ELECTRONEGATIVITY MODELS FOR THE INFRARED VIBRATIONAL INTENSITIES OF THE HALOMETHANES

Ab initio molecular orbital calculations and empirical electronegativi ty models are used to understand the linear electronegativity relation ships observed for the carbon mean dipole moment derivatives and atomi c effective charges calculated from the experimental infrared vibratio nal intensities of the halomethanes. The charge-charge flux-overlap in terpretation of the molecular orbital results shows that only the char ge contribution is important in explaining the variations in these par ameters for the fluoromethanes. For this reason a simple electrostatic model is sufficient to explain their fundamental infrared intensity s ums. The mean dipole moment derivative values determined from the expe rimental intensities suggest the absence of a saturation effect on the ability of substituted fluorine atoms to drain electron density from the carbon atoms. A similar model has been used by others to explain t he increasing thermodynamic stabilities of the fluoromethanes with inc reasing fluorine substitution. In contrast intramolecular charge trans fer is predominant in determining the chloromethane intensities. The f luorochloromethane intensities can only be explained using models comb ining characteristics of the fluoro- and chloromethane models. The cha rge equilibration procedure introduced recently in the literature is f ound to be significantly superior to the simpler electronegativity equ alization method for calculating atomic charges for the prediction of the infrared intensity sums of the halomethanes.