Nucleophilic Substitution Reactions at Liquid/Liquid Interfaces:  Molecular Dynamics Simulation of a Model S_N1 Dissociation Reaction at the Water/Carbon Tetrachloride Interface

Abstract

The ionic dissociation step of the nucleophilic substitution reaction t-BuCl --> t-Bu(+) + Cl(-) is studied at the water/carbon tetrachloride interface using molecular dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. The umbrella sampling technique is used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) at several interface regions of varying distances from the Gibbs dividing surface. We find a significant increase of the ionic dissociation barrier height and of the reaction free energy at the interface relative to bulk water. This is shown to be due to the reduced polarity of the interface which causes a destabilization of the pure ionic state. However, deformation to the neat interface structure in the form of water protrusions into the organic phase may provide partial stabilization of the ionic species. The importance of these structural effects is examined by repeating the calculations with an artificially smooth interface. The destabilization of the ionic state at the interface also manifests itself with a rapid (picosecond time scale) recombination dynamics of the ions to form the parent molecule followed by a slow vibrational relaxation.

Calculation of the Intrinsic Solvation Free Energy Profile of an Ionic Penetrant Across a Liquid–Liquid Interface with Computer Simulations

“…To circumvent this problem, several methods providing enhanced sampling of rare events, such as harmonic and adaptive umbrella sampling, thermodynamic integration, metadynamics, , the Widom test particle insertion method and its cavity insertion variant, or potential of mean force (PMF) calculation by, for example, constrained molecular dynamics, , have been proposed over the years. These methods of solvation free energy calculation have been applied for a number of ionic − and nonionic penetrants ,− at various fluid interfaces in the past decades, complementing theoretical predictions based on various continuum dielectric calculations. , …”

Solvation Free Energy Profile of the SCN– Ion across the Water–1,2-Dichloroethane Liquid/Liquid Interface. A Computer Simulation Study

“…Computer simulation techniques, such as molecular dynamics or Monte Carlo, are handy tools to provide us with an in-depth molecular level picture of our systems of interest. Bearing such advantages, these methods have recently been widely used to investigate the behavior of different ions both at the liquid/vapor interface of their solutions − and at liquid/liquid interfaces formed by the aqueous ionic solution and various organic compounds immiscible with the aqueous phase. − Some of these computational studies address directly the question of ionic distribution in the interface normal direction by placing a number of ions into the aqueous phase and calculating their density distribution from the resulting trajectory . Such studies showed that only the compact monatomic ions characterized by low polarizability (e.g., F – ) obey the original theory of Onsager and Samaras .…”

Importance of Interfacial Adsorption in the Biphasic Hydroformylation of Higher Olefins Promoted by Cyclodextrins: A Molecular Dynamics Study at the Decene/Water Interface

“…Some papers are of more relevance to phase‐transfer catalysis 36. 37 Concerning the hydroformylation of olefins, we recently reported an exploratory MD study of biphasic solutions of the main species of the reaction and these were found to be surface‐active. Furthermore, a complex formed between a native cyclodextrin and the key reaction intermediate [RhH(CO)(TPPTS) 2 (decene)] 6− was also adsorbed at the interface 38.…”