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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2001-01, Vol.105 (1), p.1-4
• Main Authors: Ruscic, Branko, Feller, David, Dixon, David A, Peterson, Kirk A, Harding, Lawrence B, Asher, Robert L, Wagner, Albert F
• Format: Article
• Language: English
• Subjects: CHEMICAL BONDS; DATA; DISSOCIATION; ELECTRONIC STRUCTURE; FORMATION HEAT; HYDROXYL RADICALS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; POTENTIAL ENERGY; WATER
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp003711s

There are two experimental approaches to determining ΔH f0°(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 ΔH f0°(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 ΔH f0°(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 ΔH f0°(OH) based on the spectroscopic approach, the correct value is that of the positive ion cycle, ΔH f0°(OH) = 8.83 ± 0.09 kcal/mol, D 0(H−OH) = 117.57 ± 0.09 kcal/mol, and D 0(OH) = 101.79 ± 0.09 kcal/mol.