Kinetics study on reactions of C6H5O with C6H5O and O3 at 298 K

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:1<65::AID-KIN8>3.0.CO;2-J

Kinetics for reactions of phenoxy radical, C₆H₅O, with itself and with O₃ were examined at 298 K and low pressure (1 Torr) using discharge flow coupled with mass spectrometry (DF/MS). The rate constant for the phenoxy radical self-reaction was determined to be k₁ = (1.49±0.53)×10^-11 cm³ molecule^-1 s^-1 defined by d[C₆H₅O]/dt = -2 k₁[C₆H₅O]². The rate constant for the C₆H₅O reaction with O₃ was measured to be k₂ = (2.86±0.35)×10^-13 cm³ molecule^-1 s^-1, which may be a lower limit value. Because of much higher atmospheric abundance of ozone than that of both NO and phenoxy, the reaction of C₆H₅O with ozone may represent the principal fate of the phenoxy radical in the atmosphere. Products from reaction of C₆H₅O+C₆H₅O, NO, and NO₂ were also investigated, and (C₆H₅O)₂ (m/e = 186), C₆H₅O(NO) (m/e = 123), and C₆H₅O(NO₂) (m/e = 139) adducts were observed as products for the reactions of C₆H₅O with itself, NO, and NO₂, respectively.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:1<65::AID-KIN8>3.0.CO;2-J

A Kinetics Study on
Reactions of C H O with
6
5
C H O and O at 298 K
6
5
3
ZHINING TAO, ZHUANGJIE LI
The Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, 105 South Gregory Avenue,
Urbana, Illinois 61801
Received 6 May 1998; accepted 28 July 1998
ABSTRACT: Kinetics for reactions of phenoxy radical, C H O, with itself and with O were
6
5
3
examined at 298 K and low pressure (1 Torr) using discharge flow coupled with mass spec-
trometry (DF/MS). The rate constant for the phenoxy radical self-reaction was determined to
Ϫ
11
3
Ϫ
1
Ϫ
1
2
be k ϭ (1.49 Ϯ 0.53) ϫ 10 cm molecule
s
defined by d[C H O]/dt ϭ Ϫ2 k [C H O] . The
6 5 1 6 5
1
Ϫ
13
rate constant for the C H O reaction with O was measured to be k ϭ (2.86 Ϯ 0.35) ϫ 10
cm molecule s , which may be a lower limit value. Because of much higher atmospheric
abundance of ozone than that of both NO and phenoxy, the reaction of C H O with ozone may
6
5
3
2
3
Ϫ
1
Ϫ1
6
5
represent the principal fate of the phenoxy radical in the atmosphere. Products from reaction
of C H O ϩ C H O, NO, and NO were also investigated, and (C H O) (m/e ϭ 186), C H O(NO)
6
5
6
5
2
6
5
2
6
5
(
m/e ϭ 123), and C H O(NO ) (m/e ϭ 139) adducts were observed as products for the reactions
6 5 2
of C H O with itself, NO, and NO , respectively. ᭧ 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31:
6
5
2
6
5–72, 1999
INTRODUCTION
Phenoxy radical, C H O, is one of the key intermediate
may lead to formation of C H O [7]. In this process
6 5
the toluene molecule is attacked by hydroxyl radical
and then interacts with O2 and NO to produce
C H CHO. Further oxidation of C H CHO can lead to
6
5
species in the oxidation of small aromatic compounds
such as benzene and toluene, which are important both
as ingredients of reformulated fossil fuel and as par-
ticipants in the formation of polycyclic aromatic hy-
drocarbons (PAHs) and secondary organic aerosol par-
ticles [1–4]. The phenoxy radical can be formed in
both combustion and atmospheric processes [5–7]. It
is well known that the C H O radical is formed in high
6
5
6
5
the formation of phenyl and then phenoxy radical. It
has been suggested that the phenoxy radical further
participates in subsequent reactions in the combustion
system and in the atmosphere [5,7,8]. The identity of
the products and information on the kinetics for each
possible reaction of C H O are crucial in the under-
6
5
standing of its chemical fate. There have been an in-
creasing number of investigations of reactions involv-
ing phenoxy radicals. Besides the unimolecular
decomposition of C H O into C H and CO, which has
6
5
temperature combustion processes involving benzene
and toluene, in which the hydrogen atom or methyl
group on the aromatic ring is substituted by an oxygen
atom [5,6]. It has also been postulated that the minor
atmospheric oxidative degradation process of toluene
6
5
5
5
been extensively studied and well characterized [5,9–
12], kinetics studies regarding some binary reactions
of the phenoxy radical are also found in recent litera-
ture [13–17]. For example, Buth et al. [13] investi-
gated the self-reaction of phenoxy radical as well as
Correspondence to: Z. Li
Contract grant sponsor: EPA Research Grant, NSF
Contract grant number: X825967-01-0 EPA, NSF ATM-
the reaction of C H O with H, O, CH , and C H using
9
813331
6
5
3
2
5
᭧
1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/010065-08
a discharge flow technique. Yu et al. examined the

6
6
TAO AND LI
Figure 1 Experimental apparatus for DF/MS study of C H O ϩ C H O, O , NO, and NO . For
6
5
6
5
3
2
the C H O self-reaction, the C H O was produced in the double sliding injector. For other reactions
6
5
6
5
the C H O radical was introduced through a side arm as a minor reactant, while the other reactants
6
5
were added through the sliding injector.
