Is HO2+ a Detectable Interstellar Molecule?

Susanna L. Widicus WeaverDepartments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

Current address: Department of Chemistry, Emory University

David E. WoonDepartment of Chemistry, University of Illinois at Urbana-Champaign

Branko RuscicChemical Sciences and Engineering Division, Argonne National Laboratory

Benjamin J. McCallDepartments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

Larsson et al., 2007, A&A 466, 999

Interstellar O2

X(O2)

Predicted: 5-10 × 10-6

Odin, -Oph: 5 × 10-8

SWAS limits: < 3 × 10-7

O2 Detection Difficulties

O2

• No permanent electric dipole moment

• Weak magnetic dipole-allowed transitions

• Atmospheric spectral interference

An O2 tracer? HO2+

• a = 1.518 Db = 1.934 D

• Strong millimeter and submillimeter spectrum

• Can be observed from ground-based observatories

Is HO2+ observable?

Interstellar HO2+ Chemistry

H3+ + O2 HO2

+ + H2

Formation:

Destruction: Reverse of this same reaction.

)H(

)O()H()HO(

2

23

1

12

n

nn

k

kn

When steady-state is reached, a true

chemical equilibrium exists!

)H(

)O()H(

2

23n

nnKT

)HO( 2n

kT

E

)O()H(

)H()HO(570.0

23

220

intint

intintT

qq

qqeK

Required Information?

)H(

)O()H(

2

23n

nnKT

)HO( 2n

TkT

E

T QeK0

1.

)O()H(

)H()HO(

23

22

qq

qqQT

2.

)X(2

)X()X()X(2

3

2 intinttr qVh

mkTqqq

3.

)O()H(

)H()HO(570.0

23

22

intint

intint

qq

qq

)O()H(

)H()HO(

)O()H(

)H()HO(

23

222

3

23

22

intint

intint

Tqq

qq

mm

mmQ

4.

where E0/k = rH°/R

Experimental HO2+

Thermochemistry Results

rH°298

(kJ/mol)rG°298

(kJ/mol)Reference

-0.16 ± 0.57 -0.16 ± 0.57 Fennelly et al. (1973)

-1.5 ± 1.4 Fehsenfeld et al. (1975)

-2.1 ± 1.9 Fehsenfeld et al. (1975)

-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 rH°298 = - 0.2092 kJ/molRuscic 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+

rH°298 = 1.31 ± 0.11 kJ/mol

rG°298 = -1.75 ± 0.11 kJ/mol

Partition Functions

Equilibrium Constant

Nuclear Spin Selection Rules

O2 +

22.8 cm-1

0 cm-1

o-H3+

p-H3+

o-H2

p-H2

0 cm-1

118.5 cm-1

+ HO2+

2/3

1/2

1/21/3

1

1

O2 + p-H3+ → p-H2 + HO2

+ k = k1/4so 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()H(

2

23n

nnKT

)HO( 2n

(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

ConclusionsMost accurate theoretical investigation of HO2

+ to date:

• Thermochemistry- HO2

+ formation rH°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 K- 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