Study of the N-H center dot center dot center dot O=C proton transfer reaction in aqueous solution using classical free energy curves

The classical free energy surfaces of the reactive and product states invol ved in the proton transfer reaction of the N-H . . . O=C molecular associat ion in aqueous solution were obtained by means of molecular dynamics simula tions using ab initio MP2-CP potentials. The values of the reaction free en ergy (DeltaG = -1.53 kJ mol(-1), which is opposed to the non-spontaneous ch aracter of the reaction in the gas phase) and the rate constant (k(TST) = 7 .92 x 10(11) s(-1)) of the process show the viability of this proton transf er in aqueous medium. The change in spontaneity due to the surrounding wate r reflects a major redistribution of the solvent (DeltaG(r) = 14.18 kJ mol( -1)), which leads to a decrease of the energy barrier to a value of DeltaG( #) = 5.05 kJ mol(-1). Also, the higher solvent force constant and solvent f requency derived from the free energy curve associated with the non-transfe rred proton system (k(R) = 4.01 x 10(-4) cm and v(R) = 10.13 ps(-1)) reflec t a weakening of the water-water interactions when the solute has the trans ferred proton. (C) 2001 Elsevier Science B.V. All rights reserved.