Products of the gas-phase reaction of O3 with cyclohexene

Article abstract of DOI:10.1021/jp022122e

In the troposphere, alkenes react with OH radicals, NO₃ radicals, and O₃, with the O₃ reactions often being an important transformation process during both daytime and nighttime. The reactions of O₃ with alkenes lead to the production of OH radicals, often in high yield, and these reactions also lead to the formation of secondary organic aerosol. Products of the gas-phase reactions of O₃ with cyclohexane and cyclohexene-d₁₀ were studied in the presence of OH radical scavengers by gas. Cyclohexane and cyclohexene-d₁₂ were used as OH radicals. In the cyclohexene reaction in the presence of sufficient cyclohexane to scavenge >95% of the OH radicals formed, 1.47 × 10¹⁴ molecule/cu cm of O₃ consumed 1.62 × 10¹⁴ molecule/cu cm of cyclohexene, with a 3.5% yield of HC(O)OH which increased to 4.5% after 17 min. Addition of butanal to the reactant mixtures resulted in changes in the post-reaction API-MS spectra. Adipaldehyde could be formed from reaction of the thermalized Criegee intermediate (presumably the anti-intermediate) with water vapor. OH (or OD) radical formation yields were also measured from the reactions of O₃ with propylene (40 ± 6%).

Full text of DOI:10.1021/jp022122e

J. Phys. Chem. A 2003, 107, 2247-2255
2247
Products of the Gas-Phase Reaction of O3 with Cyclohexene
Sara M. Aschmann, Ernesto C. Tuazon, Janet Arey,† and Roger Atkinson*,†,‡
Air Pollution Research Center, UniVersity of California, RiVerside, California 92521
ReceiVed: September 24, 2002; In Final Form: January 15, 2003
Products of the gas-phase reactions of O3 with cyclohexene and cyclohexene-d10 were investigated in the
presence of OH radical scavengers by gas chromatography with flame ionization detection, combined gas
chromatography-mass spectrometry, in situ Fourier transform infrared spectroscopy, and in situ atmospheric
pressure ionization mass spectrometry (API-MS). Cyclohexane and cyclohexane-d12 were used as OH radical
scavengers in the experiments using API-MS analyses to allow products formed from the O3 reactions with
cyclohexene and cyclohexene-d10 to be differentiated from those from the reactions of OH radicals with
cyclohexane and cyclohexane-d12. The gas-phase products observed from the reaction of O3 with cyclohexene
in the presence of an OH radical scavenger were pentanal (23.6 ( 1.8%); OH radicals (54 ( 8%); formic
acid (3.5% initial yield); glutaraldehyde [HC(O)CH2CH2CH2CHO]; adipaldehyde [HC(O)CH2CH2CH2CH2-
CHO], a C6H10O3 product attributed to the secondary ozonide; a C5H7O2(OOH) product, a molecular weight
130 hydroxydicarbonyl; and a molecular weight 116 carbonyl compound. Yields of pentanal-d10, OD radicals,
and DC(O)OH from the reaction of O3 with cyclohexene-d10, of 16.4 ( 1.4%, 50 ( 7%, and 1.6% (initial),
respectively, were obtained. Our data indicate that reactions of the Criegee intermediate to form pentanal
(plus CO2) and an OH radical plus organic radical coproduct account for 78 ( 9% of the reaction pathways,
with the organic radical coproduct reacting to form (in part) glutaraldehyde. Adipaldehyde can be formed
from reaction of the thermalized Criegee intermediate (presumably the anti-intermediate) with water vapor.
OH (or OD) radical formation yields were also measured from the reactions of O3 with propene (40 ( 6%),
propene-d6 (27 ( 4%) R-pinene (86 ( 13%), and 2,3-dimethyl-2-butene (107 ( 16%).
Introduction
mass spectrometry (GC-MS), in situ Fourier transform infrared
spectroscopy (FT-IR), and in situ atmospheric pressure ioniza-
Alkenes are emitted into the atmosphere from anthropogenic
and biogenic sources.1 In the troposphere, alkenes react with
OH radicals, NO3 radicals, and O3,1-3 with the O3 reactions
often being an important transformation process during both
daytime and nighttime.1,3 The reactions of O3 with alkenes lead
to the production of OH radicals, often in high yield,2,3 and
these reactions also lead to the formation of secondary organic
aerosol.4-8 While the initial steps involved in the reactions of
O3 with alkenes are understood,1-3 there are many details of
the complete reaction schemes which require investigation, and
these include the identity of the reaction products which initiate
formation of secondary organic aerosol, and the reactions of
the stabilized Criegee intermediates. For example, it has been
reported that the reaction of O3 with cyclohexene (a symmetrical
cycloalkene which can serve as a model compound for several
monoterpenes emitted from vegetation) forms C5- and C6-
dicarboxylic acids which nucleate and/or partition into seed
particles and are important components of secondary organic
aerosol.5,7,8 However, the routes leading to formation of these
dicarboxylic acids are at present speculative.
tion tandem mass spectrometry (API-MS) to investigate the gas-
phase products formed from the reactions of cyclohexene and
cyclohexene-d10 in the presence of OH radical scavengers. The
use of cyclohexane and cyclohexane-d12 as OH radical scav-
engers in the experiments with API-MS analyses allowed the
products arising from the reactions of O3 with cyclohexene to
be differentiated from those formed from the OH radical reaction
with cyclohexane, and investigation of the cyclohexene-d10
reaction further aided in the elucidation of products using API-
MS. As part of this work, the use of 2,3-butanediol as a radical
scavenger to determine OH radical formation from the reactions
of O3 with cyclohexene and cyclohexene-d10 was validated.
Experimental Section
Experiments were carried out at atmospheric pressure of air
and at 296-298 K in a 5800 L evacuable, Teflon-coated
chamber equipped with a multiple-reflection optical system
interfaced to a Nicolet 7199 FT-IR spectrometer, in a ∼8000 L
Teflon chamber with off-line analyses by GC-FID, and in a
∼7000 L Teflon chamber interfaced to a PE SCIEX API III
MS/MS direct air sampling, atmospheric pressure ionization
tandem mass spectrometer (API-MS). The experiments with GC-
FID, GC-MS and API-MS analyses were carried out at ∼5%
relative humidity (∼3.4 × 1016 molecule cm-3 of water).
GC-FID Analyses. Two series of O3/cyclohexene (or cyclo-
hexene-d10)/scavenger/air reactions were carried out, with
analysis by gas chromatography with flame ionization detection
(GC-FID) and with the scavenger being present to (a) react with
To complement a recent study by Ziemann8 of the aerosol-
phase products formed from the reaction of O3 with cyclohex-
ene, in this work we have used gas chromatography with flame
ionization detection (GC-FID), combined gas chromatography-
* To whom correspondence should be addressed. Phone: (909-787-4191.
E-mail: ratkins@mail.ucr.edu.
† Also Department of Environmental Sciences and Interdepartmental
Graduate Program in Environmental Toxicology.
‡ Also Department of Chemistry.
10.1021/jp022122e CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/06/2003

