Toward an Understanding of the Hydrolysis of Mg-PPi. An ab Initio Study of the Isomerization Reactions of Neutral and Anionic Mg-Pyrophosphate Complexes
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- 作者：Humberto Saint-Martin ; Luis E. Ruiz-Vicent ; Alejandro Ramí ; rez-Solí ; s ; and Ivá ; n Ortega-Blake
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：December 4, 1996
- 年：1996
- 卷：118
- 期：48
- 页码：12167 - 12173
- 全文大小：422K
- 年卷期：v.118,no.48(December 4, 1996)
- ISSN：1520-5126
文摘
Ab initio calculations were performed tostudy the stability of various pyrophosphate species in thegasphase: H4P2O7,H3P2O7-,H2P2O72-,HP2O73-,P2O74-, and theircomplexes with Mg2+. It is found that themetalcation allows the existence of highly charged anions in the gas phase.We also study the isomerization reactionsMg·H2P2O7 
(H2PO4·Mg·PO3),(Mg·HP2O7)- (HPO4·Mg·PO3)-, and(Mg·P2O7)2- (PO4·Mg·PO3)2-,at theself-consistent-field (SCF) and second-order perturbation (MP2) levelsof the theory, using a 6-31+G** basis setwith diffuse and polarization functions. Other basis sets,including one of valence triple 
plus polarization(vTZP)quality, were employed to check for the convergence of the results.It is found that the same mechanism occurs forthe isomerizations of the three species: one of the P-O bridgingbonds of the reactant is longer than the other, andthe route to the products proceeds through its elongation. Thisasymmetry is induced by the metal cation in the caseof the evenly charged anions. In all cases the metal cationcoordinates the transition states and the leaving groups.The structures found for the complexes(H2PO4·Mg·PO3),(HPO4·Mg·PO3)-, and(PO4·Mg·PO3)2-are different fromthose reported previously, the metal cation being enclosed by the twophosphates. The activation barrier increaseswith the charge of the anion, from 
G
= 5.6 kcal/mol for the neutral complexMg·H2P2O7, toG = 10.4kcal/mol for the monoanion(Mg·HP2O7)-, toG = 13.5 kcal/mol for the dianion(Mg·P2O7)2-.The positivevalue found for the energy of the isomerization(Mg·P2O7)2- (PO4·Mg·PO3)2-,G = 1.8 kcal/mol, predictsthesynthesis to be spontaneous in the gas phase, opposite of what occursin the aqueous solution. This result supportsthe view that the hydration energy makes a large contribution to theenergy of hydrolysis. The gas-phase hydrolysisreaction H2O + Mg2+ +H2P2O72- Mg2+ +H2PO4- +H2PO4- is also studied as amultistep reaction, involvingthe isomerization of H2O +(Mg·H2P2O7) H2O+ (PO3·Mg·H2PO4) as anintermediate step. It is found that theequilibrium in the gas phase yieldsH2PO4·Mg·H2PO4as the final species; an energy input is required forseparatingthe metal cation from the phosphate anions.
(H2PO4·Mg·PO3),(Mg·HP2O7)- (HPO4·Mg·PO3)-, and(Mg·P2O7)2- (PO4·Mg·PO3)2-,at theself-consistent-field (SCF) and second-order perturbation (MP2) levelsof the theory, using a 6-31+G** basis setwith diffuse and polarization functions. Other basis sets,including one of valence triple plus polarization(vTZP)quality, were employed to check for the convergence of the results.It is found that the same mechanism occurs forthe isomerizations of the three species: one of the P-O bridgingbonds of the reactant is longer than the other, andthe route to the products proceeds through its elongation. Thisasymmetry is induced by the metal cation in the caseof the evenly charged anions. In all cases the metal cationcoordinates the transition states and the leaving groups.The structures found for the complexes(H2PO4·Mg·PO3),(HPO4·Mg·PO3)-, and(PO4·Mg·PO3)2-are different fromthose reported previously, the metal cation being enclosed by the twophosphates. The activation barrier increaseswith the charge of the anion, from G= 5.6 kcal/mol for the neutral complexMg·H2P2O7, toG = 10.4kcal/mol for the monoanion(Mg·HP2O7)-, toG = 13.5 kcal/mol for the dianion(Mg·P2O7)2-.The positivevalue found for the energy of the isomerization(Mg·P2O7)2- (PO4·Mg·PO3)2-,G = 1.8 kcal/mol, predictsthesynthesis to be spontaneous in the gas phase, opposite of what occursin the aqueous solution. This result supportsthe view that the hydration energy makes a large contribution to theenergy of hydrolysis. The gas-phase hydrolysisreaction H2O + Mg2+ +H2P2O72- Mg2+ +H2PO4- +H2PO4- is also studied as amultistep reaction, involvingthe isomerization of H2O +(Mg·H2P2O7) H2O+ (PO3·Mg·H2PO4) as anintermediate step. It is found that theequilibrium in the gas phase yieldsH2PO4·Mg·H2PO4as the final species; an energy input is required forseparatingthe metal cation from the phosphate anions.