Ab initio study of the hydrolysis reactions of neutral and anionic Mg-pyrophosphate complexes in the gas phase

Ab initio calculations were performed to study the stability of various anh ydrous and monohydrated complexes of Mg2+ with pyrophosphates and orthophos phates at the self-consistent-field (SCF) and second-order perturbation (MP 2) levels of the theory, using a 6-31+G** basis set with diffuse and polari zation functions. New equilibrium geometries were found for the anhydrous M g-pyrophosphate complexes, that modify previous estimates of the isomerizat ion energies. It is found that the interaction with the water molecule stab ilizes the pyrophosphates with respect to the metaphosphate-containing comp lexes, thus modifying the reaction energies to such an extent that the isom erization results are endothermic and nonspontaneous for the dianionic comp lex. However, the hydrolysis reactions are all exothermic and spontaneous. Moreover, it is found that the water molecule readily breaks upon interacti ng with the dianionic complex, producing a hydroxide anion: H2O + [Mg . P2O 7](2-) --> [HO . Mg . HP2O7](2-), and the resulting dianionic complex is mu ch mon stable than the isomer with a metaphosphate. It is shown that this r esult is consistent with both an associative and a dissociative mechanism f or the hydrolysis. Finally, the estimate of free energies of solvation by m eans of the polarizable continuum model (PCM) yield values for the free ene rgies of hydrolysis that are close to the experimental data.