Temperature and Density Effects on an SN2 Reaction in Supercritical Water
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Molecular dynamics computer simulation is used to relate the thermodynamic properties along the reaction coordinate to microscopic solvation for the SN2 reaction of the chloride ion with methyl chloride as a function of temperature and density. Extreme conditions (e.g. reduced densities of 0.05 and 0.3 for reduced temperatures of 1.0 and 1.3, respectively) are found to be necessary to remove half of the water molecules in the first solvation shell about the chloride ion. As the temperature is increased and density decreased, the number of Cl- water hydrogen bonds decays faster than the coordination number. By analogy to adsorption phenomena, augmentation in the local solvent density relative to the bulk (clustering) is interpreted in three regions corresponding to gas, near-critical, and liquid-like densities. The lifetime of a water molecule in the first coordination sphere is found to be about 4 times shorter than under ambient conditions; the lifetime of a hydrogen bond between Cl and water decreases by a comparable factor of about 6. The values of A, E, and -TS associated with conversion from the reactant state to the transition state are explained in terms of the variations in the average coordination numbers and hydrogen bonding. The combined effects of changes in temperature and solvation lead to an increase in the rate constant by 9-12 orders of magnitude under supercritical conditions compared to ambient conditions.