C H O reaction with NO using a cavity-ring down
Products from reactions of C H O with NO and NO
6 5
6
5
2
method [14]. More recently, Berho et al. [16] per-
formed a kinetics and thermochemistry study on the
phenoxy radical reaction with NO at 280–328 K, and
they found that the rate constant of this reaction was
not significantly affected by temperature in this tem-
perature range. Berho and Lesclaux also studied both
the phenoxy radical recombination reaction at 280–
were also examined in this work:
C H O ϩ NO !: C H O(NO)
(3)
(4)
6
5
6
5
C H O ϩ NO !: C H O(NO )
6
5
2
6
5
2
4
23 K and the interaction between the C H O and O
We also briefly comment on the atmospheric impli-
cation of our experimental results.
6
5
2
at up to 500 K using UV absorption spectroscopy [17].
The self-reaction of C H O was found to yield phen-
6
5
oxy dimer, (C H O) [13]. The reactions of phenoxy
6
5
2
radical with H, CH , and C H are also association
3
2
5
EXPERIMENTAL
reactions with products being phenol or cyclohexadi-
enone, anisol or cresols, and phenetol or ethylphenol,
respectively [13]. The reaction of phenoxy radical with
NO has been suggested to yield C H O(NO) adduct as
The experimental apparatus used in the DF/MS study
of reactions 1–4 is shown in Figure 1. The DF/MS
technique is well established, and has been described
previously [18–21]. Thus, it is only briefly discussed
here. The reactor consists of a 100 cm long, 5.08-cm
o.d. Pyrex tube with an internal surface covered with
a layer of 0.05-cm thick TFE Teflon sheet to reduce
6
5
a product [14,16], with a characterized absorption
cross-section at 240 nm [16]. Finally, the reaction of
C H O with the oxygen atom was found to be a binary
6
5
reaction with quinone as primary product [13]. Despite
the progress made in these studies, more information
is needed to understand the sink of the phenoxy radical
in both combustion and atmospheric processes. In this
article, we report our kinetics study on two reactions
involving phenoxy radicals using a discharge flow
coupled with mass spectrometry (DF/MS) at room
temperature:
the C H O radical wall loss. A steady-state gas flow
6
5
(total pressure of 1–10 Torr) was maintained in the
flow tube with a mechanical pump (Edwards
E2M175). Helium was introduced into the reactor as
a carrier gas upstream of the reactor. The mean gas
velocity in the flow tube varied between 500 and 1000
Ϫ1
cm s , resulting in residence times ranging from 160
to 80 ms in the 80-cm reaction zone. Kinetic mea-
surements were carried out with the employment of a
double sliding injector, which consists of two concen-
C H O ϩ C H O !: (C H O)
2
(1)
(2)
6
5
6
5
6
5
C H O ϩ O !: products
6
5
3

REACTIONS OF C H O WITH C H O AND O
3
67
6
5
6
5
9
10
tric Pyrex tubes with the internal diameters of 9 mm
and 12.7 mm, respectively.
found to range between 3 ϫ 10 and 5 ϫ 10 molecule
Ϫ3
cm .
