S. L. Widicus Weaver, D. E. Woon, B. Ruscic, and B. J. McCall, Astrophys. J. 697, 601-609 (2009)

Is HO₂⁺ a Detectable Interstellar Molecule?

Susanna L. Widicus Weaver,^a,b David E. Woon,^b Branko Ruscic,^c and Benjamin J. McCall^a,b

^a Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
^b Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
^c Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

Astrophys. J. 697(1), 601-609 (2009)

Although molecular oxygen, O₂, has long been thought to be present in interstellar environments, it has only been tentatively detected toward one molecular cloud. The fractional abundance of O₂ determined from these observations is well below that predicted by astrochemical models. Given the difficulty of O₂ observations from ground-based telescopes, identification of a molecule that could be used as a tracer of O₂ in interstellar environments would be quite useful. To this end, we have undertaken a collaborative examination of HO₂⁺ in an attempt to evaluate the feasibility of its detection in interstellar clouds. We have conducted high-level ab initio calculations of its structure to obtain its molecular parameters. The reaction responsible for the formation of HO₂⁺ is nearly thermoneutral, and so a careful analysis of its thermochemistry was also required. Using the Active Thermochemical Tables approach, we have determined the most accurate values available to date for the proton affinities of O₂ and H₂, and the enthalpy, Gibbs energy, and equilibrium constant for the reaction H₃⁺ + O₂ → HO₂⁺ + H₂. We find that while this reaction is endothermic by 50 � 9 cm^-1 at 0 K, its equilibrium is shifted toward HO₂⁺ at the higher temperatures of hot cores. We have examined the potential formation and destruction pathways for HO₂⁺ in interstellar environments. Combining this information, we estimate the HO₂⁺ column density in dense clouds to be ~10⁹ cm^-2, which corresponds to line brightness temperatures of ≤ 0.2 mK. If our results prove correct, HO₂⁺ is clearly not a detectable interstellar molecule.

comment:
The paper reports accurate ATcT values for the proton affinities at 298 K (PA₂₉₈) and 0 K (PA₀) and gas-phase basicities (GB₂₉₈ and GB₀ ≡ PA₀) of molecular hydrogen (H₂) and molecular oxygen (O₂):
H₂ + H⁺ → H₃⁺
O₂ + H⁺ → HO₂⁺
as well as the complete thermochemistry (enthalpy, Δ_rH°_T, Gibbs energy, Δ_rG°_T, equilibrium constant, K_eq(T)) for the nearly thermoneutral proton exchange reaction between hydrogen and oxygen:
H₃⁺ + O₂ ↔ HO₂⁺ + H₂.