Ab initio study in the hydration process of metaphosphoric acid: the importance of the pnictogen interactions

A theoretical study of the hydration of metaphosphoric acid to yield phosphoric acid has been carried out by means of MP2/6-31+G(d,p) and MP2/augcc-pVTZ computational levels. Up to three explicit water molecules have been considered as well as the PCM solvation model to account for the effect of the bulk water. The reaction profile has been analyzed using the conceptual DFT methodology. The reactant structure is very dependent on the number of water molecules. The inclusion of more than one water molecule produces important cooperative effects and a shortening of the O center dot center dot center dot P pnictogen interaction besides the reaction barrier drops about 50 kJ mol(-1). Reaction force at xi(1) indicates the decreasing in the angular stress in the reaction site before reaching the TS as more explicit water molecules are taken into account. The analysis of the reaction electronic flux shows that for the three mechanisms studied, the principal reactive changes occur in the TS zone, while reactants and products remain in a zero-flux regime.