Molecular Energy States and the Thermodynamic Properties of Solid Isotopic Methanes

The heat capacities of the partially deuterated methanes, CH3D, CH2D2, and CHD3, have been measured between 2.5° and 27°K. In addition to the known transitions (two in each methane in the region 16°<T<26°K), each substance displays a large heat-capacity anomaly at lower temperatures (T<8°K). Time effects were only observed with CH3D and it is suggested that these were caused by CH4 impurity in this methane. None of the methanes was isotopically pure; the low-temperature properties of the ``pure'' methanes were estimated by assuming the heat capacities of the different isotopic species to be additive. On this basis, and neglecting the anomalies at the lowest temperatures, the residual entropies (i.e., the differences between the practical and the calorimetric entropies) are found to be very close to R ln4 for CH3D and CHD3 and to R ln6 for CH2D2. These results are to be compared with the values ∼0 and −0.58 cal/mole·deg for CD4 and CH4, respectively. Results of magnetic resonance and spectroscopic experiments and information derivable from the thermodynamic properties of CH4 and CD4 at higher temperatures all indicate that intermolecular coupling in the methane lattices is relatively weak. For this reason, the properties of the solid methanes at the lowest temperatures are interpreted as resulting from separate contributions of lattice vibrational and perturbed molecular (rotational) energy levels. It is concluded that the residual entropies and the heat-capacity anomalies are related and that their origin is in the molecular energy states of the systems. We make a quantitative analysis of the thermodynamic data for all of the isotopic methanes, taking explicit account of the degeneracies of the different nuclear-spin species. The results are shown to be consistent with the known molecular symmetries and two particularly simple crystal site symmetries.

Перевод пока недоступен