Thermodynamic Properties of CH4 and CD4. Interpretation of the Properties of the Solids

Abstract

The paper gives the results of calorimetric measurements of the heat capacities of solid and liquid CH4 and CD4, the heats of fusion of both substances and the heat of vaporization of CD4. The results for the solids, which extend from 2.3°K to the triple points, are analyzed in some detail. In order to account for the apparent zero-point entropies of both CH4 and CD4 quantitatively, it is necessary to recognize the existence of different nuclear spin species (as with ortho and para hydrogen) and to say that, as T approaches 0°K, each species tends to occupy its lowest available molecular energy level. No conversion between the different species has been observed. The relevance of these findings to the usual applications of the third law of thermodynamics is discussed. The thermal transitions which occur in solid CH4 and CD4 (as well as in the partially deuterated methanes) have also been examined and an attempt made to correlate the thermodynamic results with information derived from other experiments (e.g., spectroscopic, neutron scattering) and from theory. While correlations are possible, the fundamental causes of the transitions can still not be established.