Redox Potentials for Hydrogen Species in Acetonitrile and Tetrahydrofuran

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 pK_a(H₂) is predicted to be 43.0 in MeCN and 49.3 in THF. The proton-coupled electron transfer (PCET) accounting term C_G 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, pK_a, and C_G.