Vibrational properties of glyoxilic acid oxime dimers and tetramer - Ab initio and DFT study

Vibrational study of the lowest energy monomer, three dimers and tetramer of glyoxilic acid oxime (GAO) has been performed at HF/6-31G*, B3LYP/6-31G* and B3LYP/6-31++G** levels of the theory. O-H...N and O-H...O intermolecular hydrogen bondings in the cyclic tetramer have been evaluated from intermolecular distances, harmonic frequency shifts and hydrogen-bonding energies. The results at all levels showed that NOH and COOH vibrational modes changed significantly going from monomer to tetramer unit. The O-H redshifts (Deltav(O-H)NOH and Deltav(O-H)COOH) suggested relatively strong hydrogen O-H...OHO and O-H...N bondings and provided evidence for substantial cooperative contributions to the stabilization of cyclic tetramer. The vibrational analysis showed that in the cyclic dimer and tetramer, where NOH and COOH groups act both as a proton donor and as a proton acceptor, the O-H redshifts are complex perturbed and should be used with caution for the estimation of the O-H...O and O-H...N bond strengths. (C) 2002 Elsevier Science B.V. All rights reserved.