The gases used in this work were obtained mainly
from S. J. Smith Welding Supply: He, 99.99%; O2,
99.98%; NO, 99%; NO , 99.5%; Cl , 5.02% in He;
HCl, Ͼ99.0%. Two methods were employed to purify
the NO sample: (1) multiple low-temperature (195 K)
distillations to remove the volatile NO, and (2) addi-
tion of O to the sample to convert the NO impurity
into NO . All other gases were used as received. Phe-
Mass spectrometric detection of both reactants and
products was conducted by continuous sampling
downstream of the flow tube through a two-stage dif-
ferentially pumped beam inlet system. Beam modu-
lation was achieved with a 200-Hz tuning fork-type
chopper placed inside the second stage before the mo-
lecular beam entered the mass spectrometer (Extrel
Model C50). Ion signals were sent to a lock-in ampli-
fier (Model SR510, Stanford Research Systems, Inc.)
that was referenced to the chopper frequency. The am-
plified analog signals were digitized and recorded by
a microcomputer.
2
2
2
2
2
nol (Ͼ99%) was purchased from Fisher Scientific. O3
was produced by an ozone gas generator (Pacific
Ozone Technology L21) and stored on silica gel at 195
K. During the experiments, O was maintained at 195
3
C H O radical was generated via the following re-
action schemes [13]:
K and bubbled into the reactor with a measured flow
of He (100–500 sccm). A cold trap cooled by liquid
nitrogen was placed before the inlet of the mechanical
pump to avoid the contamination of the pump by the
corrosive chemicals. For experiments involving O3,
6
5
Cl ϩ microwave discharge !: 2Cl
2
(5)
Cl ϩ C H OH !: C H O ϩ HCl
k6 ϭ (2.37 Ϯ 0.42) ϫ 10 cm molecule
the ozone was catalytically converted into O by pass-
6
5
6
5
Ϫ
2
10
3
Ϫ
1
Ϫ1
s
ing through a heated U-tube containing copper fibers
as catalyst before arriving at the trap. It was found that
when the U-tube temperature reached 200–250ЊC,
Ͼ99% of the ozone was converted and very little
ozone was trapped under liquid nitrogen temperature.
Because our set-up was newly built, it was neces-
sary to test our experimental apparatus for accuracy
before performing the phenoxy radical study. We mea-
sured the rate constant for the reaction of ClO with
NO at room temperature as a calibration experiment
(
6)
1
3
where phenol ([C H OH] Յ 7 ϫ 10 molecule
6
5
Ϫ3
cm ) was carried into the reactor with He flowing
through a glass bubbler containing phenol. The
amount of phenol was controlled by the flow rate of
He which varied from 200 sccm to 400 sccm in this
1
4
Ϫ3
work. Chlorine gas ([Cl ] Ͻ 10 molecule cm )
2
flowed through a microwave discharge device (Op-
thos, Model MPG 4) and was atomized to produce Cl
using the DF/MS system, and obtained k
ϭ
(ClO ϩ NO)
1
3
Ϫ3
Ϫ
11
3
Ϫ1
Ϫ1
(
[Cl] Յ 2 ϫ 10 molecule cm ). The initial concen-
(1.63 Ϯ 0.21) ϫ 10 cm molecule s , which was
in very good agreement with the value of 1.7 ϫ
tration of phenol was always in excess over that of the
chlorine atom by a factor of at least 5 to ensure the
complete titration of the chlorine atoms. The phenoxy
radical produced from reactions 5 and 6 was intro-
duced into the system either from the sliding injector
for phenoxy radical self-reaction study or from the side
arm of the flow reactor for the investigation of other
reactions.
Ϫ11
3
Ϫ1
Ϫ1
10
cm molecule
s
recommended by NASA
Panel [22] for the same reaction.
RESULTS AND DISCUSSION
Quantitative Determination of Phenoxy
Radical
ϩ
The mass spectra signal from the C H O parent
6
5
ion was observed at m/e ϭ 93. The m/e ϭ 93 signal
from the fragmentation of phenol was found to be neg-
ligible under our mass spectrometer operation condi-
tions of 18.5 eV electron impact energy and 1.0 mA
emission current. The concentration measurements of
NO (m/e ϭ 46), NO (m/e ϭ 30), Cl (m/e ϭ 70), and
The quantitative determination of the phenoxy radical
concentration is crucial in kinetics studies of reactions
involving C H O, especially for reaction 1, in which
6
5
the accuracy of the rate constant depends on the ac-
curacy of the measurement of the absolute C H O con-
2
2
6
5
HCl (m/e ϭ 36) were calibrated by comparison of their
signals with the measured flow rates of these species.
