Proton magnetic resonance absorption and relaxation of dimethylamine in the condensed and clathrate deuterate phases

Abstract

Proton magnetic resonance second moment and spin-lattice relaxation data are reported for polycrystalline solid dimethylamine and dimethylamine clathrate deuterate from 77 K up to the melting points of the respective compounds. The second moment results for solid dimethylamine indicate a hindered rotation of the methyl groups around their threefold axes, while the proton T1 data show a broad distribution of reorientational correlation times among the methyl groups. A quantitative interpretation of this distribution based on the Davidson-Cole distribution function is given, and a reorientational energy barrier of 2.70±0.20 kcal mole−1 is obtained from the relaxation data using a Davidson-Cole distribution parameter δ=0.8. In the dimethylamine clathrate deuterate, a pseudothreefold axis (c-axis) reorientation of dimethylamine is observed at higher temperatures. Proton spin-lattice relaxation in this case is nonexponential over most of the temperature range studied, and an analysis of the ``null'' time (t0) data in terms of the Hilt-Hubbard theory leads to an activation energy barrier of 2.26 ± 0.02 kcal mole−1 for reorientation around the c axis. Measurements of the rotating frame spin-lattice relaxation (T1ρ) in the temperature region above the t0 minimum for the clathrate disclose the onset of slow tumbling and lattice diffusion processes, governed by an activation energy of 14.0 ±0.6 kcal mole−1.