POTENTIAL ENERGY CONSTANTS FOR SOME HALOMETHANES FROM MOLECULAR PARAMETERS

Abstract

A theory is presented for the calculation of the potential energy constants (f’s) of some halomethane molecules. The potential energy function is assumed to be that of a harmonic oscillator with interactions between pairs of internal displacement coordinates. The f for the stretching of a bond is assumed to be a linear function of the charge density. It is assumed that a small amount of charge may be transferred between bonds. The shape of the bonds is assumed to be that of a prolate spheroid. The total amount of charge for bonding present in a molecule is assumed to be constant. An angle deformation is viewed as the bending of two bonds. The f for the bending of a bond is assumed to be a fraction of the f for the stretching of that bond. The interaction f’s between the different coordinates are assumed to arise from changes in the charge densities of the bonds involved. The f’s are used in a Wilson FG matrix normal coordinate treatment. A set of computer programs facilitated the investigation of the values of the empirical constants introduced in the theory. One program calculated the f’s which reproduced the experimental spectrum by use of an iteration procedure. The molecules investigated were $CH_{2}F_{2}$, $CH_{2}Cl_{2}$, $CH_{2}Br_{2}$, $CH_{2}I_{2}$ and $CH_{2}ClBr$. The average difference between the f’s from theory and those from experimental data for the stretching f’s is ${\pm}\ 0.0932\ mdyn/${\AA}, that for the bending f’s is ${\pm}\ 0.162\ mdyn{\AA}/rad^{2}$, that for the stretching-stretching interaction f’s is ${\pm}\ 0.027\ mdyn/${\AA}, that for the stretching-bending interaction f’s is ${\pm}\ 0.019\ mdyn/rad$, and that for the bending-bending interaction f’s is ${\pm}\ 0.038\ mdyn${\AA}$/rad_{2}$. Calculations are continuing.