Molecular Dynamics with a Quantum‐Chemical Potential: Solvent Effects on an SN2 Reaction at Nitrogen

Abstract

Listen

Translate article

AbstractSolvent effects on an SN2 reaction at nitrogen (Cl− + NH2Cl → ClNH2 + Cl−) in dimethyl ether solution were studied by means of molecular dynamics simulation with a combined quantum‐chemical and molecular‐mechanical potential. The energetics and geometrical parameters of the reaction in the gas phase, calculated by means of the semiempirical model PM3(the quantum chemical part of the combined potential), were compared with ab initio calculations up to the 6‐311 + G* */MP2 and 6‐311 + G(2 d,p)/MP2 levels of theory. Compared with the gas phase potential energy surface, the free energy profile of the reaction in dimethyl ether solution shows that the solvent makes the ion‐dipole complex well shallower by approximately 6.4 kcal mol−1 and raises the height of the effective barrier from the complex to the transition state by about 2.2 kcal mol−1. The overall transition barrier between the separated reactants and the products is raised from 6.4 kcal mol−1 to 15.0 kcal mol−1 upon solvation. The radial distribution functions between solvent‐solute atom pairs at different stages of the reaction course were compared. Results show that better solvation of the charge‐localised separated reactants is responsible for the increase in the barrier height. Polarisation of the solute by its surroundings is also discussed.