The gas-phase acidity and intramolecular hydrogen bonding in oxalic acid

Ab inito calculations at MP2/6-311++G** and G2 level were carried out to study the structures and the gas-phase acidities of different conformations in oxalic acid. The gas-phase acidities (free energies of first ionization reactions) at G2 level vary from 1297.27 kJ/mol for the most acidic conformer to 1359.53 kJ/mol for the least acidic one at 298.15 K. Analyzing intramolecular hydrogen bonding in different conformers, shows that the gas-phase acidity has a direct relation with hydrogen bond stabilization of both acid and its conjugated base. The results indicate that the reaction in which the acidic hydrogen is less involved and the hydrogen of its conjugated base is more involved in the intramolecular hydrogen bond is the easiest deprotonation reaction. The atoms in molecule (AIM) theory has been also used to obtain the electronic charge distribution and some other related properties. (C) 2004 Elsevier B.V. All rights reserved.