Evidence for a Lower Enthalpy of Formation of Hydroxyl Radical and a Lower Gas-Phase Bond Dissociation Energy of Water

There are two experimental approaches to determining ΔHf0°(OH), which produce values of this key thermodynamic quantity that differ by >0.5 kcal/mol. The apparent uncertainty of the positive ion cycle approach resides in the measurement of the appearance energy of OH+ from H2O, while the uncertainty of the spectroscopic approach resides in the determination of the dissociation energy of OH(A2Σ+). In this note we present an independent experimental determination of the appearance energy that confirms the accuracy and enhances the precision of the existing positive ion cycle value for ΔHf0°(OH). We also present electronic structure calculations of the OH(A2Σ+) potential energy curve, which suggest that the extrapolation method used to obtain the spectroscopic dissociation energy is in error. Finally, we present the largest ab initio electronic structure calculations ever performed for ΔHf0°(OH) that have an apparent uncertainty much less than 0.5 kcal/mol and support only the positive ion cycle value. Although all major thermochemical tables recommend a value of ΔHf0°(OH) based on the spectroscopic approach, the correct value is that of the positive ion cycle, ΔHf0°(OH) = 8.83 ± 0.09 kcal/mol, D0(H−OH) = 117.57 ± 0.09 kcal/mol, and D0(OH) = 101.79 ± 0.09 kcal/mol.