EFFECTS OF ANHARMONICITY OF MOLECULAR VIBRATIONS ON THE IXFFRACTION OF ELECTRONS Part III. Predictive models

It has long been known how to relate anharmonic vibrational distribution functions to scattered electron intensities when deriving molecular parameters from gas-phase electron diffraction patterns. What bas been lacking is a convenient procedure for estimating the characteristic asymmetry parameters of radial distribution functions for polyatomic molecules, particularly in the case of non-bonded internuclear distances _ In the present work alternative models of bond distribution functions are discussed briefly and a plausible model is proposed for geminal non-bonded distances. Numerical examples are worked out for the groundstates of CO,, CS,, H,O, and D,O. Variations of the asymmetry parameters of SO, and SF, with temperature are examined. It is shown that the effect of asymmetry can become quite large at elevated temperatures. Molecular vibrations complicate determinations of molecular structure, whether spectroscopic or diffraction, and substantially interfere with the accuracy of determining physically meaningful structural parameters. Many years ago [l] , preliminary attempts were made to treat the effects o