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

Factorial design and principal component models are analyzed to determine how calculated C-H stretching and bending vibrational frequencies and infrared intensities depend on characteristics of the molecular orbital wave functions of cis- and trans- difluoro- and dichloroethylene. All the calculated C-H stretching frequencies can be described by the same factorial model indicating that their values depend on the wave function modifications in the same way. Application of Moller-Plesset MP2 perturbation treatment lowers these frequencies by 146 cm(-1) whereas the effect of adding more valence orbitals provokes only a 42 cm(-1) decrease. A polarization orbital-electron correlation interaction effect is also important. Considering the results for the sixteen wave functions of the factorial design all the observed C-H stretching frequencies are in best agreement with the MP2/6-311++G results. The calculated C-H stretching intensities all follow separate factorial models indicating different dependencies on the wave function characteristics. Diffuse, polarization and electron correlation effects as well as several binary interaction effects are important. In contrast to the results for the C-H frequencies, best agreements between observed and calculated intensities occur for different wave functions. The calculated in-plane bending frequency values of the difluoro- and dichloroethylenes show somewhat different behavior with wave function modifications and appear to follow separate models. Wave function modifications involving valence, diffuse and polarization orbitals and the use of MP2 treatment result in significant changes in the calculated frequencies. The calculated in-plane bending intensities all follow separate models and all the wave function factors studied here are important. Factorial designs for the out-of-plane C-H bending frequencies and intensities as functions of the wave function model are also described. he calculated C-H stretching frequencies call be adequately described by a single principal component. Bidimensional principal component graphs are sufficiently accurate for quantitative comparison of the results of trial wave functions with the observed results of the vibrational bending frequencies. The calculated intensity values follow more complex principal component models than do the frequencies, reflecting the greater difficulties involved in their theoretical determination. (C) 1997 Elsevier Science B.V.