Semicontinuum (Cluster-Continuum) Modeling of Acid-Catalyzed Aqueous Reactions: Alkene Hydration.

An effort is made to reduce the errors of continuum solvation models (CSMs) with semicontinuum modeling to achieve 3 kcal mol-1 agreement with experiment for acid-catalysis activation Gibbs energies. First, two underappreciated CSM issues are reviewed: errors in the CSM solvation Gibbs energies grow beyond 5 kcal mol-1 (i) as ions are made smaller and (ii) as water clusters grow larger. Second, the computational reproduction of the known Gibbs energies (ΔrG and Δ‡G) of the paradigmatic reaction ethene + H2O + H3O+ → TS+ → ethanol + H3O+ is attempted. It is argued that, despite the >5 kcal mol-1 solvation errors for ions, it is possible to employ error cancellation strategies to reduce the errors in the reaction and activation Gibbs energies to 3 kcal mol-1 accuracy. A new 3 kcal mol-1 effect due to solvent-molecule "placement" (confinement from 1 M bulk concentration) was isolated and proved useful. Third, computational reproduction of the known entropies (ΔrS and Δ‡S) of the paradigmatic reaction is attempted using Trouton's constant and neglect of solvent structure reorganization effects (which must cancel well for this reaction); this worked well for ΔrS but needs empirical correction of ∼11 cal mol-1 K-1 for Δ‡S due to solvent disorientation when H3O+ is consumed. These entropy estimates allow for enthalpy (ΔrH and Δ‡H) estimation from the Gibbs energy values. Fourth, two recommended options, including A + H3O+·2W → [AHOH2+·2W]‡, are shown to also work well for the activations of propene and isobutene.