and acetaldehyde) are formed from both the (CH
3
)
2
CHCH-
C-
radical (Scheme 2). Summing the
the n-octane reaction (9), where none of the products formed
arose from alkoxy radical decomposition (rather all of the
first-generation alkoxy radicals appeared to undergo isomer-
•
(
(
CH
O )CH(CH
3
)CH(CH
3
)CH
2
O radical (Scheme 1) and the (CH
3
)
2
•
3
)CH(CH
3
)
2
formation yields of 2-propyl nitrate and acetone (because
ization to form hydroxycarbonyls and hydroxynitrates, in
•
•
the CH
3
CH(OO )CH
3
radical forms either acetone or 2-propyl
addition to the octyl nitrates formed from the RO
2
+ NO
nitrate), then our observed products arising from the (CH
3
)
3
2
-
)-
reactions). Clearly, the higher the degree of branching, the
greater the importance of alkoxy radical decomposition and
the higher the yields of carbonyl compounds with less carbon
atoms than the parent alkane. This may have implications
for secondary organic aerosol formation from alkane atmo-
spheric photooxidations, with branched alkanes leading to
a higher fraction of products containing fewer carbon atoms
than their precursor alkane and thus potentially forming less
aerosol than the corresponding n-alkane of the same carbon
number.
•
•
CHCH(CH
3
)CH(CH
3
)CH
2
O (Scheme 1), (CH
CHCH(CH
3
)
2
C(O )CH(CH
•
CH(CH (Scheme 2), and (CH
3
)
2
3
)
2
2
O )CH(CH
3
)
2
(no
Scheme shown) radicals are: acetone + 2-propyl nitrate (41
13%, after subtracting the 41% attributed to the coproducts
(
of 3-methyl-2-butanone in Scheme 3), 3-methyl-2-butyl
nitrate (1.6 ( 0.2%), and acetaldehyde (47 ( 6%). These
correspond to 28% (as carbon) of the reaction products. While
•
some C
CH(CH
8
-alkyl nitrate will be formed from the (CH
)CH(CH
3
)
2
C(OO )-
3
3
)
2
+ NO reaction, this 41 ( 13% yield of
acetone plus 2-propyl nitrate (i.e., other than that formed as
a coproduct to 3-methyl-2-butanone) indicates that H-atom
abstraction from the 2- and 4-position CH groups cannot
account for more than 25-30% of the overall reaction,
significantly less than the predicted value of 56% (29) and
consistent with the above-noted low yield of 3-methyl-2-
butyl nitrate (see Scheme 2).
Acknowledgments
The authors thank the California Air Resources Board (CARB)
for supporting this research through Contract 99-330. While
this research has been funded by the CARB, the results and
contents of this publication do not necessarily reflect the
views and opinions of the agency.
A plausible range of product distributions is shown in
•
Table 1, assuming the (CH
3
)
2
CHC(CH
3
)(O )CH(CH
3
)
2
radical
Literature Cited
shown in Scheme 3 is formed in 40% yield (see above) and
3 2
considering two possible formation yields of the (CH ) -
(
1) Hoekman, S. K. Environ. Sci. Technol. 1992, 26, 1206.
•
•
(2) Lurmann, F. W.; Main, H. H. Analysis of the Ambient VOC Data
Collected in the Southern California Air Quality Study; Final
Report to California Air Resources Board Contract A832-130,
Sacramento, CA, February 1992.
CHCH(CH
3 3 2 3 2 3
)CH(CH )CH O and (CH ) C(O )CH(CH )CH-
(
CH radicals shown in Schemes 1 and 2, respectively. The
3 2
)
distribution of the products for the individual radicals are
those given above for the individual alkoxy radicals and were
predicted using the methods described by Atkinson (5), as
revised by Aschmann and Atkinson (31) [see also Supporting
Information in Aschmann et al. (32)].
(
3) Calvert, J. G.; Atkinson, R.; Becker, K. H.; Kamens, R. M.; Seinfeld,
J. H.; Wallington, T. J.; Yarwood, G. The Mechanisms of
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(
(
(
4) Atkinson, R.; Arey, J. Chem. Rev. 2003, 103, 4605.
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Technol. 1995, 29, 232.
