Reaction of NO2 with selected conjugated alkenes

Article abstract of DOI:10.1021/jp408771r

The gas phase reactions of selected alkenes (isoprene, myrcene, ocimene, and 1,3-cyclohexadiene) with NO₂ under dark condition have been investigated at T ～ 298 K and P ～ 760 Torr of purified air. The kinetic studies were performed under pseudo-first-order conditions using a large excess of NO₂ concentration to those of the alkenes. The rate coefficients (in 10^-19 cm³ molecule^-1 s^-1) obtained are 1.1 ± 0.2 for isoprene, 2.5 ± 0.3 for myrcene, 8.5 ± 1.2 for ocimene, and 15 ± 1 for 1,3-cyclohexadiene. Several products were identified by using in situ Fourier transform infrared (FT-IR) spectrometry, and acetone was found to be the major product from the reactions of NO₂ with myrcene and ocimene, with a formation yield of 22 ± 3% and 26 ± 7%, respectively. The oxidation products from the reactions of NO₂ with isoprene and 1,3-cyclohexadiene were found to be mainly nitro compounds identified by FT-IR spectroscopy. Reaction mechanisms were proposed to account for the products observed.

Full text of DOI:10.1021/jp408771r

Article
pubs.acs.org/JPCA
Reaction of NO with Selected Conjugated Alkenes
2
†
†
‡
§
e,† Jianmin Chen,∥
Franco
̧
is Bernard, Mathieu Cazaunau, Yujing Mu, Xinming Wang, Ver
́
onique Dael
̈
,
†,∥
and Abdelwahid Mellouki*
†
Institut de Combustion, Aer
́
othermique, Rea
́ ́
ctivite et Environnement (ICARE), CNRS (UPR 3021)/OSUC, 1C Avenue de la
Recherche Scientifique, 45071 Orlea
́
ns Cedex 2, France
‡
Research Center for Eco-environmental of Sciences, Chinese Academy of Sciences, Beijing 100085, China
§
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou
5
10640, China
∥
Environment Research Institute/School of Environmental Science & Engineering, Shandong University, Shandong 250100, China
*
S Supporting Information
ABSTRACT: The gas phase reactions of selected alkenes (isoprene, myrcene, ocimene,
and 1,3-cyclohexadiene) with NO under dark condition have been investigated at T ∼
2
2
98 K and P ∼ 760 Torr of purified air. The kinetic studies were performed under pseudo-
first-order conditions using a large excess of NO concentration to those of the alkenes.
The rate coefficients (in 10 cm molecule s ) obtained are 1.1 ± 0.2 for isoprene, 2.5
2
−19
3
−1 −1
±
0.3 for myrcene, 8.5 ± 1.2 for ocimene, and 15 ± 1 for 1,3-cyclohexadiene. Several
products were identified by using in situ Fourier transform infrared (FT-IR) spectrometry,
and acetone was found to be the major product from the reactions of NO with myrcene
2
and ocimene, with a formation yield of 22 ± 3% and 26 ± 7%, respectively. The oxidation
products from the reactions of NO with isoprene and 1,3-cyclohexadiene were found to
2
be mainly nitro compounds identified by FT-IR spectroscopy. Reaction mechanisms were
proposed to account for the products observed.
1
. INTRODUCTION
2. EXPERIMENTAL METHODS
The experiments have been carried out using the 7300 L
In the atmosphere, alkenes are known to react with the main
gas phase oxidants (OH and nitrate radicals, ozone, and
chlorine atoms). It is commonly assumed that the reaction
with NO is a negligible atmospheric removal process for these
1
2,13
ICARE Teflon chamber
in P ≈ 1013 mbar of purified air at
1
,2
T = 298 ± 2 K with a relative humidity of ∼5%. Reactants that
are liquids under ambient conditions were introduced into the
chamber by injecting aliquot amounts of liquid in a stream of
purified air. Gaseous reactants were injected into the chamber
using a calibrated gas cylinder equipped with pressure sensors.
Two fans installed into the chamber ensured rapid mixing of
2
species because the reaction rate coefficients of NO with
2
−20
3
alkenes are slow under atmospheric conditions (<10
cm
molecule− s ).
