PROTON-TRANSFER BETWEEN ACETIC-ACID AND METHYLAMINE IN AQUEOUS-SOLUTION - ABINITIO AND MOLECULAR-DYNAMICS STUDY OF FREE-ENERGIES OF HYDRATION

The free energy difference between the neutral and ionic forms of the complex of acetic acid with methylamine, surrounded by 253 SPC water molecules, is calculated by a combination of molecular dynamics (MD) and ab initio methods. The ab initio energy calculations (4-31G* basis set) were carried out on instantaneous values, extracted from MD trajectories. The ionic form turns out to be more stable than the neutral form by 6.7 +/- 1.9 kcal mol-1. This result compares well with the difference in pK(a) values. The stabilization of the ionic form is mainly due to the superior water solute interaction energy of this form.