Potential Functions of Molecular Groups and the Vibrations of the Halogen Derivatives of Methane

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Abstract
On the assumption of the invariance of the molecular groups YZ3 and YX formulas are given connecting the potential energy constants of the molecules YZ4, XYZ3, ZYX3, and ZX4. The validity of the assumption is tested for the series CH4, ClCH3, HCCl3, and CCl4; the consistency of the results definitely shows it to be a very good approximation. The application leads to a determination of the general potential functions of all four molecules. Frequencies of HCCl3 and DCCl3 are calculated from those of CH4, ClCH3, and CCl4, and are found to be in excellent agreement with the observed data. Predictions are made for the frequencies of ClCD3. A certain ambiguity exists for the constants for the perpendicular frequencies of both HCCl3 and ClCH3, since two different chemically acceptable sets will account for the observed frequencies. A discussion of the methane constants shows that, while individual constants are quite sensitive to various methods of determination, certain linear combinations of them, to which a definite physical meaning can be attached, are practically invariant. The methane constants are used to redetermine the constants computed by Sutherland and Dennison for ethane; they yield an entirely different set, which, however, is equally suited to an explanation of the frequencies.