Theoretical studies on the anionic association of phenol and its derivatives in acetonitrile

Abstract

The potentials of mean force (PMF) were determined for homoconjugated systems of anionic complexes composed of phenol and its derivatives with their conjugate anionic bases in acetonitrile (ACN). For each acid-base pair studied, a series of umbrella-sampling molecular dynamics simulations using the AMBER force field with explicit solvent was carried out; the respective PMF was calculated using the Weighted Histogram Analysis Method (WHAM). Subsequently, association equilibrium constants were calculated by numerical integration of the PMF profiles. The PMF curves had a typical shape, i.e., a very deep and narrow contact-minimum, a desolvation maximum, and a shallow solvent-separated minimum. All pairs formed complexes in solution, which was confirmed by the presence of contact minima corresponding to the existence of hydrogen bonds in the PMF curves. The calculated association constants in acetonitrile were subsequently compared with experimentally obtained values for the same solvent.Ab initio methods at the RHF level (utilizing the Gaussian 6-31++G** basis set) enabled the in vacuo calculation of energies and Gibbs free energies of interaction for each phenol derivative and anionic base pair along with a molecule of acetonitrile; this allowed the contribution of the solvent molecule to the PMF to be assessed. It was found that the interaction energies of anionic bases in acetonitrile are stronger than their related neutral phenol derivatives.