LINEAR FREE-ENERGY RELATIONSHIPS WITH QUANTUM-MECHANICAL CORRECTIONS - CLASSICAL AND QUANTUM-MECHANICAL RATE CONSTANTS FOR HYDRIDE TRANSFER BETWEEN NAD(+) ANALOGS IN SOLUTIONS

The microscopic validity of linear free energy relationships for adiabatic reactions in solutions is examined using computer simulation methods and realistic potential surfaces. The simulations consider hydride transfer reactions within a class of NAD(+) analogues. The potential surfaces of the reacting systems are evaluated by the empirical valence bond approach and the corresponding diabetic and adiabatic free energy functions are calculated by a free energy perturbation/umbrella-sampling approach. Quantum mechanical corrections-of the activation energies are evaluated by the quantized classical path method. It is demonstrated that a single adjustable parameter that scales the off-diagonal valence bond mixing term reproduces the observed linear free energy relationship with a fully microscopic approach without assuming a priori any Marcus-like relationship. Interestingly, the calculated solvent reorganization energies are quite different than those deduced by phenomenological approaches. This reflects the contribution of the solute reorganization energy, the coupling between the diabatic states bf the solute, and the effect of quantum mechanical nuclear factors. The present-study demonstrates the effectiveness of the empirical valence bond approach for studies of chemical processes in solutions as;well as the insight provided by applying the valence bond description to chemical processes in general.