Studies And Applications Of A Combined Discrete-continuum Solvation Model

Many chemical reactions proceed in solution. The environment plays a key role in the determination of the properties and reactivity of substances in condensed phase. Therefore the theoretical simulation of solvent effects is always an important target of computational chemistry. Theories for both implicit and explicit solvation models have been developed and applied widely. The explicit solvation model, also known as cluster or supermolecule model, is too simple due to including several solvent molecules and ignoring the long rang solute-solvent interactions. In the implicit continuum theory, the solute is treated by the quantum mechanics directly, but embedded in a dielectric continuum of permittivity equal to that of the solvent. The continuum models have been the most popular approaches for the description of solvents in quantum chemistry due to its high efficiency and implementation in popular quantum mechanical calculation software. These models provide a very easy and accurate way to treat the extremely important and strong long rang solute-solvent interactions that dominate many solvation phenomena. However, it is also known that the strong and specific solute-solvent interactions are not completely accounted for by the continuum models. Of particular, hydrogen bonding which is a short-range force is not completely predictable from bulk electrostatics.A compromise between the implicit and explicit models, known as discrete-continuum, cluster-continuum, semicontinuum, orsupermolecule-continuum model, is to include several solvent molecules around the solute into the QM part, but the rest bulk solvent is treated with the continuum theory. In this case specific solute-solvent interactions, mainly the hydrogen bonding, are retained at the quantum level, while long-range interactions are introduced through the continuum model. In the contest the discrete-continuum model has been discussed and applied to study solvent effects of several systems. The results are as follows.(1) Such the combined discrete-continuum model, which includes both the short-range interactions between a solute and solvent molecules in the first solvation shell, and long-range solute-solvent interactions, is more accurate and efficient. Furthermore, contrary to cluster model and continuum model, its performance is more stable, and it is suitable for more extensive systems.(2) The combined discrete-continuum model, which including the solvent molecules in the first solvation shell, is enough to simulate main effects. Nevertheless, it is necessary to include all solvent molecules in the first solvation shell, or several solvent molecules that interact strongly with solute. Furthermore, the results show that geometries of solvated ions closer to the real case would lead more accurate solvation properties.(3) The combined discrete-continuum model overcomes the lack and annihilates the error in continuum methods. In addition, the computation of the short-range solute-solvent interaction also causes errors. Therefore a high-level quantum calculation is necessary.IV(4) This model can also provide insight into many fundamental details where cluster model and continuum model fail to provide unique pictures.