COMPUTER-SIMULATION STUDY OF AN S(N)2 REACTION IN SUPERCRITICAL WATER

Molecular dynamics computer simulation is used to examine solvation of species of varying polarity along the reaction coordinate for the S(N)2 reaction of Cl- and CH3Cl in supercritical water. At reduced densities down to 0.5, the solute-solvent interactions are sufficiently strong to preserve solvent configurations with solute coordination numbers nearly as high as in ambient water. Hydrogen-bonding interactions are converted to less specific polar interactions. This loss of hydrogen bonds from ambient water to supercritical water decreases as the hydrogen bond strength increases for the series of solutes from the equivalent Cl's in the transition state complex, to Cl- in the ion dipole complex, to free Cl- in the reactant state. Two-dimensional angle averaged cylindrical and (1-D) spherically averaged pair distribution functions, along with energy distribution functions, provide a clear explanation of the origins of Delta A, Delta E, and T Delta S along the reaction coordinate.