Is HO2+ a Detectable Interstellar Molecule? Susanna L. Widicus Weaver Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign Current address: Department of Chemistry, Emory University David E. Woon Department of Chemistry, University of Illinois at Urbana-Champaign Branko Ruscic Chemical Sciences and Engineering Division, Argonne National Laboratory Benjamin J. McCall

Interstellar O2 X(O2) Predicted: 5-10 × 10-6 Odin, r-Oph: 5 × 10-8 SWAS limits: < 3 × 10-7 Larsson et al., 2007, A&A 466, 999

O2 Detection Difficulties No permanent electric dipole moment Weak magnetic dipole-allowed transitions Atmospheric spectral interference An O2 tracer? HO2+ ma = 1.518 D mb = 1.934 D Strong millimeter and submillimeter spectrum Can be observed from ground-based observatories Is HO2+ observable?

When steady-state is reached, a true chemical equilibrium exists! Interstellar HO2+ Chemistry Formation: Destruction: Reverse of this same reaction. H3+ + O2 HO2+ + H2 ) H ( O HO 2 3 1 n k + - = When steady-state is reached, a true chemical equilibrium exists! ) H ( O 2 3 n K T + = HO

Required Information? ) H ( O n K = HO 2 3 T + ) X ( 2 q V h mkT ÷ ø ö int tr q V h mkT ÷ ø ö ç è æ = p 3. T kT E Q e K - = 1. + ) O ( H HO 570 . 2 3 int q = T m Q ÷ ø ö ç è æ 4. ) O ( H HO 2 3 q Q T + = 2. kT E - ) O ( H HO 570 . 2 3 int T q e K + = where E0/k = DrH°/R

Experimental HO2+ Thermochemistry Results DrH°298 (kJ/mol) DrG°298 (kJ/mol) Reference -0.16 ± 0.57 Fennelly et al. (1973) -1.5 ± 1.4 Fehsenfeld et al. (1975) -2.1 ± 1.9 -2.0 ± 1.0 -1.7 ± 1.0 Kim et al. (1975) -0.0 ± 0.15 -3.0 ± 1.5 Hiraoka et al. (1979) 1.4 ± 3.3 -1.7 ± 1.5 Bohme et la. (1980) 1.4 ± 0.3 -1.48 ± 0.49 Adams & Smith (1984) 1.3 ± 11 -1.6 ± 11 Hunter & Lias (2005) Ruscic et al. suggest DrH°298 = - 0.2092 kJ/mol Ruscic et al., J Phys Chem A (2006) 110, 6592

Active Thermochemical Tables Traditional compilations – sequential approach available information used only partially propensity to develop cumulative errors assigned uncertainties do not properly reflect the available knowledge contain a hidden maze of progenitor-progeny dependencies ATcT approach – simultaneous analysis of interdependencies solutions reflect cumulative knowledge of network propagates new knowledge by solving entire network from scratch points to new experiments by isolating “weak links” complete covariance matrix available: prevents inflation of uncertainties

ATcT Results for HO2+ DrH°298 = 1.31 ± 0.11 kJ/mol DrG°298 = -1.75 ± 0.11 kJ/mol

Partition Functions

Equilibrium Constant

Nuclear Spin Selection Rules 22.8 cm-1 0 cm-1 o-H3+ p-H3+ o-H2 p-H2 118.5 cm-1 + HO2+ 2/3 1/2 1/3 1 O2 + p-H3+ → p-H2 + HO2+ k = k1/4 so K = KT/4

Spectral Prediction T = 100 K Simulated with PGopher (Western 2007) using constants from previous talk.

Is Interstellar HO2+ Detectable? Transitions? - B, C are known to ~40 MHz - A is known to ~5 GHz Temperature? - intensities scale as (KT/QT)e-Eu/kT Sources? - high n(H3+) - T~100 K 1 0 1 0 0 0 at 47.2, 102.5, 412.9 GHz 100 K hot cores

Is Interstellar HO2+ Detectable? ) H ( O 2 3 n K T + = HO (10-4 cm-3) (10-5) 0.6765 = 7×10-10 cm-3 = For L = 1 pc, NT = 2×10-9 cm-2 TMB ~ 6×10-5 K!! Clearly, HO2+ is not detectable

Conclusions Acknowledgements Most accurate theoretical investigation of HO2+ to date: Thermochemistry - HO2+ formation DrH°298 = 1.31 ± 0.11 kJ/mol Molecular constants, dipole moment - Rotational spectral prediction Examination of interstellar chemistry, likelihood of detection - Unusual case of interstellar chemical equilibrium - Line intensities ~ 60 mK - HO2+ not detectable Acknowledgements NSF CAREER award (NSF CHE-0449592) UIUC Critical Research Initiative program Prof. Thom H. Dunning, Jr. Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, US Department of Energy, contract number DEAC02-06CH11357 Task Group of the International Union of Pure and Applied Chemistry (IUPAC) on `Selected Free Radicals and Critical Intermediates: Thermodynamic Properties from Theory and Experiment' [IUPAC Project 2003-024-1-100