Theoretical and spectroscopic studies of vibrational spectra of hydrogen bonds in molecular crystal of β-oxalic acid

A theoretical model is proposed for the crystal of beta-oxalic acid, describing vibrational couplings between high- and low-frequency stretching modes in hydrogen bonds, resonance interactions between different hydrogen bonds in the unit cell, and Fermi resonance between the fundamental O-H(D) stretching and the overtone of the O-H(D) in-plane bending vibrations. The model is used for theoretical simulation of the O-H stretching bands of beta-oxalic acid and its O-D derivative at 300 K. The reproduction of the experimental bands is satisfactory. The presented model correctly fits the experimental line shape and fine structure. Also the effect of deuteration is well reproduced by this model. These results demonstrate that the proposed mechanisms are the principal ones for determining the dynamic interactions in H-bonded systems. Infrared spectra of beta-oxalic acid and its deuterated derivative have been measured. Experimental geometry and frequencies are compared with the results of DFT and ab initio calculations performed at the B3LYP/6-31++G**, B3LYP/cc-pVTZ, B3PW91/6-31++G**, B3PW91/cc-pVTZ and MP2/6-31++G** levels. The calculated geometries and frequencies agree well with the experimental data. (C) 2009 Elsevier B.V. All rights reserved.