Predicting the vibrational spectra of some simple fluorocarbons by direct scaling of primitive valence force constants

Abstract

Using the scaled quantum-mechanical (SQM) force field approach with direct scaling of individual primitive force constants, we derive optimal scaling factors by a least-squares fit to the experimentally observed fundamentals of some selected “simple” fluorocarbons. We use the derived scaling factors to predict the vibrational spectra of all possible fluoromethanes, fluoroethylenes, fluoroethanes, and monofluoropropenes, proposing a reassignment of some experimental fundamentals. Two separate sets of scaling factors are derived for both traditional Hartree–Fock (HF) calculations and density functional theory (DFT) calculations using the hybrid three-parameter B3-PW91 density functional. With the split-valence 6-31G(d) basis set, our scaling procedure gives an average error of less than 9 cm−1 in the scaled frequencies with the B3-PW91 functional. The average percentage error is around 1%. The HF results are not as good—the average error is 12.6 cm−1—showing that hybrid density functional SQM schemes are better for predicting vibrational spectra than basic Hartree–Fock. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1187–1204, 1998