Effects of hydration on scale factors for ab initio force constants II

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 non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To achieve this goal, it has been necessary to obtain scale factors for smaller hydrated molecules that can be used as a starting point for calculations on peptides. To this end, we have calculated scale factors for ab initio force constants for methylamine and protonated methylamine using a least-squares fit of calculated and experimental frequencies. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. We compare the scale factors derived from our measurements 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, 4-31G**, 6-31G+, and 6–311G ∗∗ basis sets. The results here are similar in consistency with those found in our previous calculations on formic acid, acetic acid and acetone. The differences in scale factors between vapor and aqueous phase molecules are smaller than those previously found for polar groups, indicating that methylamine exhibits relatively non-polar behavior in solution with respect to its vibrational spectrum.