A CHEMOMETRIC ANALYSIS OF AB-INITIO VIBRATIONAL FREQUENCIES AND INFRARED INTENSITIES OF METHYL-FLUORIDE

Factorial design and principal component analyses are applied to CH3F infrared frequencies and intensities calculated from ab initio wave fu nctions. In the factorial analysis, the quantitative effects of changi ng from a 6-31G to a 6-311G basis, of including polarization and diffu se orbitals, and of correcting for electron correlation using the seco nd-order Moller-Plesset procedure are determined for all frequencies a nd intensities. The most significant main effect observed for the freq uencies corresponds to the shift from Hartree-Fock to MP2 calculations , which tends to lower all frequency values by approximately 100 cm(-1 ). For the intensities, the main effects are larger for the CF stretch ing and the CH3 asymmetric stretching modes. Interaction effects betwe en two or more of the four factors are found to be of minor importance , except for the interaction between correlation and polarization. The principal component analysis indicates that wave functions with polar ization and diffuse orbitals at the second-order Moller-Plesset level provide the best estimates for the harmonic frequencies, but not for t he intensities. For the frequencies, the first principal component dis tinguishes between MP2 and Hartree-Fock calculations, while the second component separates the wave functions with polarization orbitals fro m those without these orbitals. For the intensities, the separation is similar but less well defined. This analysis also shows that wave fun ction optimization to calculate accurate intensities is more difficult than an optimization for frequencies. (C) 1996 by John Wiley & Sons, Inc.