Polyatomic molecules in condensed phase: bond order index and solvation energy studied by RISM-SCF theory

Abstract

The reference interaction site model - self-consistent field (RISM-SCF) theory was applied to a series of polyatomic molecules in aqueous solution, including CH4, NH3, H2O and HF. The bond order index, which characterizes the chemical bond, and its change by the solvation were studied by comparison with a dielectric continuum model. The change in the electronic structure was also associated with the solvation free energy. We find that the bond order indexes are not sensitive to the solvation effect even if the charge assigned to the atom is considerably changed. The distortion in the electronic energy is in proportion to the change in the solvation energy, showing that the linear response regime is a good expression of the solvation process examined here, in spite of solving the non-linear RISM-SCF equation. It is well-known that the electronic structure of a molecule is significantly changed when the molecule is dissolved into solvent. In the last few decades, theory for the electronic structure of solvated molecules has attracted a lot of researchers' attention, and various types of combination methods have been developed. These can be roughly classified into three categories, namely, the dielectric continuum model such as polarizable continuum model (PCM), quantum mechanical- molecular mechanics (QM-MM), and the method based on the integral equation theory for liquids. RISM-SCF (1,2), which is a representative of the third category, compiles two fundamental methods in theoretical chemistry: one is the reference interaction site model (RISM) (3,4), and the other is ab initio molecular orbital theory. The method determines the electronic structure and the statisti- cal solvent distribution around the solute in a self-consistent manner. It is a remarkable advantage that the RISM-SCF method can provide information on the microscopic solvation structure based on the statistical mechanics. We would like to emphasize that the RISM-SCF theory can be re- garded as an extension of RISM theory as well as that of ab initio molecular orbital theory. The RISM-SCF has been successfully applied to numerous molecular phenomena including chemical reactions, chemical equilibria, charge transfer processes and so on (5). The aim of this review is to characterize the chemical bond in condensed phase, which is definitely one of the central subjects in physical chemistry. The solvation effect by all means modifies the character of a chemical bond in the solute molecule, and, at the same time, this modification in the electronic structure affects the structure of the solvent molecules surrounding the solute. For the system we choose a series of hydrogenated compounds of the second period elements in aqueous solution, which are the most fundamental and ubiquitous molecules in the universe. Since all of them have only 'single' bond and their electronic structures in the gas phase are well known, they are thought to be the most suitable for a systematic comparison for the preset purpose.