Calculation of ionization constants of methylamines in aqueous solution

With the specific aim of calculating the ionization constants of ammonia and methylamines in aqueous solution we investigated the molecular conformations and solute-solvent interactions of the bases and their corresponding conjugate acids, applying the B3LYP/6-31+G(d) method with the Tomasi's model. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-acceptor capability, are proposed. It was considered that the solvation of the hydrogen, hydroxyl and ammonium ions occurs by means of intermolecular hydrogen bonds that involve one molecule of the ion and one molecule of water. The mentioned reactions and equilibria constituted the indispensable theoretical basis to calculate the ionization constants of the compounds. The agreement between the ionization constants calculated theoretically and the experimental values reported in the literature demonstrates the utility of the method here used. Furthermore, the agreement between the experimental and theoretically calculated values for the ionization constants of ammonia and methylamines provides solid support for the acid-base reactions proposed in this work.