The Entropy of Polyatomic Molecules

Abstract

By counting the total number of completely antisymmetric eigenfunctions which can be formed from the rotational states, the a priori probabilities of the different symmetry varieties of ammonia (nuclear doublet and quadruplet) and of methane (nuclear singlet, triplet and quintet) are derived and used to compute the corresponding entropies at 298.1°K. The relative proportions of the different varieties at high temperatures are found to be equal for ammonia and in the ratio of 2:9:5 for methane (respectively). The absolute entropy of the mixture of the different forms of ammonia is calculated to be 51.5 cal. ${\mathrm{degree}}^{-1\mathrm{}}$, the virtual entropy (the absolute minus the nuclear spin minus the frozen rotational entropy) 44.1; the absolute entropy of methane, 50.1, and its virtual entropy 44.2 cal. ${\mathrm{degree}}^{-1\mathrm{}}$