Ab initio and DFT studies on conformations, hydrogen bonding and electronic structures of glyoxalmonoxime and its methyl derivatives

All the possible carbonyl-nitroso, enol-nitroso and carbonyl-oxime conformations of glyoxalmonoxime and all the rotamers of carbonyloxime tautomer of 1-methyl and 2-methylglyoxalmonoximes have been investigated at ab initio HF and correlated level (Moller-Plesset second order) using 6-31G** basis set. Density functional theory calculations have also been done with Becke's three-parameter hybrid method. The chelated enol-nitroso form is non-existent and converges upon the carbonyl form at both DFT and MP2 calculations. In gas phase, fully linear carbonyl-oxime conformer has been found to be the most favored one over the enol-nitroso and carbonyl-nitro so conformations, unlike the chelated form in the case of malondialdehyde (MDA). The hydrogen bond energy (E-HB) of (OH)-H-...-O framework in the closed hexatomic chelate of carbonyl-oxime is affected by the methyl group at the carbonyl carbon atom at all levels (ca. 8 kJ/mol HF; ca. 5.2 kJ/mol MP2; ca. 5.3 kJ/mol B3LYP). A model compound has been chosen for the quantification of EHB. Even though enol-nitroso form is less stable, its hydrogen bond strength is much higher than in MDA. The investigation in aqueous medium was also carried out by means of PCM-SCRF method. Effect of adding one water molecule to the various GM rotamers has also been studied, incorporating the corrections to the basis set super position error. (C) 2004 Elsevier B.V. All rights reserved.