Molecular Reorientation and Nuclear-Spin Conversion in the Solid Deuteromethanes at Low Temperatures

Abstract

The heat capacities of the solid deuteromethanes have been measured down to 0.3°K, tracing out the low‐temperature heat‐capacity anomalies which had been detected in the earlier work of Colwell, Gill, and Morrison. The anomalies in CH₃D and CHD₃ have maxima at 0.43 and 0.48°K, respectively, but the CH₂D₂ heat capacity is still increasing with decreasing temperature at 0.3°K. A small anomaly was found in CD₄ but this is ascribed to chemical and isotopic impurities present in the sample. The heatcapacity anomalies are of the Schottky form and are believed to arise from the reorientation of the molecules on the lattice sites. This results in the removal of the orientational degeneracies of the molecules which give rise to the zero‐point entropies of $R$ ln4 for CH₃D and CHD₃ and $R$ ln6 for CH₂D₂. In molecules having identical protons, conversion between the different nuclear‐spin states also occurs. This is substantiated in the case of CH₃D where the measured entropy change in the range of the measurements, 0.3–4.0°K, is $\text{ΔS\hspace{0.17em}=\hspace{0.17em}1.614R}$ , which exceeds the maximum value of $R$ ln4 (1.386R) for molecular reorientation occurring separately. The occurrence of conversion was also indicated by a thermal relaxation process characterized by relaxation times ranging from 110 to 50 sec in going from 0.3 to 4.0°K. Analysis of the CH₂D₂ data indicates that nuclear‐spin conversion is also taking place in this material but the result is less well established. There is no indication of conversion occurring in CHD₃. Reasonably precise values of the energy‐level splittings have been derived for each of the partially deuterated methanes.