Semicontinuum (Cluster-Continuum) Modeling of Acid-Catalyzed Aqueous Reactions: Alkene Hydration
Darpan H. Patel, Allan L. L. East · The Journal of Physical Chemistry A · 2020
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 (Δ r G and Δ ‡ G ) of the paradigmatic reaction ethene + H 2 O + H 3 O + → TS + → ethanol + H 3 O + 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 (Δ r S 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 Δ r S but needs empirical correction of ∼11 cal mol –1 K –1 for Δ ‡ S due to solvent disorientation when H 3 O + is consumed. These entropy estimates allow for enthalpy (Δ r H and Δ ‡ H ) estimation from the Gibbs energy values. Fourth, two recommended options, including A + H 3 O + ·2W → [AHOH 2 + ·2W] ‡, are shown to also work well for the activations of propene and isobutene.