Calibration of ozone measurements was achieved by
the chemical conversion of O into NO , and the cal-
centration. According to reaction 6, for every C H O
6 5
generated in the system, a molecule of HCl is also
produced. Thus, the concentration of phenoxy radical
can be calibrated by quantitative measurement of HCl,
assuming that only the hydrogen atom in the 9OH
group of the phenol molecule is abstracted. By doing
so, the absolute concentration of phenoxy radical was
3
2
ibration of C H O was based on the stoichiometry of
6
5
reaction 6, which will be discussed later. Detection
limits for the molecules and radicals of interest were

6
8
TAO AND LI
obtained and then used to calibrate the mass spectral
signal at m/e ϭ 93. There is one potential problem
associated with this calibration method. The C H O
6
5
radical is known to react with Cl2 to produce
C H O(Cl) (m/e ϭ 128) and Cl [13,16]:
6
5
C H O ϩ Cl !: C H O(Cl) ϩ Cl
(7)
6
5
2
6
5
and although this reaction has been estimated to be a
Ϫ14
3
Ϫ1
Ϫ1
slow process (k Յ 10 cm molecule
s
[16]) the
7
Cl atom produced from reaction 7 may provide addi-
tional HCl. To minimize this effect on the C H O cal-
6
5
ibration, we produced the C H O in the sliding injector
6
5
and placed the injector very close to the downstream
inlet pinhole, so that the contact time between C H O
6
5
and Cl in the reactor is less than 0.5 ms. Very little
2
C H O(Cl) was formed under such conditions, and the
6
5
HCl was mainly produced by the Cl atoms generated
from reaction 6. We also monitored the loss of Cl2
during the C H O calibration, and found that the
Figure 2 Typical plot of 1/[C H O] vs. the sliding injector
6
5
6
5
position at 298 K. The initial concentration of C H O was
amount of Cl loss was essentially equal to the amount
6
5
2
1
2
Ϫ3
3
.44 ϫ 10 molecule cm , the total pressure in the reactor
of HCl produced, indicating that the interference of
reaction 7 on the C H O concentration determination
Ϫ
1
was 1.0 Torr, and the flow velocity was 1033 cm s .
6
5
was insignificant.
concentration of phenoxy radical ranging 0.16–1.20
ϫ 10 molecule cm . The average rate constant for
1
3
Ϫ3
The Phenoxy Radical Self-Reaction
the phenoxy self-reaction was determined from the
The self-reaction of the C H O is known to be a re-
6
5
collected data to be k ϭ (1.49 Ϯ 0.53) ϫ 10 cm3
Ϫ
11
1
combination process with phenoxy dimer as a product
13]. The dimer formation was confirmed with obser-
vation of a C H O dimer signal at m/e ϭ 186 in our
Ϫ1
Ϫ1
molecule s , where the reported error is taken as
␴ and reflects the scatter of the data and the uncer-
[
2
6
5
tainties of experimental parameters such as tempera-
ture, flow rate, and pressure.
experiment. Berho and Lesclaux [17] suggested that
the dimer might have several possible isomers that can
be formed by combination of the different resonance
structures of the phenoxy radical followed by the sub-
sequent rearrangements to more stable molecules.
Kinetic measurement of the self reaction was ac-
complished by monitoring the decay of phenoxy rad-
icals that was introduced via the double sliding injec-
tor. The kinetics expression for reaction 1 is given by:
The reaction of Cl with phenoxy radical (reaction
2
7
below) could potentially compete with reaction 1
and contribute to the C H O decay in the kinetics study
6
5
for reaction 1:
C H O ϩ Cl !: C H O(Cl) ϩ Cl
(7)
6
5
2
6
5
However, this effect is expected to be small in the
present study for two reasons. First, reaction 7 is sev-
eral orders of magnitude slower than the C H O self-
1
/[C H O] Ϫ 1/[C H O] ϭ 2k t
[I]
6
5
t
6
5
0
l
6
5
reaction, thus the C H O decay in our study of reaction
where [C H O] and [C H O] are the concentration of
6
5
6
5
0
6
5
t
1
should be mainly due to reaction 1. Second, the Cl
C H O at time zero and any time t, and k is the rate
6
5
1
atom produced from reaction 7 will rapidly react with
constant for reaction 1. Plotting 1/[C H O] as a func-
6
5
t
the excess amount of phenol to regenerate the C H O
tion of reaction time yields a straight line, which slope
6
5
radical:
would be the rate constant k . Figure 2 shows such a
1
typical plot for the phenoxy radical self-reaction. The
linear relationship between 1/[C H O] and time within
Cl ϩ C
H
5
OH !: C
H
5
O ϩ HCl
(6)
6
5
t
6
6
the time domain studied (approximately 30 ms) proved
the second order reaction mechanism for reaction 1. A
total of 20 runs were performed at 298 K for the
C H O self-reaction kinetics measurement, with initial
Because reaction 6 took place very rapidly (under our
experimental conditions in which the concentrations
1
3
14
of phenol and Cl were 5 ϫ 10 and 10 molecule
6
5
2

REACTIONS OF C H O WITH C H O AND O
3
69
6
5
6
5
Ϫ
3
cm , respectively, the phenoxy recovery time from
rex injector that was concentric with the reactor tube.