1
−1 3−6
However, this process could be important
with dienes and polyconjugated alkenes during the smog
chamber experiments where very often high concentrations of
reactants are used. Indeed, some of the previous studies on the
reactions of NO with dienes and polyconjugated alkenes
indicate that these reactions could contribute to their
reactants in few minutes. The concentrations of NO (NO and
X
NO ) were continuously monitored by a NO −NO−NO
2
X
2
2
analyzer using chemiluminescence method (Thermo Environ-
ment 42i or/and Horiba APNA-360). SF was added to the gas
4
,5
6
consumption under high NO conditions.
X
mixture to assess the dilution rate due to added flow in the
chamber compensating sampling flows by the monitors.
Organic reactants and products were monitored by an in situ
Fourier transform infrared spectrometer (Nicolet 5700 Magna)
coupled to a white-type mirror system, resulting in a 129−148
m optical path length. Infrared spectra were recorded every 2−
In this work, we report a kinetic and mechanistic study of the
reaction of NO with isoprene, 1,3-cyclohexadiene, myrcene,
and ocimene. These VOCs have been chosen according to their
emission rates into the atmosphere, and their reactivities
toward NO2 under dark conditions. The reactions rate
coefficients were determined and the oxidation products
tentatively assigned and quantified using FT-IR spectroscopy.
So far, the reactions of NO with ocimene and myrcene have
been investigated only as interfering reactions but not as central
2
7
8
,9
1
1
0 min by coadding 40−300 interferograms with a resolution of
−1
cm . A gas chromatograph coupled to photoionization
2
detector (GC-PID, DXDZ GC-4400) was also used for the
measurements of the reactants and gas phase products. The
1
0,11
systems.
both reactions in the dark.
Acetone has been identified as a product from
10,11
However, the NO -initiated
2
degradation mechanism of these two compounds is still not
Received: September 2, 2013
Revised: December 7, 2013
Published: December 9, 2013
defined. Each of the NO -initiated oxidation of isoprene and
2
3
,8
1
,3-cyclohexadiene has been subject to only one study.
©
2013 American Chemical Society
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GC-PID is equipped with a VUV lamp with transparent light as
low as 110 nm. The organic compounds were separated on a
chromatographic column of 10% SE 30, PAW-HMPS (capillary
column, 20 m length and 0.5 mm i.d.). An acquisition time of
GC-PID was fixed at 30 min. Synthetic air (>99.999%) was
linear and pass through the origin and have a slope kocimene/
kmyrcene. The quoted uncertainties on the obtained rate
coefficients originate from the uncertainties of the reference
rate coefficient associated to the value of the slope (two
standard deviation, 2σ).
−1
used as the carrier gas with a flow rate of 10−20 mL min . A
six-port valve equipped with an injection loop of 1 mL
connected to the GC, and the loop was flushed by the reaction
2.2. Product Identification. Experiments were performed
in conditions similar to those of the kinetic studies (pseudo-
first-order conditions). Time−concentration profiles of identi-
fied products have been corrected from dilution losses using the
dilution rate constant of SF . When SF was not added to the
−1
gas mixture at 350 mL min for 1 min before injection. The
temperature of the column was controlled from 40 to 60 °C
according to the studied chemical systems.
6
6
−
5 −1
mixture, a value of 1.1 × 10
s
was used. Experimental time
2.1. Kinetic Measurements. Most of the kinetic experi-
duration was extended to 4 h. Quantitative measurement of
oxidation products using FT-IR spectroscopy was based on
calibrated IR spectra when available. Time−concentration
profiles of nitro compounds have not been corrected for the
loss due to further reaction and loss to the wall. The molar
product yields were derived from the average of each individual
experiment and the resulted error corresponds to one-standard
deviation (1σ) on this average.
ments were carried out under pseudo-first-order conditions
with [NO2]0 ≫ [unsaturated organics] . Under these
0
conditions, the unsaturated organics are subject to the following
removal processes:
unsaturated org + NO → products
k
2
unsaturated org → losses (dilution and wall loss)
k′L
3
. CHEMICALS
where k is the bimolecular rate coefficient of the NO reaction
2
with the unsaturated organic and k′ is the loss rate of the
The purity and the origin of the chemicals used in this work are
isoprene (99%, Sigma-Aldrich), 1,3-cyclohexadiene (98%, Alfa
Aesar), myrcene (90%, Sigma-Aldrich), ocimene (≥90%,
composed of ≥25% of the cis-ocimene and ≥58% for the
trans-ocimene, International Flavors and Fragrances), di-n-butyl
ether (≥99%, Sigma Aldrich), and NO (1% in N , Air liquide).
