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À propos de : Solvation Effects on the SN2 Reaction between CH3Cl and Cl- in Water        

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  • Solvation Effects on the SN2 Reaction between CH3Cl and Cl- in Water
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  • Car−Parrinello molecular dynamics simulations have been performed to investigate the solvation effects onthe prototype SN2 reaction between Cl- and CH3Cl. The free energy barrier for this reaction in water wascalculated using constrained dynamics at a constant temperature of 300 K and constant volume. Calculationson the isolated system (reaction in the gas phase at zero temperature) were performed for reference purposes.Qualitatively, the calculations confirm that the double-well free energy profile of the reaction in the gasphase is converted into a single barrier by solvation and that the height of the barrier increases significantly.Quantitatively, there are two error sources. At the electronic structure side, the Becke−Perdew functionalunderestimates the barrier height by 8 kcal/mol. At the dynamics side, there is a “hysteresis” effecttoo slowan adaptation of the solvent structure to changes in the reaction coordinateyielding an estimated error of 3kcal/mol in the free energy barrier height. After correction for these errors, the calculated value of the freeenergy barrier is 27 kcal/mol. Considering the accuracy of the solvent−solvent and solvent−reactant interactionsof ca. 1 kcal/mol, this compares very well with the experimental estimate of 26.6 kcal/mol. This indicatesthat the ab initio (DFT) MD very well captures the differential energetic as well as entropic effects of thesolvation when going from the (solvated) reactants to the initial ion−dipole complex to the transition state.
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