PNNL

Abstract

A supermolecule-continuum approach with water clusters up to n = 16 H2O molecules has been used to predict the absolute hydration free energies at 298 K (?Ghyd) of both hydrogen (H•) and hydride (H-) to be 4.6 and -78 ± 2 kcal/mol, respectively. These values are combined with a high accuracy prediction of the gas phase electron affinity (?Ggas,298K = -16.9 kcal/mol) to determine the aqueous electron affinity of H• of 99.5 ± 2 kcal/mol, which yields a reduction potential for H• vs SHE of 0.04 ± 0.1 V. This value is in agreement within 0.2 V with most estimates obtained using a wide variety of approaches. These results can be used to improve the absolute hydricity scale in water which provides additional insights into how a putative hydride interacts with solvent, but do not change the ability to predict the relative reactivity of two species using relative hydricity scales. Research by A.S.M., A.M.A., and D.A.D. was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under the DOE BES Catalysis Center Program. A portion of the computational work was performed using the resources in the Molecular Sciences Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a U.S. DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research, DE-AC06-76RLO 1830. D.A.D. also thanks the Robert Ramsay Chair Fund of The University of Alabama for support.