ELECTRONEGATIVITY MODELS FOR THE INFRARED VIBRATIONAL INTENSITIES OF THE HALOMETHANES

Ab initio molecular orbital calculations and empirical electronegativity models are used to understand the linear electronegativity relationships observed for the carbon mean dipole moment derivatives and atomic effective charges calculated from the experimental infrared vibrational intensities of the halomethanes. The charge-charge flux-overlap interpretation of the molecular orbital results shows that only the charge contribution is important in explaining the variations in these parameters for the fluoromethanes. For this reason a simple electrostatic model is sufficient to explain their fundamental infrared intensity sums. The mean dipole moment derivative values determined from the experimental 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 the increasing thermodynamic stabilities of the fluoromethanes with increasing fluorine substitution. In contrast intramolecular charge transfer is predominant in determining the chloromethane intensities. The fluorochloromethane intensities can only be explained using models combining characteristics of the fluoro- and chloromethane models. The charge equilibration procedure introduced recently in the literature is found to be significantly superior to the simpler electronegativity equalization method for calculating atomic charges for the prediction of the infrared intensity sums of the halomethanes.