The observed first-order decay rate was corrected for
axial diffusion and for loss of C H O on the injector
Ϫ4
reaction 6 is about 10 s, which is three orders of
magnitude faster than its consumption time estimated
from reaction 7), the Cl atom formed in reaction 7
would be titrated completely and essentially immedi-
6
5
according to eq. [II] [18,19]:
ately, and the C H O radical loss due to reaction 7 was
2
6
5
kЈ ϭ kЈ(1 ϩ kЈD/v ) ϩ k
[II]
corr
p
quickly regained by reaction 6. Therefore, the inter-
ference of reaction 7 on the kinetic measurement of
phenoxy radical self-reaction can be neglected.
where D is the diffusion coefficient, v is the mean bulk
flow velocity and k is the first-order loss of C H O on
the surface of the sliding injector. The diffusion co-
p
6
5
Buth et al. measured the rate constant of phenoxy
radical self-reaction indirectly by competing the
C H O ϩ O reaction with the C H O ϩ C H O reac-
efficient D for C H O in the helium was estimated
6
5
6
5
6
5
6
5
based on the empirical equation developed by Fuller
tion. They reported a rate constant of k ϭ (3.32 ϩ
2
1
et al. [23], which yielded approximately 0.30 atm cm
s
Ϫ11
3
Ϫ1
Ϫ1
3
.32/Ϫ1.66) ϫ 10
cm molecule
s
[13]. Re-
Ϫ1
at 298 K. The corrections for axial diffusion were
cently, Berho et al. determined the rate coefficient for
always less than 1%. Finally, the bimolecular rate con-
Ϫ
11
the C H O self-reaction to be (1.22 Ϯ 0.02) ϫ 10
6
5
stant for reaction 2, k , was obtained from slopes of
2
3
Ϫ1
Ϫ1
cm molecule
with UV absorption spectrometry at 1 atm and 298 K
the error bar was later modified to be Ϯ 0.24) [17].
Our rate constant value for reaction 1 (k ϭ (1.49 Ϯ
s
using a flash photolysis coupled
linear least-squares fits to the plots of kЈcorr vs. the ex-
cess reactant concentration.
The determination of absolute concentration of O3
is important in measuring the rate constant for reaction
(
1
Ϫ11
3
Ϫ1
Ϫ1
0
.53) ϫ 10 cm molecule s ) is in good agree-
2
. In this work, O measurement was calibrated by the
3
ment with that measured by Berho et al., but is about
a factor of two lower than that reported by Buth et al.
The loss of the C H O on the wall was suggested
chemical conversion of O to NO by reacting O with
3
2
3
excess of NO:
O ϩ NO !: NO ϩ O
2
6
5
to contribute to the large uncertainty in Buth et al.’s
measurement [13]. In our experiment, however, no
significant decrease of the C H O signal was observed
3
2
Ϫ14
3
Ϫ1
Ϫ1
k8 ϭ 1.8 ϫ 10
cm molecule
s
[22] (8)
6
5
when the radical was introduced to the reactor from
the side arm and exposed to the sliding injector sur-
face, indicating a negligible, if any, loss of the C H O
This was accomplished by introducing NO from the
sliding injector and O from the side arm of the reactor
6
5
3
onto the Pyrex wall. The coverage of a Teflon sheet
to the internal surface of the reactor should further help
in reducing the wall loss of the phenoxy radical. Thus
the contribution of the wall loss to the C H O decay
tube, with the injector placed in a downstream position
to minimize the further reaction of NO with O . The
conversion factor was determined from the ratio of the
2
3
change in O signal (m/e ϭ 48) to that of NO (m/e ϭ
6
5
3
2
in our C H O self-reaction rate constant determination
46), which yielded ⌬S /⌬S ϭ 0.49. Then the ab-
6
5
48 46
should be negligible.
solute calibration of the mass spectra signal at m/e ϭ
6 allowed the evaluation of the absolute O concen-
4
3
tration.
