Microhydration of oxalic acid leading to dissociation

The effect of microhydration on the simplest dicarboxylic acid, namely oxalic acid, leading to the dissociation of its proton, is studied using first principle-based electronic structure calculations. The geometry of the hydrated clusters of oxalic acid considering up to seven water molecules is determined at omega B97X-D/aug-cc-pVDZ level of theory. Solvent stabilisation and interaction energy parameters are calculated applying CCSD(T) level of theory. The calculated free energy of formation shows that the hydrated oxalic acid clusters are stable only at low temperature and pressure. Though the solvent stabilisation energy increases linearly with an increase in the size of the hydrated cluster, the calculated interaction energy, acidic O-H bond dipole moment and hydrogen bond energy show characteristic features of ion pair formation. The spectral manifestation of the weakening hydroxyl bond is observed as red shift in its stretching frequency. A rigid potential energy scan, altering the dissociating O-H bond length of the oxalic acid molecule, shows an energy barrier for acid to water proton transfer in all cases except hepta-hydrate of oxalic acid, where a barrier-less proton transfer occurs. The number of water molecules (n) needed for dissociation of oxalic acid molecule is consistent with the value obtained from recently reported emperical correlation between n and pKa. [GRAPHICS] .