Theoretical study for the basicities of methylamines in aqueous solution: A RISM-SCF calculation of solvation thermodynamics

An ab initio quantum chemical calculation combined with the extended reference interaction site method in statistical mechanics of molecular liquids (the RISM-SCF method) is used to examine the basicities of methylamines in aqueous solution. Free energy changes upon protonation are calculated for a series of methylamines in the gas phase and in aqueous solution, respectively. The gas phase results show a monotonical increase in the basicity with successive methyl substitutions in accord with experiment and earlier theoretical studies. In aqueous solution, however, the results show an irregular order in basicity, which is in qualitative agreement with well established experimental data. Several contributions to free energy changes such as those resulting from solvation and from substitutions are analyzed entirely from a microscopic point of view. It is shown that solvation free energies play an essential role in the determination of the basicity in the solution. Further, it is shown that the solvation structures and the solute electronic structures are altered by the self-consistent treatment in the determination of solute-solvent interactions and that such alterations cause substantial effects on the basicity.