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

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.