A QUANTUM MOLECULAR-DYNAMICS STUDY OF PROTON-TRANSFER REACTIONS ALONG ASYMMETRICAL H-BONDS IN SOLUTION

A molecular-dynamics study of a model for AH - B half arrow right over half arrow left A- H+ B reactions in liquid chloromethane is presented. The parameters of the model are fitted to those of typical OH-N proton-transfer complexes. The rate constant is computed at a quantum level for complexes of various H-bond strength and A-B equilibrium distance. The influence of the properties of the complex on the proton-transfer mechanism is outlined. Also the static and dynamical role of the solvent, the tunneling contribution to the rate, and the associated kinetic isotope effect are discussed. The rate calculations are based on two independent methods. First a curve-crossing, transition-state rate formula which, although related to standard charge-transfer theories, presents some original features and allows the determination of the rate at very low computational cost is developed. The curve-crossing results are compared to those of a path-integral, quantum transition-state calculation. The overall agreement between the two methods is satisfactory, although there is a discrepancy in the adiabatic reaction regime; a rigorous estimation of the transmission coefficients would be needed then. Finally, it is shown that zero-point energy and parabolic barrier tunneling factors added to the classical transition-state-theory rate constant are unable to describe properly the quantum effects in the present case.