L
unsaturated organic due to both dilution and wall losses. Under
these conditions, the unsaturated organics concentration−time
profiles followed the pseudo-first-order rate law:
[unsaturated org]
2
2
The sample of ocimene contains about 68% for the trans-
ocimene and 32% for the cis-ocimene.
=
[unsaturated org] × exp(−(k′ + k′)) × t
(I)
0
L
with k′ = k[NO ] . (k′ + k′ ) is derived from the slope of the
2
0
L
4
. RESULTS AND DISCUSSION
plot of ln([unsaturated org] /[unsaturated org] ) versus
0
t
reaction time. k′ is obtained by following the decay rate of
4.1. Kinetic Measurements. The loss rates of the organic
L
the unsaturated organics in the absence of NO . Plots of (k′ +
compounds in absence of NO were on the order of k = (1.1−
2
2
L
1.6) × 10− s . To achieve the pseudo-first-order conditions,
5 −1
k′ ) versus the concentration of the unsaturated organics for
L
−3
the reaction of NO were linear and the absolute rate
the initial concentrations (in molecules cm ) were [alkenes]
= (1.8−16.3) × 10 ; [NO ] = (0.31−3.0) × 10 . Unsaturated
organics were monitored using FT-IR spectroscopy over the
following wavenumbers (in cm ): isoprene, 900; myrcene,
900; ocimene, 900 and 989; 1,3-cyclohexadiene, 3055 and
2837. Experimental durations were from 10 to 110 min. Typical
plots of ln([alkenes] /[alkenes] ) versus time under different
2
0
13
15
coefficients, k, were derived from the least-squares fit of the
straight lines. The quoted error originates from two-standard
deviation from the slope (2σ).
The rate coefficients of both isomers of ocimene (cis and
trans) were also measured using the relative rate method in
which the relative disappearance rate of these species and that
of myrcene used as reference compound were monitored in
2
0
−1
0
t
concentrations of NO are displayed in Figure 1. The rate
2
parallel in the presence of NO . The reaction rate coefficients
coefficients, k, are derived from the slopes of the linear fits
2
can then be derived from the equation:
originated from the plots of k′ versus NO concentrations
2
(
Figure 2). Experimental conditions for each run and the
⎛
⎝
[ocimene] ⎞
t
0
corresponding results can be found in the Supporting
ln⎜
⎟ − k (ocimene) × t
⎜
⎟
L
3
[ocimene]t ⎠
Information. The rate coefficient values derived (in cm
molecule− s ) at T = 299 ± 3 K and P = 760 Torr are
1
−1
⎛
⎛
⎞
⎞
⎟
[
myrcene]t0
kocimene
= k
ln
− k (myrcene) × t⎟
isoprene + NO → products
k = (1.1 ± 0.2) × 10−19
k = (2.5 ± 0.3) × 10−
k = (8.5 ± 1.2) × 10−
⎜
⎜
⎜
⎜
⎟
⎟
L
2
myrcene ⎝ ⎝
[myrcene]
t ⎠
⎠
19
myrcene + NO → products
2
where [ocimene] , [ocimene] , [myrcene] and [myrcene]
t
0
t
t
0
t
0
19
represent the concentrations of ocimene (cis- or trans-ocimene)
and myrcene at reaction time t , [ocimene] , and [myrcene] are
ocimene + NO
2
→ products
0
t
t
18
1
,3‐cyclohexadiene + NO → products
k = (1.5 ± 0.1) × 10−
the corresponding concentrations at time t, kocimene and kmyrcene
2
are the rate constants of the reaction of NO with cis- or trans-
2
The quoted errors represent two-standard deviation (2σ) from
the least-squares analysis.
ocimene and myrcene, respectively. To account for dilution and
wall processes, the pseudo-first-order decay rates of cis- and
trans-ocimene and myrcene, k (ocimene) and k (myrcene),
In the gas phase, NO coexists with its dimer N O according
2
2
4
L
L
to the equilibrium reaction 2NO2 ↔ N O . Using the
2
4
respectively, have been estimated in the absence of NO . Plots
−19
3
−1 −1
2
equilibrium constant, K = 2.5 × 10
cm molecule s ,
of ln[(ocimene) /(ocimene) ] − k (ocimene) × t versus
t
0
t
L
for this equilibrium, it is estimated that N O in our system
2
4
ln[(myrcene) /(myrcene) ] − k (myrcene) × t should be
under the highest NO concentrations used in this work (up to
t
0
t
L
2
1
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Article
study contained both isomers and from the IR analysis we
could not distinguish between these two species. Hence, we
performed further measurements using the relative rate method
and GC-PID as analytical tool. However, a significant overlap of
both chromatographic peaks was observed. Therefore, the
quantification has been performed using the peak height.
Assuming the same response factors for both isomers, the ratios
cis- and trans-ocimene in the reagent were found to be 28% and
7
2%, respectively, which was in agreement with the
composition stated by the supplier. The initial concentrations
1
3
of ocimene, myrcene, and NO used were (in 10 molecules
2
−3
cm ) 4.9, 6.2, and 6.4, respectively. The initial concentration
of NO (as impurity) in the gas mixture was about 2.5 × 10
molecules cm . The presence of NO may lead to the possible
1
1
−3
formation of OH radicals through the reaction NO + HO →
2
OH + NO , which could contribute to an additional
2
consumption of the organic reactants, resulting in an extra
uncertainty on the measured rate coefficients. Therefore, di-n-
1
4
−3
butyl ether was added in excess (3.8 × 10 molecules cm ) to
Figure 1. Pseudo-first-order experiments from the NO -initiated
2
the gas mixture to suppress OH radicals formation (k(di-n-
reaction of myrcene.
−11
3
−1 −1 14
butyl ether + OH) = 2.8 × 10 cm molecule s ). The
plot of (ln([ocimene] /[ocimene] ) − k (ocimene) ) versus
0
t
L
t
15
3
−1
3
× 10 cm molecule ) was less than 0.08% of the NO
(ln([myrcene] /[myrcene] ) − k (myrcene) ) is shown in the
2
0
t
L
t
levels. At these concentrations and assuming the same
reactivity, the dimer would not impact the chemistry occurring
in our experimental system. The absolute method did not
enable us to make a difference between the reactivity of the
ocimene isomers (cis and trans) as the samples used in our
Supporting Information. k (cis/trans-ocimene) and
L
kL(myrcene) represent the first-order decay rates of cis/trans-
ocimene and myrcene in the absence of NO , which were
2
−5
−1
estimated to be ∼1.4 × 10 s . Under these experimental
conditions, the reaction of ocimene and myrcene with NO
2
Figure 2. Plot of absolute rate data for the NO reactions with isoprene (a), myrcene (b), ocimene (c), and 1,3-cyclohexadiene (d).
2
1
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Table 1. Summary of the Rate Coefficients of Studied Alkenes by Dark Reaction with NO in the Gas Phase and Compared to
2
a
the Literature
k (cm molecule s−1
3
−1
)
ref
alkenes
isoprene
T (K)
no. of runs
(1.1 ± 0.2) × 10−
(1.15 ± 0.08) × 10
(1.8 ± 0.3) × 10−
19
19
19
19
19
19
19
19
19
19
19
19
18
19
19
19
18
18
18
18
18
this work, b
3, b
298 ± 2
NI
6
−
5
NI
1
15, c
−
AT
7
(1.8 ± 0.3) × 10
4, c
295 ± 2
295 ± 2
296
NI
NI
8
(1.63 ± 0.13) × 10−
(1.03 ± 0.03) × 10−
16, d
5, b
−
(1.14 ± 0.07) × 10
(2.5 ± 0.3) × 10−
(2.9 ± 0.3) × 10
6, d
myrcene
ocimene
298 ± 1
NI
7
this work, b
10
−
NI
NI
1
2
2
97 ± 2
94 ± 2
2.9 × 10−
11
−
(2.6 ± 0.2) × 10
17, b
299 ± 2
7
(8.5 ± 1.2) × 10−
1.0 × 10−
this work, b
11
2
97 ± 2
94 ± 2
1
−
2
NI
1
(8.9 ± 0.4) × 10
(7.3 ± 1.3) × 10−
17, d, f
this work, e
this work, e
this work, b
8, b
cis-ocimene
297 ± 1
297 ± 1
298 ± 1
−
trans-ocimene
1
(7.3 ± 1.8) × 10
1,3-cyclohexadiene
7
(1.5 ± 0.1) × 10−
2
96
5
(1.75 ± 0.15) × 10−
(
(
1.7 ± 0.2) × 10−
4, e
1.9 ± 0.3) × 10−
4, c
−
(
1.78 ± 0.22) × 10
5, b
a
b
c
AT: ambient temperature. NI: not indicated. Pseudo-first-order conditions (loss rate of the alkene). Second-order conditions (loss rates of both
d
e
f
reactants). Pseudo-first-order conditions (loss rate of NO ). Relative rate conditions. Rate coefficients of isomers cis and trans identical within
2
∼7%.
accounted, respectively, for 84% and 63% of their total decay.
