Effect of microhydration on dissociation of trifluoroacetic acid.

First-principle-based electronic structure calculations were carried out on microhydrated trifluoroacetic acid clusters (CF3COOH, tfa) to understand its molecular level interaction with water and subsequent ionic dissociation to form CF3COO(-) ion. From several geometrical inputs, the global minimum energy structure of hydrated cluster, tfa · nH2O (n = 1-7), was obtained adopting dispersion-corrected density functional, namely, ωB97X-D, and a set of correlated atomic basis function, aug-cc-pVDZ. It was predicted that tfa requires at least six H2O molecules to dissociate. Energy parameters of these hydrated clusters were improved by applying MP2 as well as CCSD(T) methods. A linear variation was observed for calculated solvent stabilization energy profile with the number of solvent H2O molecules present in the hydrated cluster. However, the calculated interaction energy profile showed the characteristic feature indicating the formation of contact ion-pair on the addition of six H2O molecules to tfa. On the basis of energy decomposition analysis, it was observed that the major interaction between tfa and H2O molecules was of electrostatic nature. On successive addition of water molecules, the electrostatic component of the interaction between solute and solvent molecules depicted a sudden increase when moving from penta- to hexahydrated cluster. This observed nature of energy profile coincided with the formation of hydronium ion in the case of hexahydrated cluster. The formation of H3O(+) was manifested in simulated IR spectra of tfa·6H2O and tfa · 7H2O clusters. A large red shift in IR peak positions corresponding to O-H stretching of tfa was predicted on microhydration.