Solvent Effects on the Energetics of the Phenol O−H Bond: Differential Solvation of Phenol and Phenoxy Radical in Benzene and Acetonitrile

Abstract

Monte Carlo statistical mechanics simulations, density-functional theory calculations, time-resolved photoacoustic calorimetry, and isoperibol reaction-solution calorimetry experiments were carried out to investigate the solvation enthalpies and solvent effects on the energetics of the phenol O−H bond in benzene and acetonitrile. A good agreement between theoretical and experimental results is obtained for the solvation enthalpies of phenol in benzene and acetonitrile. The theoretical calculations also indicate that the differences between the solvation enthalpies of phenol (PhOH) and phenoxy radical (PhO•) in both benzene and acetonitrile are significantly smaller than previous estimations based on the ECW model. The results for the solvation enthalpies are used to obtain the O−H bond dissociation enthalpies in benzene and acetonitrile. For benzene and acetonitrile, the theoretical results of 89.4 ± 1.2 and 90.5 ± 1.7 kcal mol-1, respectively, are in good agreement with the experimental values (90.9 ± 1.3 and 92.9 ± 0.9 kcal mol-1), obtained by photoacoustic calorimetry. The solute−solvent interaction energies of phenol and phenoxy radical with both acetonitrile and benzene differ by less than 2 kcal mol-1. A detailed analysis of the solvent contributions to the differential solvation enthalpy is made in terms of the hydrogen bonds and the solute−solvent interactions. Both PhOH and PhO• induce a significant, although equivalent, solvent reorganization enthalpy. Finally, the convergence of the solute−solvent interaction is analyzed as a function of the distance to the solute and illustrates the advantages and limitations of local models such as microsolvation and hydrogen-bond-only models.