Multiscale Treatment for the Molecular Mechanism of a Diels-Alder Reaction in Solution: A QM/MM-MD Study

Thermodynamics and the solvent role in the acceleration of the Diels-Alder reaction between cyclopentadiene (CPD) and methyl vinyl ketone (MVK) have been revisited. In this work we use an ab initio hybrid QM/MM-MD scheme combined with multiple steered molecular dynamics to extract the free energy pofile in water and methanol using the bidirectional Minh-Adib estimator. We obtain 18.7 kcal mol^-1 and 20.8 kcal mol^-1 free energy barrier for the reaction in water and methanol, respectively. This methodology reproduces experimental values with an absolute error of about 0.8 kcal mol^-1. The experimental difference between the activation free-energy barriers of water and methanol is also reproduced with an absolute error of about 0.1 kcal mol^-1. We explore the charge transfer evolution along reaction coordinates to characterize the electronic behavior for this reaction. It is shown that the solvent molecules around the reaction system produce a global polarization along the reaction coordinate which is consistent with the solvent polarity. The results highlight the role of hydrogen bonding formed in the transition state to stabilize the system charge reorganization in the reaction process.