Sn2 reactions in water

Mo Y, Gao J (2000) Ab initio QM/MM simulations with a molecular orbital-valence bond (MOVB) method application to an SN2 reaction in water. J Comput Chem 21(16) 1458—1469... 

In the study of reactivity, Jorgensen and col. have normally used both, the OPLS model and potential functions derived from ab initio calculations. As we have already indicated, when intermolecular pair potentials are applied to the study of a chemical process, the evolution of charges, as well as the Lennard-Jones terms, along the reaction coordinate, has to be considered. For the SN2 reaction in water between chloride anion... 

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

SN2 reaction in water 215 10 Summary and outlook 218 Acknowledgments 218 References 218... 

The Sn2 reaction in solution. We saw above the application of microsolvation to Sn2 reactions ([14, 15]). Let us now look at the chloride ion-chloromethane Sn2 reaction in water, as studied by a continuum method. Figure 8.2 shows a calculated reaction profile (potential energy surface) from a continuum solvent study of the Sn2 attack of chloride ion on chloromethane (methyl chloride) in water. Calculations were by the author using B3LYP/6-31+G (plus or diffuse functions in the basis set are considered to be very important where anions are involved Section 5.3.3) with the continuum solvent method SM8 [22] as implemented in Spartan [31] some of the data for Fig. 8.2 are given in Table 8.1. Using as the reaction coordinate r the deviation from the transition state C-Cl... 

A typical example of such reactions is the exothermic Sn2 nucleophilic displacement reaction Cl -I- CH3—Br Cl—CH3 - - Br . Table 5-2 provides a comparison of Arrhenius activation energies and specific rate constants for this Finkelstein reaction in both the gas phase and solution. The new techniques described above cf. Sections 4.2.2 and 5.1) have made it possible to determine the rate constant of this ion-molecule reaction in the absence of any solvent molecules in the gas phase. The result is surprising on going from a protic solvent to a non-HBD solvent and then further to the gas phase, the ratio of the rate constants is approximately 1 10 10 The activation energy of this Sn2 reaction in water is about ten times larger than in the gas phase. The suppression of the Sn2 rate constant in aqueous solution by up to 15 orders of magnitude demonstrates the vital role of the solvent. 

Berne, Molecular Dynamics Study of an Isomerizing Diatomic in a Lennard-Jones Fluid, J. Chem. Phys., 89, 4833 (1988) b) B. J. Gertner, K. R. Wilson, J. T. Hynes, Nonequilibrium Solvation Effects on Reaction Rates for Model SN2 Reactions in Water,... 

Ohmine s work is one of the earliest studies where the motion of individual solvent molecules and the energy flow to those molecules are considered in an attempt to understand the mechanism of a reactive process. As previously discussed (with reference to the simulations of an Sn2 reaction in water by Gertner et al. ), such studies prove quite useful in understanding the mecha-... 

Hwang et al. studied the energetics and dynamics of the Sn2 reaction in water by a combination of the EVB method and free enei y perturbation calculation.33 In this study, the solvent molecules were treated by the surface-constrained all-atom solvent (SCAAS), while the reactants were described by the EVB. These investigators employed an elaborate mapping function to drive the reactant state (e,) to the product state ( 2). The free energy of activation... 

This raises a point that connects these studies of solvation dynamics with general theories of reactivity. It has been pointed out that the energy changes associated with solvation changes on this time-scale are important in, for example, Sn2 reactions in water they increase the rate of reaction, by stabilising the product. The usual theory of Sn2 reactions in solution assumes that there is equilibrium solvation at all points on the reaction coordinate, implying that the reorientation of solvent molecules is very fast compared with motion along the reaction coordinate. The ultrafast motion of water molecules just described sup-... 

Bergsma JP, Gertner BJ, Wilson KR, Hynes JT (1987) Molecular-dynamics of a model Sn2 reaction in water. J Chem Phys 86(3) 1356-1376... 

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