TOWARD AN UNDERSTANDING OF THE HYDROLYSIS OF MG-PPI - AN AB-INITIO STUDY OF THE ISOMERIZATION-REACTIONS OF NEUTRAL AND ANIONIC MG-PYROPHOSPHATE COMPLEXES

Ab initio calculations were performed to study the stability of variou s pyrophosphate species in the gas phase: H4P2O7, H3P2O7-, H2P2O72-, H P2O73-, P2O74-, and their complexes with Mg2+. It is found that the me tal cation allows the existence of highly charged anions in the gas ph ase. We also study the isomerization reactions Mg . H2P2O7 --> (H2PO4 . Mg . PO3), (Mg . HP2O7)(-) --> (HPO4 . Mg . PO3)(-), and (Mg . P2O7) (2-) --> (PO4 . Mg . PO3)(2-), at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G* basis set with diffuse and polarization functions. Other basis sets, including one of valence triple zeta plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is foun d that the same mechanism occurs for the isomerizations of the three s pecies: one of the P-O bridging bonds of the reactant is longer than t he other, and the route to the products proceeds through its elongatio n. This asymmetry is induced by the metal cation in the case of the ev enly charged anions. In all cases the metal cation coordinates the tra nsition states and the leaving groups. The structures found for the co mplexes (H2PO4 . Mg . PO3), HPO4 . Mg . PO3)(-), and (PO4 . Mg . PO3)( 2-) are different from those reported previously, the metal cation bei ng enclosed by the two phosphates. The activation barrier increases wi th the charge of the anion, from Delta G degrees(double dagger)=5.6 kc al/mol for the neutral complex Mg . H2P2O7, to Delta G degrees(double dagger)=10.4 kcal/mol for the monoanion (Mg . HP2O7)(-), to Delta G de grees(double dagger)=13.5 kcal/mol for the dianion (Mg . P2O7)(2-). Th e positive value found for the energy of the isomerization (Mg . P2O7) (2-) --> (PO4 . Mg . PO3)(2-), Delta G degrees(double dagger)=1.8 kcal /mol, predicts the synthesis to be spontaneous in the gas phase, oppos ite of what occurs in the aqueous solution. This result supports the v iew that the hydration energy makes a large contribution to the energy of hydrolysis. The gas-phase hydrolysis reaction H2O + Mg2+ + H2P2O72 - --> Mg2+ + H2PO4- + H2PO4- is also studied as a multistep reaction, involving the isomerization of H2O + (Mg . H2P2O7) --> H2O + (PO3 . Mg . H2PO4) as an intermediate step. It is found that the equilibrium in the gas phase yields H2PO4 . Mg . H2PO4 as the final species; an ener gy input is required for separating the metal cation from the phosphat e anions.