The properties of the fundamental hydrogen species, proton (H+), hydrogen atom (H•), and hydride anion (H–) are critical to a vast range of chemical processes, yet their thermodynamic properties in nonaqueous solvents are not well established. A hybrid supermolecule-continuum approach is used to predict the Gibbs free energies of solvation (ΔG°solv) and standard redox potentials (E°) for the 2H+/H2 and H•/H– couples in acetonitrile (MeCN) and tetrahydrofuran (THF) following the approach previously used for water. Gas phase and solution phase structures were optimized with ωB97X-D/aug-cc-pVTZ. Solvation was treated with the SMD self-consistent reaction field model, and CCSD(T)/aug-cc-pVTZ gas phase calculations were used as a benchmark for method sensitivity. For H+, the ΔG°solv values are predicted to be −258.7 kcal/mol in MeCN and −259.2 kcal/mol in THF. The standard redox potentials for the 2H+/H2 couple are +0.21 V (±0.1 V) in MeCN and +0.18 V (±0.15 V) in THF relative to the aqueous SHE (E°SHE = 4.28 V on the absolute scale), consistent with the available experimental data. The ΔG°solv for H– is predicted to be −76.9 kcal/mol in MeCN and −67.8 kcal/mol in THF, and for H•, ΔG°solv is predicted to be 2.1 kcal/mol in both MeCN and THF. These solvation energies yield calculated redox potentials for the H•/H– couple of −0.12 V (±0.3 V) in MeCN and −0.52 V (±0.1 V) in THF relative to the aqueous SHE. H2 solvation is near thermoneutral in both solvents. The pKa(H2) is predicted to be 43.0 in MeCN and 49.3 in THF. The proton-coupled electron transfer (PCET) accounting term CG was evaluated on the ferrocene scale and deviates from the reported literature values by 7.5 kcal/mol in MeCN and 5 kcal/mol in THF. These results establish an internally consistent thermodynamic framework for hydrogen redox chemistry in MeCN and THF and improve consistency across E°, ΔG°solv, pKa, and CG.
Duda et al. (Tue,) studied this question.