Thermodynamics of C3. II. General Methods for Nonrigid Molecules at High Temperature

Publisher Website
Google Scholar
Abstract
A classical expression is developed for the partition function of the coupled bending and rotational degrees of freedom of a linear triatomic molecule. The usual rigid‐rotator harmonic‐oscillator partition function can be derived as an approximation to our result. Analytical approximations to both the bending‐rotator and the rigid‐rotator partition functions for a molecule with a quadratic—quartic potential are considered and the accuracy of these approximations is assessed by numerical calculations. Values for the thermodynamic functions for C3 are calculated using the best available spectroscopic data. It is shown that for the bending and rotational degrees of freedom, quantum corrections are negligible at temperatures above a few hundred degrees while anharmonicity and vibration—rotation coupling are quite important. For example, at 2400°K, a quartic term in the potential of C3 lowers the calculated value of S° by 2.8 eu, and the vibration—rotation coupling lowers it by another 1.1 eu to a computed value of 81.4 eu. A drastic steepening of the potential gives a lower limit, more or less consistent with the spectroscopic data, of 79.8 eu. The best experimental values for S°2400 are 76 or 77 eu.

This publication has 6 references indexed in Scilit: