G sum rule applications for the vibrational intensities of the hydrocarbons

Abstract

G sum rule graphical solutions for the atomic effective charges, ξα, of the hydrocarbons, acetylene, ethylene, ethane, propane, and cyclopropane, are reported and compared with the numerical solutions. The importance of experimental error uncertainties in the effective charge values is emphasized. Polar tensor and effective charge values have been calculated for all the possible sign combinations of the ∂p/∂Qi’s of ethylene and ethane. ξα values calculated from the G sum rule and the polar tensors show excellent agreement indicating that normal coordinate and band separation approximations introduce little error into the effective charge values. Although ξH is relatively constant for all the hydrocarbons, save acetylene, ξC shows large variations with changes in molecular environment. The empirical relation nCξC = nHξH holds surprisingly well for most hydrocarbons, although the effective charges for methane provide a striking exception to this rule.