Hydration of methylamine and methylammonium ion: structural and thermodynamic properties from the data of the integral equation method in the RISM approximation

The structural and thermodynamic properties of hydration of methylamine and methyl-ammonium ion were investigated by the integral equations method in the RISM approximation. According to calculations, the average number of water molecules in the first hydration shell of CH3 group is 14.4 for aqueous methylamine and 12.7 for aqueous methylammonium solution. The first hydration shells of the NH2 group of methylamine and the NH3 (+) group of methylammonium ion contain 6.9 and 5.6 water molecules, respectively. The average number of H-bonds formed by the NH2 group is 2.4 and that formed by the NH3 (+) group is 3. The results obtained show no H-bonding between the nitrogen atom of NH3 (+) group of methylammonium and water molecules. The hydrogen atom of water participating in the hydrogen bonding with the nitrogen atom of methylamine now is a constituent of the NH3 (+) group of methylammonium ion. The hydration free energies and the ionization constant calculated within the framework of the RISM theory are in good agreement with experimental data.