Cooperativity Assisted Shortening of Hydrogen Bonds in Crystalline Oxalic Acid Dihydrate: DFT and NBO Model Studies

The distance of ∼2.49 Å separating the carboxylic OH oxygen from the water oxygen atom in the α-polymorph of crystalline oxalic acid dihydrate is by ∼0.1 Å shorter than the average distance in carboxylic acid monohydrates. It is also by ∼0.2 Å shorter than the corresponding distance presently calculated for the heterotrimer consisting of one acid and two water molecules. The large difference between RO···O in the heterotrimer and in the crystal is attributed to the cooperative effect in the latter; this is supported by calculations carried out on clusters constituted of an increasing number of acid and water molecules. The present DFT calculations with geometry optimization include seven isolated model clusters, the largest of which contains five acid and eight water molecules. The RO···O of the short hydrogen bond shortens progressively with increasing the number of cluster constituents; in the largest cluster, it reaches 2.50 Å. This is remarkably close to both the experimental distance as well as to the distance obtained by the periodic DFT calculation. The electronic effects were studied by Natural Bond Orbital analysis, revealing an enhancement of hydrogen bonding on extending the network by increased polarization of the carbonyl group and by the increased delocalization interaction between the lone electron pair on the acceptor oxygen atom and the OH antibond orbital. The formation of circular motifs appears to be the most important factor in the cooperative shortening of the hydrogen bonds. In agreement with the measured hydrogen bond distances, inspection of the electron density reveals a notable difference in hydrogen bond shrinking tendency between the two known polymorphs of the title system.