Abstract
A calculation is presented describing the coupled relaxation of the Zeeman and tunnel reservoirs of methyl groups, influenced by paramagnetic copper pairs. The transition rates are calculated following the approach introduced by Obata. Differential equations are derived which include the relaxation rates of both the intramethyl magnetic dipolar interaction and the interaction between the ${\mathrm{CH}}_{3}$-- protons and the paramagnetic copper pairs. The solution of the equations is compared with experimental data from polycrystalline copper acetate between 30 and 70 K. A time-dependent proton Zeeman spin-lattice relaxation time is observed. Its ``initial'' and ``final'' values are measured and explained. Moreover, resonant features in ${T}_{1}^{\mathrm{\ensuremath{-}}1}$(T) are observed near 40 K, which are related to the ${\mathrm{CH}}_{3}$-- tunnel splittings. The temperature dependence of the tunnel frequencies is studied. The correlation time of the ${\mathrm{CH}}_{3}$-- motion is determined from the dipolar spin-lattice relaxation. It is then used to evaluate theoretically the observed time constants of the Zeeman relaxation.
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