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 spectroscopi c studies of the Heisenberg spin-exchange reaction between nitroxide f ree radicals at near-infinite dilution in near-critical and supercriti cal ethane. We discuss the effects of correlations in the solute-solut e and solvent-solute radial distribution functions on the rate constan ts for collision and reaction. We find that the enhancements in the lo cal density of solvents around solutes strongly affect the rate consta nt for solvent-solute encounters. This result holds implications for t hose reactions where collisional-energy transfer from solvent to solut e is the rate-limiting step. While the rate of collisions between solu tes is strongly affected by solute-solute correlations for all densiti es, the reaction rate constant is affected by such local density augme ntations only for certain combinations of density and collision lenght scale. Rate constants estimated computationally and experimentally sh ow the same qualitative trend as a function of density. Collision life times estimated from the simulations show a strong density dependence. These lifetimes reflect the competing effects of the intermolecular f orce and the potential of mean force and are distinctly bimodal at the higher densities.