Computational study of the dissociation of oxalic acid in water clusters

The ionic dissociation of the oxalic acid in water was examined by performing a theoretical study of the different properties of clusters formed by the acid with up to five water molecules. Geometry optimization and frequency calculations were done at density functional theory level with the B3LYP functional using the 6-31+G* basis set. The influence of water in the acid dissociation was characterized by analyzing several cluster properties including energies, stretching frequencies and lengths of the O-H bonds involved in the hydrogen bond lattice. The relative stability of the different conformers for the oxalic acid is altered by the formation of clusters with water. Thus, the most stable conformation of oxalic acid becomes the second most stable for clusters with two water molecules; moreover, this conformer loses stability in relation to other configurations as the size of the cluster increases. The addition of water molecules to the cluster lengthens the O-H bond involved in the hydrogen bonds and simultaneously decreases the O...H intermolecular distances. This process results in the formation of ion-pairs for a cluster with five water molecules. The O...H bond stretching frequency undergoes a marked redshift as a consequence of the weakening of the O-H bond. However, the drop in the size of the frequency is gradual and no abrupt gap appears. (C) 2004 Elsevier B.V. All rights reserved.