A Multi-Level Quantum Mechanics and Molecular Mechanics Study of the Doubleinversion Mechanism at Nitrogen: F- +NH2Cl in Aqueous Solution.

Abstract

We employ a multi-level quantum mechanics and molecular mechanics method to investigate the double inversion mechanism of the nucleophilic substitution reaction at N center: the F- +NH2Cl reaction in aqueous solution. We find that the structures of the stationary points along the reaction path are quite different from the ones in gas owing to the hydrogen-bond interactions between the solute and the surrounding water molecules. The atomic-level evolutions of the structures and charge transfer along the reaction path show that this double-inversion mechanism consists of an upside-down proton inversion process and a Walden-inversion process. The computed potential of mean force at the CCSD(T)/MM level of theory has the first, second barrier heights, and reaction free energy at 11.7, 29.6, and 12.6 kcal/mol, agreeing well with the predicted ones at 12.6, 32.5, and 12.2 kcal/mol which obtained based on the gas-phase reaction path and the solvation free energies of the stationary points.