The predicted product yields are consistent with our
measured values, except that our measured 2-propyl nitrate
yield is a factor of 3 higher than that predicted. For the
predicted product distributions shown in Table 1, H-atom
abstraction from the CH groups at the 2- and 4-positions
accounts for 10-15% plus the percentage formation of the
(7) Atkinson, R.; Kwok, E. S. C.; Arey, J.; Aschmann, S. M. Faraday
Discuss. 1995, 100, 23.
(
8) Kwok, E. S. C.; Arey, J.; Atkinson, R. J. Phys. Chem. 1996, 100,
14.
2
8 3 2 2 3 3 2
C -alkyl nitrate (CH ) C(ONO )CH(CH )CH(CH ) ; hence
(
9) Arey, J.; Aschmann, S. M.; Kwok, E. S. C.; Atkinson, R. J. Phys.
Chem. A 2001, 105, 1020.
expected (33) to be e20% and a factor of ∼3 lower than
predicted from the Kwok and Atkinson (29) estimation
method. This discrepancy can be attributed to steric hin-
drance at the 2- and 4-position CH groups toward OH radical
reaction, similar to the conclusion drawn for reaction of OH
radicals with 2,2,4-trimethylpentane (23). An alternative
(
(
10) Aschmann, S. M.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2001,
05, 7598.
11) Reisen, F.; Aschmann, S. M.; Atkinson, R.; Arey, J. Environ. Sci.
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1
(12) Atkinson, R. Atmos. Chem. Phys. 2003, 3, 2233.
(13) Cox, R. A.; Derwent, R. G.; Williams, M. R. Environ. Sci. Technol.
1980, 14, 57.
explanation, but thought to be less likely, is that isomerization
•
of the (CH
3
)
2
C(O )CH(CH
3
)CH(CH
3
)
2
radical (Scheme 2) is
(
(
(
14) Takagi, H.; Washida, N.; Bandow, H.; Akimoto, H.; Okuda, M.
J. Phys. Chem. 1981, 85, 2701.
15) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.;
Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563.
much more important than predicted and accounts for ∼30-
5% of the overall reaction, leading to hydroxycarbonyl and
hydroxynitrate products that were not quantified. Note that
the formation of the observed C - and C -hydroxynitrates of
3
5
8
16) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,
molecular weight 149 and 191, respectively, is predicted (see
Scheme 1). Additionally, based on data for alkanes, including
8
7, 2012.
(17) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.;
Pitts, J. N., Jr. Int. J. Chem. Kinet. 1984, 16, 1085.
(18) Atkinson, R.; Aschmann, S. M.; Winer, A. M. J. Atmos. Chem.
branched alkanes (33), ∼15% of C
to be formed from the reactions of the various octyl peroxy
radicals with NO. The octyl nitrate, C - and C -hydroxyni-
trates, and C -hydroxycarbonyl products combined with
those quantified (Table 1) and with the HCHO anticipated
8
-alkyl nitrates are expected
1
987, 5, 91.
5
8
(
(
19) Harris, S. J.; Kerr, J. A. Int. J. Chem. Kinet. 1989, 21, 207.
20) Aschmann, S. M.; Chew, A. A.; Arey, J.; Atkinson, R. J. Phys.
Chem. A 1997, 101, 8042.
8
as a coproduct to the acetaldehyde formed from the (CH
3
)
2
-
(
(
(
(
(
21) Platz, J.; Sehested, J.; Nielsen, O. J.; Wallington, T. J. J. Phys.
Chem. A 1999, 103, 2688.
22) Orlando, J. J.; Iraci, L. T.; Tyndall, G. S. J. Phys. Chem. A 2000,
•
CHCH(CH
3
)CH(CH
3
)CH
2
O
radical (Scheme 1) may well
account for all of the reaction products.
1
04, 5072.
23) Aschmann, S. M.; Arey, J.; Atkinson, R. Environ. Sci. Technol.
002, 36, 625.
The reaction of OH radicals with 2,3,4-trimethylpen-
tane therefore leads to ∼50-55% of the product carbon
being formed from first-generation alkoxy radical decom-
position reactions. This can be compared to the 2,2,4-
trimethylpentane reaction (23), where 40 (5% of the products
were readily observed by GC methods and attributed to
decomposition of the first-generation alkoxy radicals and to
2
24) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Environ. Sci.
Technol. 1995, 29, 1674.
25) Taylor, W. D.; Allston, T. D.; Moscato, M. J.; Fazekas, G. B.;
Kozlowski, R.; Takacs, G. A. Int. J. Chem. Kinet. 1980, 12,
231.
5
0 4 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 19, 2004