Figure 3 shows the corrected pseudo-first-order de-
cay rate, kЈcorr, as a function of O concentration. With
Phenoxy Radical Reaction with Ozone
Kinetic measurement for reaction 2 was carried out by
monitoring the decay of phenoxy radical as a function
of reaction time in the presence of ozone. The rate
constant for this reaction was obtained using the well-
known steady-state flow-tube method [18–21]. Fol-
lowing this method, the experiments were carried out
under pseudo-first-order conditions, in which the con-
centration of O was in excess over that of C H O.
3
1
1
initial C H O concentrations of (3.30–10.4) ϫ 10
6
5
Ϫ
3
molecule cm and O3 concentrations of (1.45–
1
3
Ϫ3
11.0) ϫ 10 molecule cm , kЈ varies from 7 to 33
corr
Ϫ
1
s . The slope of the linear least-squares fit through all
the data yields k ϭ (2.86 Ϯ 0.35) ϫ 10 cm mol-
Ϫ
13
3
2
Ϫ1
Ϫ1
ecule
s
for the reaction of C H O with O , where
6 5 3
the quoted uncertainty corresponds to 2␴. The rapid
self-reaction of the phenoxy radical may add compli-
cation to this measurement. Because the C H O was
3
6
5
The pseudo-first-order decay rate, kЈ, was determined
from slopes of plots of the logarithm of phenoxy rad-
ical concentration against injector distance, which de-
termined the contact time of the reactants. In these
experiments C H O was introduced into the flow tube
6
5
introduced from the side arm, there may be a C H O
6
5
concentration gradient going from the upstream to the
downstream of the reactor. This potential effect was
reduced by taking the C H O decay data within the
6
5
as a minor reactant through a fixed side arm, and the
6
5
excess reagent, O , was added through the sliding Py-
first 20 ms contact time domain, which was less than
3

7
0
TAO AND LI
1
4
Ϫ3
1
0 molecule cm ), suggesting an association mech-
anism yielding a C H O(NO) species for reaction 3:
6
5
C H O ϩ NO !: C H O(NO)
(3)
6
5
6
5
The detail structural information of the adduct
could not be identified with our apparatus. There had
been theoretical calculations predicting the possible
structures of the adduct isomers including C H ONO,
6
5
o-NOC H O, and p-NOC H O, which could result
6
5
6
5
from direct NO attachment to the oxygen and the
ortho- and para-carbon of the C H O, respectively
6
5
[14,16]. Rearrangement of these isomers had been
postulated to lead to the formation of C H NO and
6
5
2
C H (OH)(NO) [14,16]. Although our product detec-
6
4
tion was unable to pinpoint the preferred isomer prod-
uct for reaction 3, phenyl nitrite, C H ONO, has been
6
5
suggested as the most likely product of the reaction 3
[14,16].
Figure 3 First-order decay rate of C H O, kЈcorr, as a func-
6
5
tion of [O ] at 298 K. The first-order decay rates were mea-
3
sured under total pressure of 1.0 Torr in the reactor and flow
The reaction of C H O with NO was also found to
6
5
2
Ϫ
1
14
velocity of 850–920 cm s , and then corrected for both
C H O axial diffusion and loss on the sliding injector.
be an association process. When ϳ2.0 ϫ 10 mole-
Ϫ3
6
5
cule cm of NO was introduced into the reactor con-
2
1
2
Ϫ3
taining ϳ3 ϫ 10 molecule cm of C H O, a signal
6
5
was observed at m/e ϭ 139 that was assigned to
C H O(NO ) , and this suggests that reaction 4 has an
the typical lifetime (Ն35 ms) of the phenoxy radical
under our experimental conditions. However, the
C H O concentration gradient could not be completely
ϩ
6
5
2
association pathway:
6
5
eliminated, and thus the rate constant reported here
should be regarded as a lower limit value for the phen-
C H O ϩ NO !: C H O(NO )
(4)
6
5
2
6
5
2
oxy radical reaction with O3.
There is no structural information available for the
adduct product in reaction 4. However, analogous to
reaction 3, the production of the isomers of
C H ONO , o-NO C H O, and p-NO C H O, as well
The possible products for reaction 2 were expected
to be phenylperoxy or O addition adduct:
3
6
5
2
2
6
5
2
6
5
C H O ϩ O !: C H O(O) [or C H O(OH)] ϩ O
as the rearrangement products such as nitrophenols,
seem likely. Such rearrangements may involve the H
atom shifts to form ortho- and para-nitrophenols.
Coombes and Diggle studied the mechanism of liquid
phase nitration of phenol in cyclohexane at 298 K [25].
