A b i n i t i o studies of the structures and energies of the H−(H2O) and H−(H2O)2 complexes

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Abstract
Accurate calculations for the H−(H2O) complex with extended basis sets are reported at the restricted Hartree–Fock (RHF) through the fourth‐order Mo/ller–Plesset (MP) perturbation levels of theory. In the equilibrium geometry of the H−(H2O) complex the H− anion is found to lie almost along one of the H–O bond directions. The H–H− distance proved to be very sensitive to electron correlation effects: it is 1.8 and 1.4 Å at the RHF and MP2 levels, respectively. The interaction energy between H− and H2O at the MP4 level including conterpoise corrections for basis set superposition error, depending upon the basis set used, is found to range from 16.2 to 16.9 kcal/mol, and the electron correlation is responsible for one‐third of this value. The enthalpy of formation of H−(H2O) is estimated to be from −15.2 to −16.0 kcal/mol compared with the experimental value of −17.3±1.2 kcal/mol. The vibrational frequencies of H−(H2O) are also reported. The H−(H2O)2 complex is also studied by using a polarized double zeta basis set. The geometry of the H−(H2O)2 complex is partly optimized at the MP2 level and the energetic effects of the addition of the second water to H−(H2O) are analyzed.