The VA and VCD spectra of various isotopomers of L-alanine in aqueous solution

Density functional theory (DFT) at the Becke 3LYP level has been used to calculate the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of various deuterated species Of L-alanine. The effect of replacing the methine hydrogen, CH1, the methyl group, CH3, and both the methine and methyl group hydrogens, CH1 and CH3, with the deuteriated species Cd1, Cd3 and Cd1 and Cd3 is investigated, both in H2O and D2O. The predicted results are then compared to the corresponding experimental spectra. Comparison of the experimental and calculated frequency shifts and intensity changes in the VA and VCD spectra are used as a check of the Becke 3LYP hybrid model to successfully reproduce these fine features in the VA and VCD intensities. Furthermore, several aspects of the isotopomers and the isotope effects on vibrational spectra are discussed in the light of the approximation, i.e., validity of Born-Oppenheimer approximation. (C) 2002 Elsevier Science B.V. All rights reserved.