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

2. 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

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

4. 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

5. 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

6. 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 = 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 = ± 0.11 kJ/mol

9. Partition Functions

10. Equilibrium Constant

11. 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

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
at 47.2, 102.5, GHz
100 K
hot cores

14. 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

15. 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 )
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