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Is HO 2 + a Detectable Interstellar Molecule?

Is HO 2 + 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

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Is HO 2 + a Detectable Interstellar Molecule?

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  1. 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 Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

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

  3. O2 Detection Difficulties • O2 • 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?

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

  5. Required Information? E 0 - = K e Q kT T T + q ( HO ) q ( H ) 2 2 = Q T + q ( H ) q ( O ) 3 2 3 æ ö p 2 mkT 2 3. = = ç ÷ q ( X ) q ( X ) q ( X ) V q ( X ) 1. tr int int 2 h è ø 3 æ ö + + 2 m ( HO ) m ( H ) q ( HO ) q ( H ) 2. 4. ç ÷ 2 2 int 2 int 2 = Q ç ÷ T + + m ( H ) m ( O ) q ( H ) q ( O ) è ø 3 2 int 3 int 2 + q ( HO ) q ( H ) int 2 int 2 = 0 . 570 + q ( H ) q ( O ) int 3 int 2 n ( O ) + = 2 K n ( H ) + T 3 n ( HO ) n ( H ) + E q ( HO ) q ( H ) 2 2 0 - where E0/k = DrH°/R int 2 int 2 = K 0 . 570 e kT T + q ( H ) q ( O ) int 3 int 2

  6. Experimental HO2+Thermochemistry Results Ruscic et al. suggest DrH°298 = - 0.2092 kJ/mol Ruscic et al., J Phys Chem A (2006) 110, 6592

  7. 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

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

  9. Partition Functions

  10. Equilibrium Constant

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

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

  13. 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

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

  15. Conclusions • 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

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