Cavity Ringdown Spectroscopy and Kinetics of n-Butoxy Isomerization: Detection of the A-X Band of HOC 4 H 8 OO Matthew K. Sprague 1, Mitchio Okumura 1, and Stanley P. Sander 2 Ohio State 66 th Molecular Spectroscopy Symposium June 21, – Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA – Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109

Atmospheric alkoxy radicals affect HO x and NO x cycles, can isomerize or react with O 2 HO x and NO x both affect tropospheric ozone production Decomposition is negligible (large barrier to reaction) Important to determine the branching ratio of isomerization to O 2 R-CH 2 O R-CH 2 OH R-CHO + HO 2 R + HCHO decomp isom + O 2

Isomerization proceeds through a six membered transition state In the presence of O 2, rapid association occurs to form a hydroxyalkylperoxy radical Smallest alkoxy that can isomerize is n-butoxy Relatively slow isomerization, good starting point for study of larger alkoxy radicals   -hydroxybutylperoxy HOC 4 H 8 OO + O 2

Previous measurements of n-butoxy kinetics made use of end product analysis Problem – assuming kinetic rate constants can add to uncertainty (25-85% for k isom /k O2, 2  ) 1 Better kinetic rate constants can be obtained if we can directly detect HOC 4 H 8 OO 1 – Atkinson et al., J. Atmos. Chem. And Phys., 2006, 3625

GOAL: Obtain a clean, unique spectrum of HOC 4 H 8 OO to measure n-butoxy kinetics Use the A  X Electronic transition At early times, no other absorbers in the NIR region Calculated freqs cm -1 (global min.), 7358 cm -1 (straight chain), B3LYP/6-31+G(d,p) Multiple conformers exist, we expect to observe a broad, unstructured absorption (similar to C 4 H 9 OO) 2 X A 2 – Glover and Miller, J. Phys. Chem. A,, 2005, 11191

We use Cavity Ringdown Spectroscopy (CRDS) to directly detect HOC 4 H 8 OO Light is injected into an optical cavity, and the ringdown time (1/e time of light leaving the cavity) is recorded In the presence of an absorber, the ringdown time decreases, can convert to absorbance Typical sensitivity 1 ppm Hz -½  abs t oo I

Cavity Ringdown Spectrometer Diagram Near IR ( cm -1 ) Nd:YAG laser with SHG = 532 nm Dye laser (DCM) = nm Excimer Laser (XeF) = 351 nm Polished Si to filter visible light Cavity Ringdown / Reaction Cell InGaAs Detector, PC Oscilloscope Raman Shifter (H 2 )  (2nd Stokes) = cm -1

Our chemistry for HOC 4 H 8 OO production Chemistry initiated by pulsed laser photolysis Measure products 10 µs after butyl nitrite photolysis Unaffected by secondary chemistry

Results: HOC 4 H 8 OO A-X Spectrum cm -1 : Increased noise due to background H 2 O, less signal averaging Similar shape to A-X spectrum of n-butyl peroxy (inset) 2 Peak at 7190 cm -1 from other, stable photolysis products (not HOC 4 H 8 OO) 2 – Glover and Miller, J. Phys. Chem. A,, 2005, 11191

For negligible prompt isomerization, relative isomerization yield (  isom ) varies with [O 2 ] according to Use peak at 7556 cm -1 as the measure of isomerization product formed Fit data to a line, y-intercept represents  isom =1 Can only fit data for 1.5e17 cm -3 < [O 2 ] < 1.5e19 cm -3 –Below 1.5e17, dominant product changes to HOC 4 H 8 –Above 1.5e19, prompt isomerization affects measurements We can obtain relative kinetics by measuring absorption as a function of [O 2 ]

Results: Relative Kinetics

k isom /k O2 (  cm -3 ) % Uncertainty Molecule methodp (torr) Ref ± HOC 4 H 8 OO CRDS, A-X330This Work 1.81 ± HOC 4 H 8 OO CRDS, OH stretch 670Garland, submitted 1.95 ± (4-hydroxy) butanal Static, FTIR700Cassanelli, ± butanal Smog, GC760Cox, ± butanal Smog, FTIR700Niki, ± butanal Slow Flow, GC 760Cassanelli, ± butanal Slow Flow, GC 760Cassanelli, ± OH+NO 2 LIF38Hein, ± butanal FTIR760Geiger, ± IUPAC Recommenda tion 760Atkinson, 2006 (All errors reported to 2  ) Our CRDS experiments give lower uncertainties than the previous studies 1 – Atkinson et al., J. Atmos. Chem. And Phys., 2006, 3625

Summary We have detected the A-X electronic transition of HOC 4 H 8 OO –Broad absorption, similar in shape to the A-X band of C 4 H 9 OO –Peaks at 7350 cm -1, 7556 cm -1, shoulder at 7500 cm -1 We have used the A-X band to measure the relative kinetics of n-butoxy isomerization –k isom /k O2 = (1.6 ± 0.4) × cm -3 –CRDS measurements can obtain lower uncertainties than end product studies

Acknowledgements Mitchio Okumura, Stan Sander – Advisors Ralph Page – improvements to optical setup Nathan Eddingsaas – FTIR analysis of C 4 H 9 ONO Funding –Department of Defense NDSEG Fellowship –California Air Resources Board Contracts , –NASA Upper Atmosphere Research Program Grants NAG , NNG06GD88G, NNX09AE21G

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