interaction between the unpaired electron and the four protons in the molecule, as well as a rotational relaxation of the anisotropic g tensor. Good quantitative agreement between theory and experiment is obtained for the coefficient of m_J² in the equation for (1/T₂) if the rotational correlation time is taken to be τ_c = 7×10^—11 sec, a value which is consistent with the molecular dimensions. The dependence of the saturation parameters on m_J is also in qualitative agreement with the theory, but quantitative discrepancies are found which have not been explained.