Infrared, Raman and electronic spectra of alanine: A comparison with ab intio calculation

Abstract

The infrared, Raman and electronic spectra of alanine molecule have been studied in solid as well as in aqueous solution. The vibrational frequencies for the fundamental modes and the energies of low lying electronic states of alanine in neutral and its zwitterionic form have been calculated using AM1, RHF and DFT method with different basis sets. RHF/6-31G, DFT/6-31G, 6-31+G* and 6-311++G** calculations for vibrational frequencies of both l and d-alanine and zwitterionic alanine(zala) have been performed in both gas phase and in aqueous solution. It is concluded that while there is no significant difference between the corresponding frequencies of l- and d-alanine in gas phase but frequencies are changed for zala and alanine in water. A solvation model(PCM) for neutral alanine and zala at DFT/6-31+G* and 6-311++G** level has also been performed. Gas phase and solvation (PCM) model calculations for alanine and zala reveal that neutral alanine is more stable in gas phase while the reverse is true in aqueous medium. A comparison between the experimentally observed IR spectra of alanine in solid and water solution does not show much variation in corresponding frequencies but theoretically some changes are predicted. The rotational constants and dipole moments have also been calculated.