Proton spin-lattice relaxation studies of reorienting methyl groups in solids

Abstract

Spin-lattice relaxation times (T₁) have been measured at 30 MHz as a function of temperature for the methyl protons in a series of compounds, each of which contains a single methyl group. In all cases except one, the measurements were carried out above 77°K, where thermally activated motion predominates. The purpose was to determine activation energies for methyl rotation and to detect phase transitions. The following activation energies were found for polycrystalline samples from the slopes of plots of ln T₁ vs T⁻¹ on the low-temperature side of the T₁ minimum: CH₃PCl₂ (2.25 kcal/mole), CH₃PSCl₂ (2.35), CH₃SCN (2.4), CH₃CD₂I (3.2), and CH₃CD₂Br (3.6). For CH₃COCl and CH₃NCS, which apparently form glasses, only the high-temperature side of the T₁ minimum could be observed. The apparent activation energies were 1.45 and 0.57 kcal/mole, respectively. Significantly nonexponential behavior was observed near and to the high-temperature side of the T₁ minima. The relationship of these activation energies to internal rotation barriers obtained from ir, microwave, and cw NMR absorption studies is discussed.