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SCRF/Monte Carlo Study of Solvent Effects on a Polar [2+2] Cycloaddition

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journal contribution
posted on 31.10.1996 by Dongchul Lim, William L. Jorgensen
Solvent effects on the polar [2+2] cycloaddition of 1,1-dicyanoethylene (DCNE) and methyl vinyl ether (MVE) have been studied using ab initio self-consistent reaction field (SCRF) and Monte Carlo (MC) statistical mechanics calculations. Emphasis is placed on the dependence of transition-state structure and energetics on solvation and on the impact of using different charge sets and geometries from the SCRF calculations in the MC simulations. The ab initio calculations were performed with dielectric constants ε of 1.0, 2.23 (CCl4), and 35.94 (CH3CN) with the 6-31G* basis set at the Hartree−Fock (HF) level and incorporating electron correlation through MP2 and Becke3LYP density functional methods. Computed dipole moments of 10−17 D for the transition structures overlap estimates based on the solvent effects observed by Steiner and Huisgen. However, the mechanistic picture that emerges is medium dependent. In the gas phase and nonpolar solvent, the results here indicate a concerted (single barrier) process with no intermediate, while at ε = 35.94, an anti zwitterion is formed and then is transformed to the product. With the Onsager and SCIPCM SCRF calculations, the activation energy is computed to be lowered by 10−13 kcal/mol in going from CCl4 to CH3CN, which overestimates observed effects for such reactions. Five sets of charges and geometries from the ab initio calculations were tested in MC free energy perturbation (FEP) simulations using 260 explicit carbon tetrachloride and acetonitrile molecules in periodic cells. The resultant predicted solvent effects of 7−20 kcal/mol on the free energy of activation are also greater than the observed ca. 5 kcal/mol. The best results are obtained using the charges and geometries from Becke3LYP/6-31G* gas-phase (ε = 1) calculations.