The values of the rate coefficient ratios obtained are k(trans-
ocimene + NO )/k(myrcene + NO ) = 2.9 ± 0.2 and k(trans-
For ocimene, the rate coefficient obtained in this work is in
11
good agreement with those obtained by Reissell et al. and
17
Atkinson et al. as shown in Table 1. In addition, we have
conducted one run to estimate the reactivity of ocimene
2
2
ocimene + NO )/k(myrcene + NO ) = 2.9 ± 0.4. These results
2
2
were placed on the absolute basis by multiplying the ratios by
isomers toward NO and obtained k(cis-ocimene) = k(trans-
2
−19
3
−1 −1
−19
3
−1 −1
k(myrcene + NO ) = (2.5 ± 0.3) × 10 cm molecule s .
ocimene) ≈ 7 × 10 cm molecule s .
2
The rate coefficients derived for the reactions of NO with
Finally, the rate coefficient value for the reaction of NO with
2
2
3
−1 −1
ocimene isomers are (in cm molecule s ):
1,3-cyclohexadiene is also in good agreement with the previous
4
,5,8
measurements.
19
cis‐ocimene + NO → products
k = (7.3 ± 1.3) × 10−
2
Examination of the reactivity of NO with the four studied
2
unsaturated VOCs shows the trend: k(1,3-cyclohexadiene) >
k(ocimene) > k(myrcene) > k(isoprene). The four species
possess a group of conjugated double carbon bonds (−C
CCC−), which are the key reactive sites. The different
reactivities observed mainly depend on the molecular structure
and sites of their substitution groups to the conjugated dienes,
revealing that the reactivity increases with the substitution
degree of −CCCC− groups which has been observed
k = (7.3 ± 1.8) × 10−19
trans‐ocimene + NO → products
2
Errors on the rate coefficients originated from the slopes of the
linear fit adding the error to the rate coefficient of the reference
compound. Absolute error on the rate coefficient is equivalent
to two standard deviations (2σ).
The rate coefficients compared to the literature data are
summarized in Table 1. For isoprene, the reported values are in
5
,6
in this study and elsewhere. The main difference between
−19
−3 −1
these four conjugated dienes is the degree of substitution. The
the range (1.0−1.8) × 10 molecules cm
s
at 297 ± 2 K
difference in the reactivity of NO with ocimene and myrcene
and P ∼ 760 Torr and could be considered in fair agreement
regarding the low reactivity and the difficulties to extract a
precise rate coefficient value for this reaction under these
conditions.
2
shows that the reaction occurs mainly on the conjugated double
bonds and the isolated −CC(CH ) does not play an
3 2
important role.
The rate coefficient value for the reaction of myrcene with
4.2. Oxidation Products. A set of three to five experiments
have been performed for each reaction. Reactants were
monitored using the same procedure as mentioned in the
kinetic section (section 4.1).
NO measured in this work is in good agreement with those
2
10
11
obtained by Orlando et al., Reissell et al., and Atkinson et
1
7
11
al. Reissell et al. indicated that in their work during the first
1
5−20 min of the reaction, the decay rate of myrcene was fast
FT-IR spectra from NO
2
-initiated reactions of myrcene and
and decreased over the course of the reaction time. These
authors suggested the OH radicals formation due to the
presence of NO (NO + HO → NO + OH) either originally
ocimene have been first subtracted from HNO
3
and HONO,
showing the presence of acetone and formic acid among the
oxidation products. Examples of IR spectra obtained are
presented in the Supporting Information. Plots of acetone
versus the consumption of myrcene and ocimene are displayed
2
2
present in the gas mixture or introduced with NO might
2
contribute to additional myrcene consumption.
1
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in Figure 3a,b, respectively. Acetone, the major identified
product from NO2 reactions with myrcene and ocimene,
at all or formed at concentration levels below the detection
limit of our analytical system.
FT-IR analyses of isoprene/NO /air and 1,3-cyclohexa-
2
diene/NO /air mixtures have shown the formation in time of
2
nitro-organic compounds, as shown the IR spectra on Figure
4
a,b. In the case of isoprene, we have observed an increase of IR
Figure 3. Acetone formation versus the consumed concentration of
myrcene (a) and ocimene (b) during the reactions with NO2.
accounted for 22 ± 3% and 26 ± 7%, respectively. HCHO,
HCOOH, and CO have been quantified at 2.0 ± 0.8%, 0.7 ±
0
.2%, and 19 ± 8%, respectively, for the reaction of NO with
2
myrcene and 2.1 ± 1.6%, 1.1 ± 0.5%, and 11 ± 5%,
respectively, for the reaction of NO with ocimene. HONO
2
formation yields have been estimated to 3.5 ± 1.3% for
myrcene and 4.1 ± 1.2% for ocimene. Previous work conducted
on the NO initiated oxidation of myrcene and ocimene
2
reported only acetone as the reaction product. The formation
yield of acetone from myrcene oxidation obtained in the
present work is in disagreement with the reported ones in the
1
0
11
literature estimated at 11 ± 3% and ∼37%. The acetone
formation yield for ocimene oxidation is in agreement with that
reported by Reissell et al. (∼28%). However, it has to be
mentioned that the data of Reissell et al. were derived from a
single experiment and the use of a preconcentration system on
adsorbent cartridge (Tenax) to collect gaseous compounds
during analysis. Indeed, this method could lead to an
11
11
Figure 4. FT-IR spectra of products recorded in the range 1900 −700
−
1
cm from the NO -initiated reaction of isoprene (a) and 1,3-
2
cyclohexadiene (b). Panels A are the recorded spectra of isoprene and
1
,3-cyclohexadiene, before NO2 addition on the Figure 4a,b,
respectively. Panels B have been recorded at 27 and 8 min after
overestimation of reaction products in the terpene−NO −air
X
NO addition for isoprene and 1,3-cyclohexadiene, respectively. Panels
1
8
2
system. The difference observed with the reported value of
C have been recorded 2 and 2 h and 35 min after NO addition for
1
0
2
Orlando et al. remains unexplained so far. We have not
observed the formation of nitrogen-containing compounds as
products of the reactions NO + myrcene and NO + ocimene.
isoprene and 1,3-cyclohexadiene, respectively. Spectra in panels B and
C have been subtracted from isoprene (Figure 4a) and 1,3-
cyclohexadiene (Figure 4b) and known products (nitrous acid, nitric
2
2
This may indicate that these types of products are not formed
acid, and formic acid for isoprene/NO system).
2
1
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−
1
absorption bands centered at 1721, 1296, and 791 cm
formed from secondary sources. Their primary formation yields
have been estimated to be <1%.
−1
assigned to −OONO and at 1373 cm assigned to −NO2
2
1
9,20
groups.
An example of the recorded spectra resulting from
4.3. Reaction Mechanism. The suggested mechanistic
the NO reaction with isoprene in 760 Torr of air is shown in
schemes of the reaction of myrcene and ocimene with NO are
2
2
Figure 4a. IR band features of identified product species have
been subtracted from these spectra. Major IR bands from
spectra B and C have been assigned to nitrogen-containing
compounds, −O NO and −NO groups. However, in the
displayed in Figures 6 and 7, respectively. The reaction
proceeds by addition of NO on conjugated double bonds
2
>CCCC<. For both compounds, the carbon in the sixth
position is supposed to be unfavored due to a steric hindrance
and a low electron delocalization. The expected addition site for
2
2
2
absence of FT-IR reference spectra, their quantifications were
not possible. Figure 5 displays an example of the temporal
NO is that on carbons 5 and 8 for ocimene and on carbons 8
2
and 10 for myrcene, leading to the formation of the allylic C
•
CC radical. Although leading to a less stable radical (C
•
CCC ) than the allylic radical, addition of NO at position
2
7
on ocimene is also considered as a pathway leading to the
observed acetone production. This leads to the formation
nitroalkyl radicals (R−NO ) through reaction 1. These addition
2
sites are expected to occur due to the high stability of the
nitroalkyl radical formed. It either reacts directly with O to
2
form a nitroperoxyalkyl radical (ROONO ) and further with
2
RO via reaction 5 or undergoes electron delocalization, leading
to two other nitroalkyl radical intermediates as suggested
through the equilibrium pathways 2.1. To lead to acetone
formation, we have proposed an intramolecular cyclization of
these intermediates (reaction 2.2 for myrcene and reactions 1.1
and 2.2 for ocimene), which would react quickly to form a
nitroalkoxyl radical (RONO ) through reaction 3 for myrcene
2
and reaction 5 for ocimene. Acetone might be formed through
the decomposition of the nitroalkoxyl radical (reaction 4 for
Figure 5. Time concentration profiles of R−OONO and R−NO
2
2
myrcene and reaction 6 for ocimene). From the NO reaction
type products from NO -initiated reaction of isoprene in the dark.
2
2
with myrcene, the decomposition of the nitroalkoxyl radical
originated from the direct addition of O , without any
2
resonance may lead to the formation of a carbonyl compound
profiles of the IR bands intensities attributed to the −OONO2
and OCH
decomposes to form NO
formation yield of formaldehyde (<1%) shows that this
reaction occurs to a minor extent.
2
NO
2
radical for myrcene + NO
2
reaction. This latter
and −NO groups, obtained from the subtraction of the
2
and formaldehyde. The low
2
residual spectra assigned as reference spectra, versus the
consumed fraction of isoprene over the course of the
experiment. Figure 5 shows a rapid formation at an earlier
stage of the reaction and a decrease over the reaction time for
the R−OONO compound while bands assigned to R−NO
The reaction of NO
NO addition on the double bond, leading to the formation of a
nitrohexenyl radicals. The addition of NO
with 1,3-cyclohexadiene proceeds via
2
2
4
,8
on the terminal
2
2
2
show a linear increase before reaching a plateau against the
consumed fraction of isoprene. HONO has been also observed
at low concentrations, and its formation has been estimated to
be 3.5 ± 1.8%. HCHO, HCOOH, and CO have been identified
among the oxidation products but are mainly formed as
secondary products. Their primary formation yields have been
estimated to be 1.3 ± 0.6%, 0.7 ± 0.5%, and 0.9 ± 0.3% for
HCHO, HCOOH, and CO, respectively. Analysis of NO2-
initiated oxidation of 1,3-cyclohexadiene data has shown
intense peaks that can also be assigned to nitrogen-containing
groups (Figure 4b). Panel B shows three strong bands located
bond is favored due the stability−resonance of the radical
formed. An unpaired electron is removed by electronic
delocalization. A rapid reaction with oxygen leads to the
formation of a nitroperoxyhexenyl radical, which further reacts
with NO
The mechanism proposed for the reaction of isoprene with
NO is based on the studies of the reaction of 1,3-butadiene
(CH
.
2
2
2
CHCHCH
) with NO proposed by Calvert et al.,
2 2
19
2
5
Atkinson et al., and Niki et al., which proceeds by
electrophilic addition on the double bond >CC<. A
proposed scheme for the reaction of isoprene with NO is
2
−
1
at 1721, 1296, and 790 cm characteristic of −O NO groups,
given in the Supporting Information. As shown in the FT-IR
NO
2
2
2
which could be attributed to the formation of nitrocyclohex-
enylperoxy nitrate. From panel C, it can be seen that these
spectra (Figure 4a), formation of −O
2
2
and −ONO
8
compounds have been identified from the beginning of the
compounds exhibit a strong reaction time variability. For a
reaction. NO will add predominantly to C1 due to the
2
longer reaction time, peaks assigned to −O NO groups
formation of the most substituted nitroalkyl radical. NO2
addition on C4 is expected to lead to the formation of a less
stable secondary alkyl radical, which may, however, also occur
to some extent. Additions to C2 and C3 appear to be unlikely
due to a low stability of the radical formed. The nitroalkyl
radical, R−NO , will form a nitroperoxy radical (RNO −OO),
2
2
−1
decreased while the IR bands at 1280 and 822 cm increased.
HONO was also identified from the reaction of NO with 1,3-
2
cyclohexadiene with a yield 2.4 ± 0.6%, in agreement with that
8
reported by Jenkin et al., estimated to be around 2%. In
8
addition, Jenkin et al. have identified benzene among the
2
2
oxidation products from the reaction of 1,3-cyclohexadiene
which in turn reacts with NO , leading to a dinitroperoxy
2
with NO at 0.41 ± 0.02%. HCHO, CO, and HCOOH have
nitrate (RNO −OONO ). The alkyl peroxynitrate may
2
2
2
been identified among the oxidation products but are mainly
decompose back to the peroxy radical. The experiments
1
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Article
Figure 6. Proposed mechanism for the reaction of NO with myrcene in the dark.
