The H•/H- Redox Couple and Absolute Hydration Energy of H.

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 (ΔGhyd) of both hydrogen (H•) and hydride (H-) to be 4.6 ± 1 kcal/mol and -78 ± 3 kcal/mol, respectively. These values are combined with a high accuracy prediction of the gas phase electron affinity (ΔGgas,298K = -16.9 kcal/mol) to determine the aqueous electron affinity of H• of 99.5 ± 3 kcal/mol, which yields a reduction potential for H• vs SHE of -0.03 ± 0.15 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.