The Effect of Rotational Distortion on the Thermodynamic Properties of Water and Other Polyatomic Molecules

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Abstract
The classical mechanical expression for the energy of a non‐vibrating, rotating polyatomic molecule is obtained which includes a correction term arising from the centrifugal distortion of the molecule caused by the rotation. Since the rotational partition function closely approaches the classical phase integral for room temperatures or higher, the latter is used to calculate the effect of centrifugal distortion on the entropy, heat capacity and free energy. The correction terms —ρRT2, 2ρRT and 2ρRT must be added to F, S and Cv, respectively. ρ is a constant characteristic of each molecule. The values of ρ calculated are: H2O, 2.04 10—5; H2S, 1.62 10—5; NH3, 1.45 10—5; CH4, 1.72 10—5; C2H4, 0.79 × 10—5. For water this correction amounts to approximately ½ percent of the heat capacity at the boiling point, or +0.032 entropy unit.