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 r eference interaction site method in statistical mechanics of molecular liquids (the RISM-SCF method) is used to examine the basicities of me thylamines in aqueous solution. Free energy changes upon protonation a re calculated for a series of methylamines in the gas phase and in aqu eous solution, respectively. The gas phase results show a monotonical increase in the basicity with successive methyl substitutions in accor d with experiment and earlier theoretical studies. In aqueous solution , however, the results show an irregular order in basicity, which is i n qualitative agreement with well established experimental data. Sever al contributions to free energy changes such as those resulting from s olvation and from substitutions are analyzed entirely from a microscop ic point of view. It is shown that solvation free energies play an ess ential role in the determination of the basicity in the solution. Furt her, it is shown that the solvation structures and the solute electron ic structures are altered by the self-consistent treatment in the dete rmination of solute-solvent interactions and that such alterations cau se substantial effects on the basicity.