A molecular-dynamics study of a model SN1 dissociation reaction at the water liquid/vapor interface

Abstract

The thermodynamics and dynamics of a model S(N)1 reaction: t-BuCl --> t-Bu+ + Cl- is studied at the water liquid/vapor 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. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation.

Valence bond theory for chemical dynamics

“…Valence bond theory has been used as a framework for modeling chemical reaction dynamics since the early days of quantum mechanics 2, 3, 20–24, 28, 29, 40, 41, 45, 48, 50, 51, 67, 72, 79, 81, 86, 87, 89, 90, 92, 95–99, 101–104, 106, 112, 210–286. In part this may be explained by the fact that valence bond theory often provides a concise description of multireference character, and transition states are more likely than stable molecules to exhibit multireference character.…”

Molecular Dynamics Study of a Model SN1 Dissociation Reaction at Liquid/Liquid Interfaces: Effect of Liquid Polarity

“…The gas phase pure co‐valent state u cov ( R ) is described by a Morse potential. The explicit forms for u ion ( R ) and u cov ( R ) (including the detailed parameterization) have been discussed previously 23,24 …”

“…The PMF can then be calculated as follows: where P b ( R ) is the probability distribution calculated with the biased Hamiltonian. Sixth degree polynomial fits to the PMF result for bulk water and the lowest density region of the water liquid/vapor interface, as calculated in a previous study, 24 work well as a biasing potential for sampling at the water/DCE interface in regions where R < 2.4 Å. The PMF in bulk DCE was similarly sampled with the aid of biasing potentials based on the water/CCl 4 PMF results with a constrained (flat) interface and the gas phase adiabatic t ‐BuCl potential.…”

“…The explicit forms for u ion (R) and u cov (R) (including the detailed parameterization) have been discussed previously. 23,24 b. Solvent potentials. The water potential is described by a fully flexible simple point charge (SPC) model that we have utilized extensively in the past, which includes the intramolecular potential of Kuchitsu and Morino.…”

“…This reaction has been studied both experimentally 15,16 and theoretically [17][18][19][20][21][22] in bulk liquids, including recently in supercritical water. 23 We have studied this S N 1 dissociation using MD at the liquid/vapor interface of water 24 and the water/CCl 4 liquid interface. 25 Our goal is to understand how the unique interfacial properties influence the thermodynamics and dynamics of t-BuCl dissociation.…”