Effects of anharmonicity of molecular vibrations on the diffraction of electrons : Part V. Tetrahedral molecules

Abstract

Unless the skewing of radial distribution peaks is properly taken into account, diffraction analyses of gas molecules may lead to geometrically inconsistent arrays of internuclear distances. It is shown how to derive the required nonbonded skew parameter a for a tetrahedral AX4 molecule from its potential constants in the case of an ensemble distributed among too many vibrational states to make the standard perturbation or variation methods feasible. Elements of the transformation tensor relating curvilinear coordinates Si to rectilinear coordinates Si are found and applied in a treatment using the "effective potential energy" approach of Miller. Illustrative calculations are carried out for hot CF4 and SiF4 molecules. It is verified that the contributions to a from nonlinear transformations and from Morse stretching deduced by a simple model in Part III of this series are in reasonably good agreement with the more rigorous contributions calculated in this research. It is shown, however, that certain other contributions are larger.