Electron- and nuclear-spin relaxation in an integer spin system, t r i s-(acetylacetonato)Mn( iii ) in solution

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.