They found the 97% yield of nitrophenols (54% o-,
6
5
3
6
5
6
4
2
(2a)
!
: C H O(O )
(2b)
6
5
3
Ab initio studies suggested that C H O(O) could exist
6
5
43% p-) at the stoichiometric equivalent of the reac-
in the form of the most stable isomer, C H O(OH)
6
4
tants by a mechanism involving the phenoxy radical
as an intermediary.
[
15,24]. However, searches for C H O(O) species at
6 5
m/e ϭ 109 and C H O(O ) species at m/e ϭ 141 from
6
5
3
We summarize our results of kinetics study in Table
I. As an intermediate species from the minor oxidative
degradation of small aromatic compounds such as tol-
uene, the phenoxy radical can further react with other
trace species in the atmosphere. Although the rate con-
stant for reaction of C H O with ozone is several times
reaction 2 were negative under our experimental con-
ditions. Further work is required to clarify the products
and the reaction mechanism for C H O reaction with
6
5
O .
3
6
5
smaller than that of C H O with NO (k ϭ 1.45 ϫ
6
5
3
3
Product Study for Reactions of C H O with
NO and NO2
6
5
Ϫ12
Ϫ
12
Ϫ1
1
0
[14] and (1.65 Ϯ 0.10) ϫ 10 cm molecule
Ϫ
1
s
[16]), the removal of C H O by ozone is expected
6 5
An adduct signal at m/e ϭ 123 was observed when
to dominate over the C H O removal by NO and itself
under atmospheric conditions due to the much higher
atmospheric abundance of ozone than those of NO and
6 5
1
2
Ϫ3
ϳ2.0 ϫ 10 molecule cm of C H O was produced
6
5
in the presence of excess amount of NO (ϳ1.2 ϫ

REACTIONS OF C H O WITH C H O AND O
3
71
6
5
6
5
Table I Summary of Kinetics Study on Reactions of C H O ϩ C H O, O
6
5
6
5
3
P
T
(K)
k
3
Ϫ
1
Ϫ
1
Methoda
Reaction
(Torr)
(cm molecule s )
Ref.
Ϫ
11
C H O ϩ C H O : (C H O)
2
1–5
295
(3.32 Ϯ 3.32/Ϫ1.66) ϫ 10
DF/MS
FP/UV
DF/MS
13
17
this work
6
5
6
5
6
5
Ϫ
12
7
60
280–423 (1.44 Ϯ 0.16) ϫ 10 exp[(631 Ϯ 37)/T]
Ϫ
11
1
298
(1.49 Ϯ 0.53) ϫ 10
Ϫ
13
C H O ϩ O : Products
1
298
(2.86 Ϯ 0.35) ϫ 10
DF/MS
this work
6
5
3
aDF/MS ϭ discharge flow/mass spectrometry; FP/UV ϭ flash photolysis/UV absorption.
phenoxy. In a clean tropospheric air, in which the
ozone represents the principal fate of the phenoxy rad-
ical in the atmosphere.
background concentrations of ozone and NO (NO ϩ
x
2
1
2
Ϫ3
NO) are 40 ppbv (1 ϫ 10 molecule cm ) and 30
8
Ϫ3
pptv (7.7 ϫ 10 molecule cm ), respectively [26], and
the concentration of phenoxy is presumably no more
This work was done with the startup support from the
UIUC Campus Research Board and the partial support from
the EPA Research grant (X825967-01-0 EPA) and National
Science Foundation (NSF ATM-9813331). The authors
would like to thank Prof. R. Lesclaux for the preprints of
their phenoxy radical study, and Prof. J. S. Francisco as well
as Dr. J. Gaffney for helpful discussion.
6
Ϫ3
than 10 molecule cm , more than 99% of the phen-
oxy radical will be removed by ozone based on a sim-
ple computer simulation involving these three chemi-
cal processes. Even in the polluted area where the NOx
concentration could be as high as 10 ppbv, the simu-
lation shows that still around 87% of phenoxy will be
removed by ozone.
BIBLIOGRAPHY
SUMMARY
1
2
3
. Frenklach, M.; Warnatz, J. Combust Sci Techol 1987,
1, 265.
. Longwell, J. P. 19th Symp. (Int.) on Combustion; The
Combustion Institute: xxxx, 1982, p. 1339.
. Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein,
S. E. 21st Symp. (Int.) on Combustion; The Combustion
Institute: Pittsburgh, 1986, p. 1067.
