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.

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The infrared spectrum of sulfur trioxide was obtained under prism resolution in the gaseous state and in xenon matrices at liquid-nitrogen temperature. In both cases a satisfactory vibrational assignment of the monomeric SO3 spectrum could be made on the basis of a simple D3h symmetry model. The observed gas phase fundamentals were v3(e′) = 1391, v4(e′) = 529, and v2(a2″) = 495 cm—1. The matrix fundamentals were essentially the same as those of the gas phase except that the v2(a2″) mode was shifted to 464 cm—1. A normal coordinate treatment of the SO3 molecule was carried out and potential constants were determined for both Urey-Bradley and simplified valence force fields. Three weak absorption bands of S3O9 were also detected in the gas-phase spectrum. From the temperature and pressure variation of the intensities of these bands it was estimated that for the trimerization reaction at 298°K, Kp was approximately 1 atm—2 and ΔH298° was approximately 30 kcal/mole of S3O9. The values of the thermodynamic properties of monomeric SO3 were computed for the ideal gaseous state using the rigid rotor harmonic oscillator approximation at 1 atm from 100° to 1500°K.