Density functional theory study of the origin of IR and Raman band shifts in H-bond complexes of triethylamine with water

The present article discusses the results of density functional calculations obtained using B3LYP functional and the standard 6-31G* basis sets for mono-, di-, and triethylamines (MEA, DEA, and TEA, respectively) and their H-bonded complexes with water molecules. In particular, the origin of the observed experimentally clear red shift for methyeène v_{C - H} mode due to the nanosecond laser T-jump in TEA/H₂O system is validated theoretically by explicit consideration of related infrared (IR) and Raman peaks appeared at the C - H absorbance region. All theoretical calculations were performed with full geometry optimizations without imposing any constraints, and followed by direct harmonic frequency calculations. The effects of nonspecific and specific solvations were accounted for using simple Onsager model and explicit water molecules. It was shown that the former continuum model does not strongly perturb the observed spectral changes in the C - H region because of shallow potential energy surface for the interacting subsystems. Both the structural changes in geometry and electron redistribution as a result of H-bonding were found to be main factors to explain the observed experimental spectral changes in IR and Raman spectra.