5
We performed a kinetics study for the phenoxy radical
self-reaction and reaction of phenoxy with ozone and
investigated the products from reactions of phenoxy
with itself, nitric oxide, and nitrogen dioxide at 298 K
and low pressures using discharge flow coupled with
mass spectrometry (DF/MS). The rate constant for the
phenoxy radical self reaction was determined to be
4. Odum, J. R.; Jungkamp, T. P. W.; Griffin, J. R.; Flagan,
R. C.; Seinfeld, J. H. Science 1997, 276, 96.
5. Brezinsky, K. Prog Energ Combust Sci 1986, 12, 1.
Ϫ
11
3
Ϫ
1
Ϫ
1
k1 ϭ (1.49 Ϯ 0.53) ϫ 10
cm molecule
s ,
6
. Glassman, I. Combustion; 3rd ed.; Academic Press:
New York, 1996.
. Atkinson, R. Atmos Environ 1990, 24A, 1.
. Mulcahy, M. F. R.; Williams, D. J. Aust J Chem 1965,
which was in good agreement to that measured by
Berho et al. [17], but significantly lower than that re-
ported by Buth et al. [13]. The rate constant for the
phenoxy radical reaction with ozone was measured to
7
8
1
8, 20.
Ϫ
13
3
Ϫ
1
Ϫ1
be k ϭ (2.86 Ϯ 0.35) ϫ 10
cm molecule s ,
2
9. Colussi, A. J.; Zabel, F.; Benson, S. W. Int J Chem
Kinet 1977, 9, 161.
10. Lin, C. Y.; Lin, M. C. Int J Chem Kinet 1985, 17, 1025.
11. Schmoltner, A. M.; Anex, D. S.; Lee, Y.T. J Phys Chem
1992, 96, 1236.
2. Liu, R.; Morokuma, K.; Mebel, A. M.; Lin, M. C. J Phys
Chem 1996, 100, 9314.
3. Buth, R.; Hoyermann, K.; Seeba, J. 25th Symp. (Int.)
on Combustion; The Combustion Institute: Pittsburgh,
994, p. 841.
4. Yu, T.; Mebel, A. M.; Lin, M. C. J Phys Org Chem
995, 8, 47.
5. Lin, M. C.; Mebel, A. M. J Phys Org Chem 1995, 8,
07.
which may be a lower limit value because of the pos-
sible phenoxy radical concentration gradient devel-
oped in the flow tube due to the rapid phenoxy radical
self-reaction. (C H O) (m/e ϭ 186), C H O(NO)
6
5
2
6
5
1
(m/e ϭ 123), and C H O(NO ) (m/e ϭ 139) were ob-
6 5 2
served as products for the reactions of the phenoxy
radical with C H O, NO and NO , respectively, sug-
1
6
5
2
gesting that reactions 1, 3, and 4 take place with pref-
erence of forming adducts. The possible products of
C H O(O) (m/e ϭ 109) or C H O(O ) adduct (m/e ϭ
1
1
1
6
5
6
5
3
1
1
41) from reaction of the phenoxy radical with ozone
were not detected by our mass spectrometer. In addi-
tion, simple computer simulation of the reaction mech-
anism indicates that the reaction of phenoxy with
4
16. Berho, F.; Caralp, F.; Rayez, M. T.; Lesclaux, R. J Phys
Chem 1998, 102, 1.

7
2
TAO AND LI
1
7. Berho, F.; Lesclaux, R.Chem Phys Lett 1997, 279, 289;
also a private communication with Prof. R. Lesclaux.
8. Friedl, R. R.; Sander, S. P. J Phys Chem 1989, 93, 4756.
9. Li, Z.; Friedl, R. R.; Sander, S. P. J Chem Soc Faraday
Trans 1997, 93, 1.
23. Fuller, E. N.; Schettler, P. D.; Giddings, J. C. Ind Eng
Chem 1966, 58, 19.
24. Mebel, A. M.; Lin, M. C. J Am Chem Soc 1994, 116,
9577.
25. Coombes, R. G.; Diggle, A. W. Tetrahedron Lett 1994,
35, 6373.
26. Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry
and Physics: From Air Pollution to Climate Change;
Jonh Wiley & Sons, Inc.: New York, 1997.
1
1
2
2
2
0. Howard, C. J. J Phys Chem 1979, 83, 3.
1. Brown, R. L. J Res Natl Bur Stand (US) 1978, 83, 1.
2. DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hamp-
son, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara,
A. R.; Kolb, C. E.; Molina, M. J. JPL Publication 97-
4
; 1997.