ISOMERIZATION OF PO3-CENTER-DOT(H2O)(N) CLUSTERS TO H2PO4-CENTER-DOT(H2O)(N-1) - TRANSITION-STATES AND BARRIER HEIGHTS

Several mechanisms for the isomerization reactions of the PO3-.(H2O)n clusters (n = 1, 2, and 3) to H2PO4-.(H2O)n-1 have been studied using ab initio quantum mechanical methods, resulting in predictions of the transition state structures, isomerization barrier heights, exothermicities, and products. Basis sets as large as triple-zeta plus double polarization plus f functions have been used with self-consistent-field, second-order perturbation theory, configuration interaction, and coupled cluster methods. The isomerization barrier (DELTAG-degrees) for PO3-.H2O to H2PO4- is 32 kcal mol-1 at the DZP+diff SCF level, 25 kcal mol-1 at the DZP CISD level, and 22 kcal mol-1 at the DZP+diff MP2 level. The isomerization barriers for PO3-.(H2O)2 and PO3-.(H2O)3 are lower by only a few kilocalories per mole than for PO3-.H2O via four-centered transition states and lower by about 5 kcal mol-1 by six-center transition states. The PO3- anion is thermodynamically stable in the gas phse only when the PO3-:H2O molar ratio is below 1:3. However, even with the 1:3 molar ratio, the PO3-.(H2O)3 cluster is expected to be a product along with H2PO4-.(H2O)2. The H2PO4- anion forms double-donor double-acceptor hydrogen bonds with H2O similar to those predicted earlier for PO3-. The C2 symmetry conformation of H2PO4- is more stable than the C(s) form, while the C2v conformation is a stationary point with two imaginary vibrational frequencies. The potential energy surfaces for H2PO4-.H2O and H2PO4-.(H2O)2 are qualitatively similar to that for isolated H2PO4-. The exothermicities (-DELTAH-degrees) of the reactions PO3-.(H2O)n --> H2PO4-.(H2O)n-1 are 18 (n = 1), 22 (n = 2), and 21 kcal mol-1 (n = 3) at the DZP+diff SCF level of theory. The DZP+diff SCF hydration exothermicity of H2PO4- (i.e. -DELTAH for H2PO4- + H2O --> H2PO4-.H2O) is 14 kcal mol-1, while that for H2PO4-.H2O is 8 kcal mol-1. The analogous hydration exothermicities for the isomers PO3-.H2O and PO3-.(H2O)2 are 11 and 10 kcal mol-1, respectively. Although much progress has been made, several questions remain concerning the relationships between the present theoretical results and existing experiments.