Transferable quantum mechanical scale factors for alanine

Abstract

We have obtained ab initio scale factors for alanine · 1H 2O supermolecules in acidic and basic solutions using the 4-31G, 4-31G ∗, 6-31G ∗∗ and 6-31+G ∗∗ basis sets. After full optimization of each supermolecule, but prior to transformation of its Cartesian coordinates and force constants to internal coordinate representations, we have deleted the Cartesian coordinates and force constants for each water molecule, leaving only the coordinates and force constants for alanine. This preserves the effect of water on the electronic structure of alanine while avoiding artifacts in normal mode calculations due to vibrational coupling with statically bound water. All but two of the 4-31G scale factors show transferability from smaller molecules. Most scale factors are also transferable between pH 1 and pH 13 for the coordinates unchanged by protonation. Force constants are scaled using the experimentally determined Raman and FTIR vibrational frequencies of eight isotopic species of alaninate in water at pH 13 and seven isotopic species of alanine at pH 1. Good agreement is obtained between the calculated and experimental frequencies for the isotopic species for all basis sets and structures. A Fermi resonance analysis of the Raman spectrum of alaninate in the region from 500 to 900 cm −1 suggests an explanation for an unassigned band at 590 cm −1 in the unsubstituted molecule.