(173a) Solvent Reaction Coordinate for an SN2 Reaction

We study the prototypical S_N2 reaction Cl^- + CH₃Cl → CH₃Cl + Cl^- in water using QM/MM computer simulations with transition path sampling [1] and inertial likelihood maximization [2], implemented in CP2K [3]. We have identified a new solute coordinate and a new solvent coordinate that both improve upon the original atom-exchange coordinate used in the classic analysis by Chandrasekhar et al. [4, 5]. The new solvent coordinate quantifies instantaneous solvent induced polarization relative to equilibrium charge density at each point along the reaction pathway. The new solute coordinate quantifies the out-of-plane motion of the central -CH₃- fragment. We will present free energy surfaces, transmission coefficients, and an analysis of the friction spectrum for the optimized coordinates.

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