ATMOSPHERIC CHEMISTRY OF ISOPROPYL FORMATE AND TERT-BUTYL FORMATE
497
•
•
radicals (e.g., CF3CFHO , HOCH2CH2O ) [43]. The
present systems have extra energy available from de-
composition of the acyloxy radical, so the energy that
is deposited can approach the barriers for the unsubsti-
The slope (0.49 ± 0.05) gives the yield of abstraction
by Cl at the formyl hydrogen.
Figure S3 Plot of the reciprocal iPOPAN yield
against the mean ratio NO/NO2 for individual pho-
tolysis periods in the reaction of IPF with Cl atoms in
the presence of NO.
•
•
tuted alkoxy radicals (CH3)2CHO and (CH3)3CO .
In the case of IPF, no products were observed that
could be unequivocally attributed to this activated de-
composition process. However, the possibility also ex-
ists that highly activated isopropoxy radicals could re-
act with O2 more rapidly than thermally equilibrated
radicals. This would explain the fact that the iso-
propyl nitrite and nitrate were typically below the de-
tection limit. Some decomposition of hot isopropoxy
to CH3CHO + CH3 radicals cannot be totally ruled
out.
NCAR is operated by the University Corporation for Atmo-
spheric Research under sponsorship from the National Sci-
ence Foundation. Computer facilities at UNT were purchased
with NSF grant CHE-0342824, and PM is grateful to the
R. A. Welch Foundation (Grant B-1174) and the UNT Fac-
ulty Research Fund for support. We thank Julia Lee Taylor,
Eric Apel, and an anonymous reviewer for their comments
on the manuscript.
Isomerization of alkoxy radicals occurs in both
schemes, leadingtoabstractionof theformyl hydrogen.
•
In the case of HC(O)OC(CH3)2O , the isomerization
BIBLIOGRAPHY
occurs via a five-membered intermediate. Normally
this transition state would not be favorable as a result
of ring strain, but the weak H C(O) bond reduces the
activation energy and enhances the reaction rate. More
theoretical work is clearly needed to understand the
role of these strained isomerization reactions.
Overall, the oxidation of isopropyl and tert-butyl
formates provides a rich chemistry. The mechanisms
proposed here, while not unique, provide a description
of the chemistry, which is in agreement with our under-
standing of ester chemistry based on previous studies.
Reaction Scheme 1 summarizes the mechanisms in-
volved in the formation of the observed products, with
branching fractions shown for the Cl atom reactions.
In the atmosphere, where reaction will proceed by OH
attack, it is expected that IPF will form AFAN (40–
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4
5%), acetone (40–45%), and formic acid (15–20%).
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9
1
1
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small amounts of the acylperoxy nitrates may also be
formed. Hydrolysis and rain-out of the soluble prod-
ucts should also be considered for a full understanding
of the atmospheric fate of these molecules.
SUPPORTING INFORMATION
1
1
1
1
Figure S1 Plot of the yield of (CH3)2CHOC(O)Cl ver-
sus consumption of IPF in the absence of O2. Data
are shown for three experiments with Cl2 in the range
1
4
−3
(
3–34) × 10 molecule cm . The slope (0.30 ± 0.03)
gives the yield of abstraction at the formyl hydrogen.
Figure S2 Plot of the yield of CO2 versus consump-
tion of TBF from three experiments in the absence of
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J. C. Phys Chem Chem Phys 2002, 4, 5789–5794.
14
O2. Filled circles: Cl2 = 6.4 × 10 ; open circles: Cl2 =
18. Ferenac, M. A.; Davis, A. J.; Holloway, A. S.; Dibble,
T. S. J Phys Chem A 2003, 107, 63–72.
15
14
−3
.
1
.3 × 10 ; triangles: Cl2 = 3.2 × 10 molecule cm
International Journal of Chemical Kinetics DOI 10.1002/kin