Experimental and theoretical investigation on the vibrational spectroscopy of l-theanine

In this work, experimental and theoretical investigations on vibrational spectroscopy of l-theanine were presented. FT-IR and Raman spectra of l-theanine powder sample were recorded and corresponding theoretical calculations were performed based on Density Functional Theory (DFT) at B3LYP level using 6-31++G(d,p) and 6-311++G(d,p) basis sets combined with the Polarized Continuum Model (PCM) with water as the solvent. The experimental vibrational bands were assigned based on the basis of calculations while the predicted geometric parameters were compared with those obtained in experiment, most of the bands measured were well reproduced in the calculations while the discrepancies are significant for the bands mainly related to the vibrations of protonated amino group ( NH 3 + ) and ionized carboxyl group (COO−), which are affected by the intramolecular hydrogen bond interaction. Good agreements between the theoretical and experimental results confirm the feasibility of the DFT method combined with PCM in the study of the molecular structure and vibrational spectra of l-theanine.