Study of the N–H⋯OC proton transfer reaction in aqueous solution using classical free energy curves

Abstract

The classical free energy surfaces of the reactive and product states involved in the proton transfer reaction of the N–H⋯OC molecular association in aqueous solution were obtained by means of molecular dynamics simulations using ab initio MP2-CP potentials. The values of the reaction free energy ( ΔG=−1.53 kJ mol −1, which is opposed to the non-spontaneous character of the reaction in the gas phase) and the rate constant ( k TST=7.92×10 11 s −1) of the process show the viability of this proton transfer in aqueous medium. The change in spontaneity due to the surrounding water reflects a major redistribution of the solvent ( ΔG r =14.18 kJ mol −1), which leads to a decrease of the energy barrier to a value of ΔG #=5.05 kJ mol −1. Also, the higher solvent force constant and solvent frequency derived from the free energy curve associated with the non-transferred proton system ( k R =4.01×10 −4 cm and ν R =10.13 ps −1) reflect a weakening of the water–water interactions when the solute has the transferred proton.