Fine structure in the transition region: reaction force analyses of water-assisted proton transfers

We have analyzed the variation of the reaction force F(xi) and the reaction force constant kappa(xi) along the intrinsic reaction coordinates xi of the water-assisted proton transfer reactions of HX-N = Y (X,Y = O,S). The profile of the force constant of the vibration associated with the reactive mode, k (xi) (xi), was also determined. We compare our results to the corresponding intramolecular proton transfers in the absence of a water molecule. The presence of water promotes the proton transfers, decreasing the energy barriers by about 12 - 15 kcal mol(-1). This is due in part to much smaller bond angle changes being needed than when water is absent. The kappa(xi) profiles along the intrinsic reaction coordinates for the water-assisted processes show striking and intriguing differences in the transition regions. For the HS-N = S and HO-N = S systems, two kappa(xi) minima are obtained, whereas for HO-N = O only one minimum is found. The k (xi) (xi) show similar behavior in the transition regions. We propose that this fine structure reflects the degree of synchronicity of the two proton migrations in each case.