Abstract
Our recently developed optimized potentials for liquid simulations-virtual site ionic liquid (OPLS-VSIL) force field has been shown to provide accurate bulk phase properties and local ion-ion interactions for a wide variety of imidazolium-based ionic liquids. The force field features a virtual site that offloads negative charge to inside the plane of the ring with careful attention given to hydrogen bonding interactions. In this study, the Diels-Alder reaction between cyclopentadiene and methyl acrylate was computationally investigated in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], as a basis for the validation of the OPLS-VSIL to properly reproduce a reaction medium environment. Mixed ab initio quantum mechanics and molecular mechanics (QM/MM) calculations coupled to free energy perturbation and Monte Carlo sampling (FEP/MC) that utilized M06-2X/6-31G(d) and OPLS-VSIL gave activation free energy barriers of 14.9 and 16.0 kcal/mol for the endo-cis and exo-cis Diels-Alder reaction pathways, respectively (exptl. ΔH‡ of 14.6 kcal/mol). The endo selectivity trend was correctly predicted with a calculated 73% endo preference. The rate and selectivity enhancements present in the endo conformation were found to arise from preferential hydrogen bonding with the exposed C4 ring hydrogen on the BMIM cation. Weaker electronic stabilization of the exo transition state was predicted. For comparison, our earlier ±0.8 charge-scaled OPLS-2009IL force field also yielded a ΔG‡ of 14.9 kcal/mol for the favorable endo reaction pathway but did not adequately capture the highly organized solvent interactions present between the cation and Diels-Alder transition state.
Highlights
Ionic liquids are massively customizable solvents [1,2,3,4,5] that can provide significant advantages in a wide range of technological applications [6,7,8,9,10,11,12,13,14]. These molten salts are often liquid at room temperature due in large part to the molecular asymmetry built into the cation, for example, 1-butyl-3-methylimidazolium [BMIM], that impedes a strong charge ordering with anions [15,16,17]
A wide range of computational methodologies have been utilized from quantum mechanics (QM) and ab initio molecular dynamics (AIMD) to empirical potential-based approaches such as molecular dynamics (MD) and coarse-grained models [32]
All calculations were run on computers and methyl acrylate in [BMIM][PF
Summary
Ionic liquids are massively customizable solvents [1,2,3,4,5] that can provide significant advantages in a wide range of technological applications [6,7,8,9,10,11,12,13,14]. The physical and chemical properties of ionic liquids are fundamentally related to their solvent structure [18,19], which in turn is largely dependent upon Coulombic forces [20,21], dispersion interactions [22,23] and ideal ion-ion hydrogen bonding geometries [24,25].
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