2
showed that despite the presence of a relatively high NO2
concentration, the infrared peroxynitrate band disappears when
the reaction goes on (Figure 4a). This behavior may indicate an
instability of this peroxynitrate, which decomposes followed by
a self-reaction of the peroxy radicals. This pathway leads to the
studied alkenes but to a minor extent compared to the addition
mechanism.
8
5. ATMOSPHERIC IMPLICATIONS
The rate coefficients for the reactions of NO with a selected
2
formation of an alkoxy radical which in turn reacts with NO to
2
series of important atmospheric alkenes (myrcene, ocimene,
isoprene, and 1,3-cyclohexadiene) have been determined and
form R−ONO , as observed by FT-IR measurement. The
2
−
18
3
−1 −1
disappearance of peroxynitrate may be due to the following
found to be in the range (0.1−1.5) × 10 cm molecule s .
5
equilibrium ROONO + NO + M ↔ ROONO −NO + M,
2
2
2
2
These values indicate that the reactivity of these species toward
or/and further reaction of RO with residual NO formed over
2
NO is rather slow compared to that with other atmospheric
2
the course of the reaction, which turns into RO and NO2.
−10
oxidants such as OH, O , and NO (k = (1−2.5) × 10 , kO3
3
3
OH
However, the peroxynitrate RO −NO formed is thermally
2
2
(0.1−12) × 10−16, and k
−11
cm3
=
= (0.07−2.2) × 10
NO
3
unstable and may promptly redissociate back to RO . The
2
−1
−1 2,23,24
molecule s ).
However, high levels of NO have been
peroxy radicals will react rapidly with NO, leading to a rapid
2
5
measured in different locations in the world, in particular in
disappearance of the peroxynitrates:
rapid economic development region such as China and India as
25−27
ROONO + NO → RONO + NO
reported by satellite measurements.
These observations
2
2
2
indicate that the contribution of this process to the atmospheric
oxidation of the studied compounds, especially in anthro-
or the self-reaction of R−O NO , leading to the formation of
2
2
28
RONO and O .
pogenically influenced rural air areas, may be not so
negligible. This process may occur under dark as well as
under light conditions and indoor or outdoor. On the other
hand, assuming the concentrations for the following atmos-
2
2
It should be noticed that the formation of oxidation products
formed from NO + alkenes in the absence of NO arise after
2
RO + RO and RO + HO reactions. Therefore, the nature of
2
2
2
2
6
−3
the products formed and their yields may be dependent on the
pheric oxidants: [OH] = 2 × 10 molecules cm for a 12 h
8
−3
presence of mix of specific RO radicals present in gas mixture
daytime average, NO = 5 × 10 molecules cm for a 12 h
nighttime average, and NO = 2 × 10 molecules cm (800
2
3
13
−3
and [RO ]/[HO ] ratios, which then differ from laboratory
2
2
2
2
1,22
experiments to ambient atmospheres.
The presence of
ppb, a very high level that could conceivably be found in a very
heavily trafficked area under stagnation conditions). The NO2
reactions with ocimene and 1,3-cyclohexadiene could, under
HONO among the products revealed that H-abstraction
mechanism also may occur in the NO reaction with the
2
1
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phellandrene, may have higher contributions to both nitro-
organic compounds and HONO.
ASSOCIATED CONTENT
Supporting Information
Summary of the experimental conditions and the results of the
■
*
S
absolute rate study from the reaction of NO with alkenes
2
(
isoprene, myrcene, ocimene, and 1,3-cyclohexadiene) is
available. A graph obtained of the relative rate measurements
from the NO -initiated reaction of ocimene using myrcene as
2
reference organic compound in the presence of an excess of di-
n-butylether can be also found. In addition, FT-IR spectra of
−1
products recorded in the range 1900−700 cm from the NO2-
initiated reaction of myrcene (a) and ocimene (b) are provided.
A proposed mechanism of the NO -initiated reaction of
2
isoprene in the dark is also included. Complete author lists
for refs 7 and 28 are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
A. Mellouki: tel, +33 (0) 2 38 25 76 12; fax, +33 (0) 2 38 69
0 04; e-mail, mellouki@cnrs-orleans.fr.
■
*
6
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by the European FP7
EUROCHAMP 2 project, the IRSES-EU project “AMIS” and
the Labex Voltaire (ANR-10-LABX-100-01).
Figure 7. Proposed mechanism for the reaction of NO with ocimene
2
in the dark.
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