Effects of Hydration on Scale Factors for ab Initio Force Constants. 9. Methanol

Abstract

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for nonpolar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFT that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.

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