Effects of wave function modifications on calculated C-F and C-Cl vibrational frequencies and infrared intensities of the dihaloethylenes

The C-F and C-Cl calculated stretching frequencies are shown to obey different factorial models. On average, Moller-Plesset 2 treatment lowers the C-F stretching frequencies by 100 cm(-1) whereas it only lowers the C-Cl ones by 37 cm(-1). Diffuse functions have negligible effects on these C-Cl frequencies but lower the C-F stretching values by 24 cm(-1). However, inclusion of polarization functions in the basis set increases both the C-F and C-Cl stretching frequencies by 90 cm(-1). The use of Moller-Plesset 2 treatment also lowers the bending frequencies and the inclusion of polarization functions increases them, although these effects are smaller than those for the stretching frequencies. The stretching and bending made fundamental intensities of these dihaloethylenes all follow very different factorial models. Low dimensional principal component projections are shown to provide accurate representations of the agreement of the calculated frequencies and intensities with the experimental values. Wave function modifications leading to more accurate characteristic frequency values appear to be relatively insensitive to the vibration's symmetry or the molecular environment of the characteristic CF or CCI groups. (C) 1997 Elsevier Science B.V.