Continuous charge distribution models of ions in polar media. Part 5.—Quantum mechanical treatment of ionic and molecular solvation

Abstract

This paper explores the possibility of establishing a computationally useful theory of ionic and molecular solvation based upon the direct use of quantum mechanically defined charge density components. The objective is to calculate free energies of solvation. The form of analysis proposed makes use of Ruedenberg's density matrix formulation with which the molecular electronic charge distribution is decomposed into quasiclassical and interference density components. Thus, the electrostatic source charges in a molecule contribute individually to the free energy. The various contributions, further, can be related to terms in a typical molecular orbital calculation. Formally, it appears possible at this stage to anticipate that general rules of solvation can be formulated, and that the rules of solvation should emerge in much the same form as the rules of bond energy additivity in thermochemistry. The concepts advanced in this paper are illustrated by the use of three transparent, simple molecular systems: H⁺₂, H₂, and H^–₂ in a structureless, continuum water solvent system.