Recommended Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species

Summary

Datasets in digital electronic formats are provided for data contained in the publication "Recommended Values for the Gas Phase Enthalpies of Formation of Hydrogen-Oxygen Species;" J. Res. Natl. Inst. Stand. Technol. 121, 108-138 (2016); DOI: 10.6028/jres.121.005.

In this work, we have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (H_xO_y) and selected values for use. The compilation consists of values derived from experimental measurements, quantum chemical calculations, and evaluations. This work updates the recommended values in the NIST-JANAF (1985) and Gurvich et al (1989) thermochemical tables for seven species. For two species, HO₃ and H₂O₃ (important in atmospheric chemistry) and not found in prior thermochemical evaluations, we also provide tables of thermochemical functions (C_p, S°, H°, and Δ_fH°) as a function of temperature. In this work, we also provide supplementary data for the species consisting of zero point energies, vibrational frequencies, and ion reaction energetics.

DESCRIPTION

In this work, we have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (HxOy) and selected values for use. Compared to uncertainties for the recommended values in the NIST-JANAF tables (1998) and the thermochemical tables of Gurvich et al (1989), the updated values are (50 to 100) times more precise for H atom, O atom, and O₃, (5 to 10) times more precise for OH and HO₂, and (25 to 35)% more precise for H2O and H2O₂. Enthalpies of formation for the species HO₃ and H2O₃, which are important in atmospheric chemistry, are not found in prior thermochemical evaluations. For these species, we also provide tables of thermochemical functions (Cp, S°, H°, and ΔH°) as a function of temperature (along with recommended molecular geometries and vibrational frequencies).

This compilation of gas phase enthalpies of formation consists of values derived from experimental measurements, quantum chemical calculations, and evaluations. We list not only recent, more precise values, but older values for archival purposes. Uncertainties (where available) are given along with the enthalpies of formation. The uncertainties listed are in general 2σ (a coverage factor of 2 having a level of confidence of approximately 95%). Some workers did not explicitly state whether the uncertainties are 1σ (standard uncertainty) or 2σ – in these cases, we provide the uncertainties as reported.

In the tables of enthalpies of formation for each species, we also identify the (experimental or computational) method used to derive the enthalpies of formation. A glossary for the notations used in the methods is provided. In the tables comments, we provide the reported values for dissociation energies, heats of reaction, appearance energies, or other quantities used to derive the enthalpy of formation. We provide these values in the original reported units. We utilized the following conversion factors: 1 kJ mol^-1 ≡ 83.593472(4) cm^-1, 1 eV ≡ 96.4853365(21) kJ mol^-1, 1 eV ≡ 8065.544005(50) cm^-1, 1 hartree ≡ 2625.49962(13) kJ mol^-1, and Rydberg Constant (R8) ≡ 109737.31568539(55) cm^-1 taken from the CODATA values for the fundamental constants by Mohr et al and 1 kcal mol^-1 ≡ 4.184 kJ mol^-1 from the NIST-JANAF tables.

Many of the values selected for the HxOy species are based on the ATcT (Active Thermochemical Tables) thermochemical network approach of Ruscic and coworkers. We only include those ATcT values from published work and not those available only online (which have no documentation nor references).

For each of the species, we provide a table of gas phase enthalpies of formation along with a short discussion. This is followed by a summary table of the selected values of the enthalpies of formation.

In addition to the compilation of gas phase enthalpies of formation for these hydrogen-oxygen species, we provide supplementary data consisting of zero point energies, vibrational frequencies, and ion reaction energetics. Zero point energies (ZPEs) are necessary when deriving ground state total atomization energies (ΣD₀) used to compute enthalpies of formation Δ_fH°(0 K) from electronic total atomization energies (ΣD_e) obtained from quantum chemical calculations. We also provide a table with vibrational frequencies, both experimental fundamental (anharmonic) frequencies and calculated (or derived) harmonic frequencies. In addition, we provide ion reaction energetics, such as ionization energies, electron affinities, and appearance energies. These quantities are utilized to compute enthalpies of formation using thermochemical cycles involving ion processes.

Datasets for Thermochemical Data for Hydrogen-Oxygen Species

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