A computational study of the hydration of the OH radical

The OH radical is an important species in natural and man made aqueous environments, influencing diverse processes such as the oxidation of atmospheric pollutants or the development of some diseases, Yet, little is known about the solvation thermodynamics and structure of the hydration shell of OH. Here, we present a computational study of the hydration of OH in small H2n+1On+1 (n = 1-5) clusters. We begin by comparing three different quantum chemical methods, UMP2, BLYP, and BHLYP. We find that BLYP does not describe correctly the OH-H2O interaction as compared to the current MP2 or other high ab initio calculations found in the literature. BLYP favors the formation of hemibonded H2O-OH structures, whereas MP2 predicts that hydrogen-bonded complexes are more stable. Mixing Becke's exchange functional with 50% Hartree-Fock exchange improves the DFT description, yielding results that are similar to those from MP2. We rind that the H2n+1On+l clusters form structures in which all species are donors and acceptors in hydrogen bonded rings similar to those of pure water clusters. OH participates in two or three hydrogen bonds. Structures in which OH forms more than three hydrogen bonds are not favored energetically. We report values of energy, enthalpy, and Gibbs free energy of complexation in the gas phase. OH(g) + H2nOn(g) --> H2n+1On+1(g), as a function of cluster size. We also estimate values of thermodynamic parameters of hydration in the liquid phase from OH(g) + H2nOn(aq) --> H2n+1On+1(aq), where the energies of the aqueous species, H2nOn(aq) and H2n+1On+1(aq), are calculated by means of a hybrid solvation model in which part of the solvent is treated explicitly and the long-range interactions are added into the Hamiltonian by means of the PCM version of the self-consistent reaction field. The implications of our work as well as the accuracy of the results are also discussed.