Cavity Ringdown Spectroscopy and Kinetics of n-Butoxy Isomerization:

Detection of the A-X Band of HOC4H8OO•

Matthew K. Sprague1, Mitchio Okumura1, and Stanley P. Sander2

Ohio State 66th Molecular Spectroscopy SymposiumJune 21, 2011

1 – Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125

2 – Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109

Atmospheric alkoxy radicals affect HOx and NOx cycles, can isomerize or react with O2

• HOx and NOx both affect tropospheric ozone production

• Decomposition is negligible (large barrier to reaction)

• Important to determine the branching ratio of isomerization to O2

R-CH2O•

•R-CH2OH

R-CHO+

HO2•

R•+

HCHO

decomp

isom

+ O2

Isomerization proceeds through a six membered transition state

• In the presence of O2, 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

g-hydroxybutylperoxyHOC4H8OO•

+ O2

Previous measurements of n-butoxy kinetics made use of end product analysis

• Problem – assuming kinetic rate constants can add to uncertainty (25-85% for kisom/kO2, 2s)1

• Better kinetic rate constants can be obtained if we can directly detect HOC4H8OO•

4 9 2 3 7 2

isom4 9 4 8

4 8 2 4 8

4 8 4 8 4 8 4 8 2

4 8 4 8 2

4 8 2 2 3 6

C H O• + O C H CHO + HO •

C H O• HOC H •

HOC H • + O HOC H OO•

HOC H OO• + HOC H OO• HOC H O• + HOC H O• + O

HOC H OO• + NO• HOC H O• + NO •

HOC H O + O HO • + HOC H CHO

1 – Atkinson et al., J. Atmos. Chem. And Phys., 2006, 3625

GOAL: Obtain a clean, unique spectrum of HOC4H8OO• to measure n-butoxy kinetics

• Use the AX Electronic transition

• At early times, no other absorbers in the NIR region

• Calculated freqs. 7777 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 C4H9OO•)2

X

A

2 – Glover and Miller, J. Phys. Chem. A,, 2005, 11191

We use Cavity Ringdown Spectroscopy (CRDS) to directly detect HOC4H8OO•

• 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

oI

0

1 1LA

c

Cavity Ringdown Spectrometer DiagramNear IR (7000-7800 cm-1)

Nd:YAG laser with SHGl = 532 nm

Dye laser (DCM)l = 620-665 nm

Excimer Laser (XeF)

l = 351 nm

Polished Si to filter visible light

Cavity Ringdown / Reaction Cell

InGaAs Detector, PC Oscilloscope

Raman Shifter (H2)D(2nd Stokes) = -8310 cm-1

Our chemistry for HOC4H8OO• production

= 351 nm4 9 4 9

4 9 2 3 7 2

isom4 9 4 8

4 8 2 4 8

4 8 4 8 4 8 4 8 2

4 8 4 8 2

C H ONO C H O• + NO•

C H O• + O C H CHO + HO •

C H O• HOC H •

HOC H • + O HOC H OO•

HOC H OO• + HOC H OO• HOC H O• + HOC H O• + O

HOC H OO• + NO• HOC H O• + NO

h

4 8 2 2 3 6

•

HOC H O + O HO • + HOC H CHO

Chemistry initiated by pulsed laser photolysis

• Measure products 10 µs after butyl nitrite photolysis

• Unaffected by secondary chemistry

Results: HOC4H8OO• A-X Spectrum

• 7150-7400 cm-1: Increased noise due to background H2O, 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 HOC4H8OO•)2 – Glover and Miller, J. Phys. Chem. A,, 2005, 11191

• For negligible prompt isomerization, relative isomerization yield (fisom) varies with [O2] according to

• Use peak at 7556 cm-1 as the measure of isomerization product formed

• Fit data to a line, y-intercept represents fisom=1

• Can only fit data for 1.5e17 cm-3 < [O2] < 1.5e19 cm-3

– Below 1.5e17, dominant product changes to HOC4H8•– Above 1.5e19, prompt isomerization affects measurements

We can obtain relative kinetics by measuring absorption as a function of [O2]

2

2

11O

isom isom

kO

k

2

2

,isom 4 9 4 8

O 4 9 2 2 3 7

: C H O• HOC H OO•

: C H O• + O HO • + C H CHO

isom Ok

k

Results: Relative Kinetics

2

2

11O

isom isom

kO

k

2

19 -31.6 0.4 10 cmisom

O

k

k

kisom/kO2

( 1019 cm-3)%

UncertaintyMolecule method p

(torr)Ref1

1.6 ± 0.4 25 HOC4H8OO• CRDS, A-X 330 This Work

1.81 ± 0.15 8 HOC4H8OO• CRDS, OH stretch

670 Garland, submitted

1.95 ± 0.4 21 (4-hydroxy) butanal Static, FTIR 700 Cassanelli, 2006

1.5 ± 0.5 33 butanal Smog, GC 760 Cox, 1981

1.9 ± 0.4 21 butanal Smog, FTIR 700 Niki,1981

2.1 ± 0.5 24 butanal Slow Flow, GC

760 Cassanelli, 2005

1.8 ± 1.1 61 butanal Slow Flow, GC

760 Cassanelli, 2005

0.25 ± 0.19 76 OH+NO2LIF 38 Hein, 1999

1.8 ± 0.6 33 butanal FTIR 760 Geiger, 2002

2.1 ± 1.8 86 IUPAC Recommenda

tion

760 Atkinson, 2006

(All errors reported to 2s)

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 HOC4H8OO•– Broad absorption, similar in shape

to the A-X band of C4H9OO•

– 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– kisom/kO2 = (1.6 ± 0.4) × 1019 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 C4H9ONO

• Funding– Department of Defense NDSEG Fellowship– California Air Resources Board Contracts 03-333, 07-730– NASA Upper Atmosphere Research Program Grants

NAG5-11657, NNG06GD88G, NNX09AE21G

Thanks for giving me the chance to share our work!