PROTON-TRANSFER IN THE MONOHYDRATED AND THE DIHYDRATED COMPLEXES OF HF AND HCL - AN MP2 6-31+G(ASTERISK-ASTERISK) AB-INITIO STUDY IN THE SELF-CONSISTENT REACTION FIELD MODEL OF SOLVATION/

A comprehensive study of the manifestation of proton transfer in the m ono- and the dihydrated complexes of hydrogen fluoride and hydrogen ch loride is presented. This includes a general ab initio study at the Ha rtree-Fock and at the second-order Moller-Plesset (MP2) levels of appr oximation, in vacuo and in simulated polar and nonpolar solvents, by m eans of the self-consistent reaction field approach, using the extende d split-valence type 6-31+G* basis set. The potential energy surface along the hydrogen-bond coordinate, for the two monohydrated complexes , in a gas-phase or in a nonpolar solution, is generally flat, display ing a single minimum, which corresponds to a hydrogen-bonded molecular complex. Unlike the weak acid/weak base H2O...HF complex, a second mi nimum is observed at the Hartree-Fock level when the strong acid/weak base H2O-HCl complex is embedded in a highly polar environment. Howeve r, the inclusion of intramolecular electron correlation causes this mi nimum to vanish, therefore indicating that (i) the acidity of hydrogen chloride is not strong enough to yield proton transfer, (ii) the mech anism accounting for the protolytic dissociation should involve more t han a single water molecule, and (iii) the stabilization of the H3O+.. .Cl- complex, characterized by the emergence of a second minimum, is l ikely to be related to the general trend of the Hartree-Fock approxima tion to neglect dispersion. The incorporation of a second water molecu le, as an attempt to increase by cooperative effect the donor characte r of the oxygen atom directly bound to the hydrogen halide, leads to a similar artifact when (H2O)(2)...HCl is surrounded by a nonpolar solv ent. In contrast, the presence of an energy minimum corresponding to a n (H2OH+...Cl- ionic complex, at both the Hartree-Fock and the MP2 lev els of approximation, in a moderately polar solution, supports the vie w that (i) two water molecules are required in this kind of acidic dis sociation and (ii) there is no explicit participation of the hydronium ion (H3O+) in the course of the process. Finally, whether the solvent is polar or not, there is no evidence of proton transfer in the dihyd rated (H2O)(2)...HF complex, hence suggesting that the protolytic diss ociation of hydrogen fluoride might imply additional water molecules.