We first explored the possible mechanism of water-catalyzed tautomerization on 1H,H as a model compound (see Fig. ). In this and all the water-catalyzed reactions considered in this study, the reaction path was followed in the exergonic direction as was previously studied. Table presents the reaction and activation energies for the six different reaction pathways for transforming P(III) → P(V) and Figure depicts their corresponding transition state (TS) structures. In the absence of water, the intramolecular (1intra) direct proton transfer (PT) and the intermolecular (1inter) dimer-assisted PT are unfavorable for the reaction to occur at RT due to their high activation free energy barriers (∆G ‡ ) of 64.3 and 31.5 kcal mol -1 , respectively, with the latter path resulting in an endothermic reaction.