Effective Molarity in

Effective molarities for diffusion-controlled r actions between spherical reactants with reactive patches are calculated analytically and by Brownian dynamics simulations. Unimolecular reaction systems with internal translational motion in one, two, and three dimensions are investigated and compared with bimolecular reactions in three dimensions. Rotational diffusion is included in all cases in which a reactant particle is anisotropically reactive. Effective molarit ies are established bycalculating the ratio kunif k61. Large rate enhancements are seen when restrictive translational constraints are imposed on the unimolecular reaction. Additional rate enhancements occur when a reduction in dimensionality accompanies the translational constraint. If the reactants are anisotropically reactive, the effective molarity is further increased if the geometric onstraints in the unimolecular system keep the reactive surfaces in a proper orientation for reaction. The presence of an attractive potential designed to represent the relief of strain in the unimolecular system also leads to rate enhancements. The results are compared with those obtained for simple models of activated (non-diffusion-controlled) r actions. Overall, these simulation results indicate that highly elevated values of effective molarity are not likely to arise from mass transport considerations alone.