Ab Initio Study of the Diels-Alder Reaction of Cyclopentadiene with Acrolein in a Ionic Liquid by KS-DFT/3D-RISM-KH Theory

We study the Diels-Alder reaction between cyclopentadiene and acrolein in a model room-temperature ionic liquid ([mmim][PF6]) as a solvent. The calculations have been performed with the KS-DFT/3D-RISM-SCF theory, where the reactants and transition state (TS) have been represented at a OM level, while the solvent is represented by a 3D distribution of classical (charge + LJ) sites obtained by solving the 3D-RISM integral equation. We show that this method, being computationally efficient, is able to reproduce the main experimental features displayed by the experiments, concerning the reaction rate enhancement and augmentation of the endo/exo ratio in ionic liquids (ILs). We find that the IL distorts noticeably the transition state geometry, inverting the order of the frontier orbitals and leading to an enhancement of the asynchronicity of the reaction. Finally, we find, in agreement with recent work, that formation of the hydrogen bond between the unique C2 hydrogen of the imidazolium ring is not essential to explain the peculiar features of these reactions in ILs.