Molecular simulation and electron paramagnetic resonance (EPR) studies of rapid bimolecular reactions in supercritical fluids

We present comparisons among Brownian dynamics simulations, molecular dynamics simulations, and electron paramagnetic resonance spectroscopic studies of the Heisenberg spin-exchange reaction between nitroxide free radicals at near-infinite dilution in near-critical and supercritical ethane. We discuss the effects of correlations in the solute-solute and solvent-solute radial distribution functions on the rate constants for collision and reaction. We find that the enhancements in the local density of solvents around solutes strongly affect the rate constant for solvent-solute encounters. This result holds implications for those reactions where collisional-energy transfer from solvent to solute is the rate-limiting step. While the rate of collisions between solutes is strongly affected by solute-solute correlations for all densities, the reaction rate constant is affected by such local density augmentations only for certain combinations of density and collision lenght scale. Rate constants estimated computationally and experimentally show the same qualitative trend as a function of density. Collision lifetimes estimated from the simulations show a strong density dependence. These lifetimes reflect the competing effects of the intermolecular force and the potential of mean force and are distinctly bimodal at the higher densities.