Calculation of Vibrational Relaxation Times of the Chloromethanes

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Abstract
The Schwartz, Slawsky, and Herzfeld method for the calculation of vibrational relaxation times of gases has been extended in its application to polyatomic molecules. A general method is developed for the calculation of the effective relaxation times and the associated constants of the sound velocity relaxation equation. Calculations have been made for the bimolecular collisions of methane and the chloromethanes involving changes of one, two, and three quanta. A normal coordinate treatment is used to obtain the atomic vibrational amplitudes. The effect of an intermolecular potential function, embodying a dipole interaction term, is included for the polar molecules of this investigation. Calculated results are compared with experimental data. For CH4 and CH3Cl the calculations agree well with experimental results. For CH2Cl2, CHCl3, and CCl4 the calculated relaxation times are higher than selected experimental values by factors of 8–9. For CH2Cl2, CHCl3, and CCl4 two major relaxation times are calculated. In the case of CH2Cl2 the two relaxation areas have been observed. The experimental data on CHCl3 and CCl4 remain inconclusive in respect to the two predicted relaxation areas. Collisions involving three quanta are shown to be important for CH2Cl2, CHCl3, and CCl4.

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