First principles computational chemistry predictions of equilibrium and kinetic constants for electrophilic reactions in water
Aqueous electrophilic reactions are broadly important in environmental chemistry, but the thermodynamic equilibrium constants, rate constants, and mechanisms describing these reactions are often difficult to determine by experiment. Here we report on our efforts to establish thermodynamic and kinetic constants of aqueous electrophilic reactions by way of ab initio computational chemistry. To accomplish this, we employ a modular suite of specialized computational techniques. We first estimate reaction free energies (for equilibrium constants) or activation free energies (for rate constants) in the gas phase, employing a combination of high-quality Coupled Cluster computations and Density Functional Theory methods. To account for the influence of aqueous solvent, we conduct additional simulations with implicit solvation models and/or microsolvated clusters of solute and water molecules, with careful attention to the handling of standard states. The resulting model predictions are relatively independent of experimental parameterization and can be applied to a very broad range of reactions, in principle. We report on comparisons of our computational results with available experimental data for several electrophilic reactions in water. These include: the equilibrium constants describing the reaction of NH3 with HOCl and HOBr to form NH2Cl and NH2Br, respectively; the equilibrium constant for the dehydration of HOCl to form Cl2O; the kinetic constant of O3 reaction with Br- to form HOBr; and the kinetic constants for the reaction of HOBr with NH3 and the reaction of HOBr with dimethylamine. These preliminary results suggest that the tested prediction approaches have order-of-magnitude uncertainty for equilibrium constants involving neutral solutes, and 2-3 orders-of-magnitude uncertainty for rate constants. As we illustrate with selected case studies, this level of accuracy is sufficient to provide useful estimates of thermodynamic and kinetic properties in cases where they have not been successfully elucidated by experiment. For example, our recent computational estimates of thermodynamic properties enabled us to ascertain the relative stabilities of bromamines, chloramines, and bromochloramines during disinfection of drinking and swimming pool water.
Record created on 2016-03-04, modified on 2016-12-27