SELF-CONSISTENT-FIELD TREATMENT OF THE DISSOCIATION OF BOUND MOLECULES IN SOLUTION

We present calculations of the dissociation of hydrogen-bonded molecul es in solution, using a mean-field theory based on a variational metho d [N.R. Werthamer, in Rare Gas Solids, edited by M. L. Klein and J. A. Venables (Academic, New York, 1976), Vol. 1, Chap. V]. The solvent is accounted for by a dielectric constant, and the effect of salt ions i n solution on hydrogen bonding is treated by means of Soumpasis's pote ntial of mean force [D. M. Soumpasis, Proc. Natl. Acad. Sci. USA 81, 5 116 (1984)]. At high concentrations, the effect of salt ions on the in teraction is dominated by an effective temperature-dependent interacti on, which results from a position-dependent term in the entropy result ing from the hard-core volume exclusion. In addition to describing the dissociation transition, this procedure provides the temperature and electrolyte-concentration dependence of the vibrational spectrum. The sample calculation by Gao and Prohofsky [J. Chem. Phys. 80, 2242 (1984 )] of two ammonia molecules bound together by a hydrogen bond in a vac uum is reconsidered in an ionic solution. Our method is also applied t o the treatment of the hydrogen-bond dissociation of a pair of water m olecules and of a hydrogen-bonded pair of negative point-charge ions. The latter is intended as a simple model for the dissociation of a sin gle hydrogen-bonded base-pair unit of a DNA double helix.