CNDO Calculation of Dipole Moment Derivatives and Infrared Intensities of C6H6 and C6F6

Abstract

The dipole moment derivatives of C6H6 and C6F6 have been calculated using the complete neglect of differential overlap (CNDO) approximate molecular orbital theory. The calculated set of derivatives with respect to the symmetry coordinates (∂ p/∂ Sj) for the e1u species of C6H6 agrees remarkably well with the unique experimental set of derivatives determined from infrared intensity measurements and the normal coordinate analysis for C6H6 and its deuterated analogues. The success of the benzene calculation suggests that we may have confidence in the CNDO results for C6F6, even though the agreement with the experimental results is much worse. An analysis of the ∂ p/∂ Sj values is given in terms of bond moment derivatives. The failure of the CNDO theory to predict a sufficiently large ``rehybridization moment'' to explain the observed intensity difference between in-plane and out-of-plane bending vibrations is discussed.