J. Am. Chem. Soc. 1999, 121, 10225-10226
10225
Experimental Study of Intermediates from
OH-Initiated Reactions of Toluene
The experimental method employed in this study is similar to
that used previously in our laboratory for chemical kinetics studies
of elementary gas-phase reactions.7 We employed a fast-flow
reactor coupled to chemical ionization mass spectrometry (CIMS)
detection, using both positive and negative reagent ions. The
CIMS apparatus was fit with an electrostatic ion guide recently
,8
,
†
‡
†
‡
M. J. Molina,* R. Zhang, K. Broekhuizen, W. Lei,
†
†
R. Navarro, and L. T. Molina
9
Department of Earth, Atmospheric, and Planetary Sciences
and Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, and
Department of Meteorology, Texas A&M UniVersity,
College Station, TX 77843
developed in our laboratory. The detection sensitivity of the
7
8
CIMS system was generally in the range from 10 to 10 molecule
-
3
cm with a S/N ratio of unity for a one second integration time.
OH radicals were generated by passing a small amount of H O
2
or H through a microwave discharge. An excess amount of ozone
2
was added downstream of the discharge cavity to scavenge the
H atoms and to convert them to OH. Alternatively, OH was
ReceiVed July 13, 1999
generated by the reaction of H-atoms with NO
with H O. The OH radical was detected in the negative ion mode
using SF6 as the reagent ion. The OH concentration was
calibrated by converting it to HNO , followed by independent
2
or of F-atoms
Aromatic hydrocarbons constitute a significant fraction of the
total volatile organic compounds (VOCs) in urban and regional
2
-
atmospheres.1,2 The primary source is emission from automobiles
and other fuel-based vehicles as well as from various industrial
3
3
calibration of the HNO with the mass spectrometer, as done
activities. Their atmospheric oxidation is largely initiated by attack
previously in our laboratory.10 The initial concentrations of OH
1
from the hydroxyl radical, OH. The toluene-OH reaction results
9
in the flow reactor were typically in the range from 5 × 10 to 7
in OH addition to the aromatic ring approximately 90% of the
time and H-atom abstraction from the methyl group about 10%
of the time. The abstraction pathway results in the formation of
benzyl radicals, which then form benzyl peroxy radicals by
addition of O . Subsequently, these radicals react with NO, leading
2
to the formation of aromatic benzylaldehyde and benzyl nitrate.
The carbonyl compound formed can further react with OH and
1
0
-3
×
10 molecule cm . Commercially available toluene (99.5%)
was introduced into the flow reactor by passing N
2
through a
toluene bubbler at 0 °C. The concentrations of toluene in the flow
1
1
12
reactor were in the range from 7 × 10 to 8 × 10 molecule
-
3
cm and were at least a factor of 10 higher than the OH
concentration to ensure pseudo-first-order conditions.
A positive ion spectrum of the OH-toluene reaction products
2
O , and the peroxy radical produced leads to the formation of
+
is shown in Figure 1(a), using O
2
as reagent ions. The prominent
carboxylic acids, thus propagating the radical chain reactions.
The addition pathway results in the formation of methyl
hydroxycyclohexadienyl radical (the adduct). Most of the ring
retaining and fragmentation species of the photooxidation of
toluene are a result of reactions of the adduct. Laboratory studies
+
7 8
peak at m/e 92 corresponds to the parent mass of toluene (C H ).
The next highest peak at m/e 109 corresponds to the OH-toluene
adduct produced by the following ion-molecule reaction:
+
+
2
indicate that the adduct reacts with NO with an estimated room
C H OH + O f C H OH + O
2
(1)
7
8
2
7
8
-
11
3
-1 -1 2,3
temperature rate constant of 3 × 10 cm molecule s . The
reaction of the adduct with O has a lower estimated rate constant
2
-16
3
-1 -1
1,3,5
We are not aware of any previous rate constant measurements
involving this radical cation. Several experiments were carried
out to verify that the ions detected at m/e 109 were indeed
attributable to the OH-toluene adduct, rather than being produced
by secondary ion-molecule reactions. First, we observed that the
signal at m/e 109 disappeared either when the toluene flow was
stopped or when the microwave discharge for generating OH
ceased. Second, we followed the evolution of the m/e 109 signal
as the injector was successively pulled upstream to increase the
reaction time. The results show that this signal rises (Figure 2b)
in accordance with OH disappearance (Figure 2a). We should
point out that the mass spectrometer signals depend only on the
mass of ions without discrimination between isomers, although
ab initio calculations indicate that the most energetically favored
of 1.8 × 10 cm molecule
s
at room temperature.
instead of reaction with NO, the aromatic peroxy radicals are
speculated to cyclicize, forming bicyclic radicals. This process
Also,
is followed by addition of another O
ring fragmentation.
2
molecule and subsequent
6
There is considerable uncertainty concerning the mechanism
of the toluene oxidation reactions initiated by OH. Few direct
experimental studies are available on the chemistry of the
intermediate radicals. Laboratory data on the detection of aromatic
intermediates in the gas phase has been reported only by Fritz et
4
3
al. and Knispel et al.; their work involves UV absorption by
the OH-benzene adduct, whose spectrum apparently displays a
broad band around 270-340 nm. Most experimental data
concerning the reactions after the initial OH attack involve the
identification of the stable products formed, which include cresols,
6
structure is that resulting from OH addition at the ortho position.
2
To study the intermediates formed in the O -OH-toluene
nitrotoluenes, benzyl nitrate, glyoxal, methylglyoxal, and per-
oxyacetyl nitrate (PAN).1 In this work we present the first
2
system, we added O to the flow reactor at concentrations on the
order of 1016 molecule cm ; several mass peaks appeared in the
-3
laboratory study to monitor directly with mass spectrometry the
aromatic intermediate radicals formed in the OH-initiated reactions
of toluene. We also report ion-molecule reaction rate constants
for these radicals using various positive and negative reagent ions.
-
negative ion spectra, using SF
6
as the reagent ion (Figure 1b).
We attribute the mass peaks at m/e 123 and 141 to the formation
of the benzyl peroxy radical and of the peroxy radical corre-
sponding to the OH-toluene adduct, according to the following
†
Massachusetts Institute of Technology.
‡
Texas A&M University
(7) Seeley, J. V.; Jayne, J. T.; Molina, M. J. Int. J. Chem. Kinet. 1993, 25,
571.
(
1) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 1994, 2, 1.
(
2) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From
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1996, 100, 4026.
Air Pollution to Climate Change; John Wiley & Sons: New York, 1997.
(
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Chem. 1990, 94, 1375.
4) Fritz, B.; Handwerk, V.; Preidel, M.; Zellner, R. Ber. Bunsen-Ges. Phys.
(9) Zhang, R.; Molina, L. T.; Molina, M. J. ReV. Sci. Instrum. 1998, 69,
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(
(10) Lipson, J.; Elrod, M. J.; Beiderhase, T. W.; Molina, L. T.; Molina,
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Chem. 1985, 89, 343.
(
(
5) Perry, R. A.; Atkinson, R.; Pitts, J. N. J. Phys. Chem. 1977, 81, 296.
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H. J. Phys. Chem. 1996, 100, 10967.
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1
0.1021/ja992461u CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/15/1999