1. Stability of the HOOO Radical via Infrared Action Spectroscopy
Timothy D. Sechler, Craig Murray,
Erika L. Derro and Marsha I. Lester
Department of Chemistry, University of Pennsylvania, Philadelphia PA 19104

2. Atmospheric Relevance
HOOO has been implicated as a key intermediate in atmospheric reactions:
O + HO2 → OH + O2
H + O3 → OH(v ≤ 9) + O2
OH(v) + O2 → OH(v – 1) + O2
More importantly, a potential sink for hydroxyl radicals
OH + O HOOO

3. Background Two predicted conformers Limited experimental studies
trans-HOOO observed in FTMW study
Suma et al. Science (2005)
Indirect measure of stability: ΔfH298 K = –1 ± 5 kcal mol–1
Speranza Inorg. Chem (1996)
Generally there is poor agreement with theory in both geometry and stability
cis-HOOO
trans-HOOO

4. Experimental Setup Infrared Action Spectroscopy
Fix UV and scan IR: collect vibrational spectrum
OH A2Σ+
HONO2 in 20% O2/Ar
photolysis UV IR
x/D = 15
Fix IR and scan UV: probe OH product states
UV (probe)
Collect LIF
predissociation
OH X2Π+ O2 X3 Σg–
IR (pump)
2νOH
HOOO X2A″

5. IR Action Spectrum of HOOO: OH stretch overtone
Fix IR
UV probe fixed A-X (1,1) P1(4)
Simulation using FTMW
rotational constants for
trans-HOOO
Band Origin cm-1
Rotational Temp. 9 K
Lorentzian Linewidth 0.20 cm → 27 ps
Unstructured feature
red-shifted ~ 7 cm-1?
Total simulation

6. OH (ν=1) Product State Distribution
P1e(8)
2P3/2
OH Internal Energy / cm -1
3500
4000
4500
5000
Relative Population 1 5 3 7 2 4 6 8
Upper Limit of D0 determined from highest observed product channel
Eint(HOOO) + hνIR – D0 = Eint(OH) + Eint(O2) + KE
Eint(HOOO) – Negligible
hνIR – cm-1
Eint(OH) – cm-1
Eint(O2) – ?
KE – ?
D0 ≤ 2140 cm–1 or kcal mol–1

7. Statistical Calculation of Kp
OH + O HOOO
Harmonic oscillator (solid) Free internal rotor (dashed)
Enthalpic contributions from thermal rotation/translation, H = E + pV, and thermal vibration
S related to partition functions

8. Atmospheric Profile of HOOO
OH + O HOOO ΔH ~ -7 kcal mol-1
Harmonic oscillator (solid)
Free internal rotor (dashed)
Stratosphere
Tropopause
Troposphere

9. Summary
Observed trans-HOOO in the OH stretch overtone region using infrared action spectroscopy
Band origin shifted 2.8 cm-1 from free OH
Lifetime 27 ps
Established an upper limit for the stability D0 ≤ 2140 cm–1 or 6.12 kcal mol–1
Statistical thermodynamics calculations suggest that up to 66% of atmospheric OH may exist as HOOO
Murray, Derro, Sechler and Lester, J. Phys. Chem. A 111, 4727 (2007).
RH03 Hua-Gen Yu, Quantum Dynamics Studies of the Vibrational States of HO3(X2A'').