Electron‐ and nuclear‐spin relaxation in an integer spin system, tris‐(acetylacetonato)Mn(iii) in solution

Abstract

Expressions are derived for the intermolecular contribution to the nuclear‐spin relaxation rate in solutions containing dissolved paramagnetic ions with spin S≥1. The calculation assumes that the electron‐spin Hamiltonian is dominated by a large axial zero‐field splitting, and it accounts for effects of Zeeman interactions to first order. The expressions are used to analyze proton‐spin relaxation of the acetone solvent in solutions of tris‐(acetylacetonato)Mn(iii)/ acetone. The main objective was to measure electron‐spin relaxation times of Mn(iii), which in this complex is a high‐spin, d4 ion with integer spin S=2. Spin‐lattice relaxation measurements were conducted over a range of magnetic field strengths (0.28–1.1 T) where the zero‐field splitting is large compared to the Zeeman energy. Electron‐spin relaxation times of Mn(iii) were found to be 8±2 ps, with little dependence on temperature over the range 215–303 K and on magnetic field strength up to 1.1 T. Use of the assumption that Zeeman splittings dominate zero‐field splittings (Solomon–Bloembergen–Morgan theory) resulted in computed electron‐spin relaxation times that are too short by a factor of 3–4.