STATISTICAL-MECHANICAL UNDERSTANDING OF CHEMICAL-REACTION MECHANISM IN SOLUTION - ENERGY FLUCTUATIONS AND HEAT-CAPACITIES FOR ISOMERIZATIONOF FORMAMIDINE IN AQUEOUS-SOLUTION

To explore the energy flow-mechanism of a solution-chemical reaction o n the basis of an energy-fluctuation analysis and the time evolution o f various kinds of energies [J. Phys. Chem. 48, 12506 (1994)], a chemi cal reaction molecular dynamics simulation was carried out in the micr ocanonical ensemble for the proton-transfer reaction of formamidine in an aqueous solution. The energy Delta E required to surmount the reac tion barrier was found to be supplied mainly from the potential energy of the solvent water rather than from the solvent kinetic energy. The ratio of the reactive energy flow from the solvent potential vs. the kinetic energy, Delta V/Delta K, was 2.34 and was found to be in good agreement with the value of 1.96 predicted from the classical constant -volume heat capacity of water, C-V(B), via the Lebowitz-Percus-Verlet relation [Phys. Rev. 153, 250 (1967)]. This finding confirmed the res ults of Wilson et al. [J. Am. Chem. Sec. 113, 74 (1991)]; namely, that the ratio should be determined only by the heat capacity of the solve nt with no relation to the kinds of solute molecules, and in aqueous s olution, the coordinate fluctuation plays a more important role in the reaction occurrence than in the momentum fluctuation. Furthermore, on the assumption that the solute internal distribution is assumed to ac complish instantaneously thermal equilibrium with the surrounding solv ent and to be characterized as an instantaneous canonical ensemble, th e instantaneous partial molar constant-volume heat capacity of solutes , C-V(S)(t), is defined at first by a simple extension of the relation ship between the equilibrium heat capacity C-V(S) and the ratio of the kinetic- and potential-energy fluctuations of the solutes. On the ave rage, C-V(S)(t) has a larger value than that evaluated within the harm onic approximation in the gas phase, i.e., 199.5 T K-1 mol(-1). The in compatibility was brought about by the intervention of the solute-solv ent interaction. In addition, an exceptionally large value of C-V(S)(t ) was observed just 0.06 ps after (or before) the barrier crossing tim e and can be explained by the smaller fluctuation in the instantaneous kinetic energy. It was also observed that, during the relaxing (or su rmounting) process of the reaction, C-V(S)(t) becomes relatively large r than those in the transition and equilibrium periods, originating fr om the simple fact that the relatively larger potential fluctuation ma kes it easier to dissipate (or supply) the reactive energy from (or to ) the reactants, as translated by a larger C-V(S)(t). (C) 1998 John Wi ley & Sons, Inc.