2248 J. Phys. Chem. A, Vol. 107, No. 13, 2003
Aschmann et al.
the OH radicals formed in the reactions of O3 with cyclohex-
ene9,10 or cyclohexene-d10 and (b) to enable the OH radical
formation yields from the O3 reactions to be derived from
measurements of a product(s) of the OH radical reaction with
the scavenger compound.9,11 Previously cyclohexane was em-
ployed as a radical scavenger,9 but because of uncertainties in
the yields of the products measured to quantify the amount of
OH radical formation, 2-butanol has more recently been
employed with measurements of the amounts of 2-butanone
formed.11,12 However, on the DB-1701 column used to separate
2-butanone from a large excess of 2-butanol, cyclohexene and
2-butanone coeluted. Therefore, a number of other diol and
hydroxycarbonyl scavengers were investigated, with the goal
being to measure the amounts of hydroxycarbonyl or dicarbonyl
products formed, respectively. For example, when 2,3-butanediol
was used as an OH radical scavenger the product monitored
was 3-hydroxy-2-butanone.13
temperature programmed at 8 °C min-1 to 200 °C. For the
analysis of pentanal (or pentanal-d10) and 3-hydroxy-2-butanone,
100 cm3 volume gas samples were collected from the chamber
onto Tenax-TA solid adsorbent, with subsequent thermal
desorption at ∼250 °C onto a 30 m DB-1701 megabore column
held at -40 °C and then temperature programmed to 200 °C at
8 °C min-1. GC-FID response factors were determined by
introducing measured amounts of the chemicals into the chamber
and conducting several replicate GC-FID analyses.
GC-MS Analyses. To identify carbonyls as their oxime
derivatives, experiments were also carried out in which a 65
µm PDMS/DVB Solid-Phase Micro Extraction (SPME) fiber14
coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFB-
HA) hydrochloride15 was exposed to the chamber reaction
products and then analyzed by combined gas chromatography-
mass spectrometry (GC-MS) with thermal desorption onto a 30
m DB-1701 fused silica capillary column in a Varian 2000 GC/
MS/MS with analysis by isobutane chemical ionization. Car-
bonyl-containing products were examined from cyclohexene and
cyclohexene-d10 reactions in which cyclohexane was used as
the OH radical scavenger. Standards of pentanal and glutaral-
dehyde were analyzed to confirm the retention times and mass
spectra of their oxime derivatives.
OH + CH3CH(OH)CH(OH)CH3 f
H2O + CH3C•(OH)CH(OH)CH3 (1)
CH3C•(OH)CH(OH)CH3 + O2 f
HO2 + CH3C(O)CH(OH)CH3 (2)
API-MS Analyses. In these experiments, the chamber
contents were sampled through a 25 mm diameter x 75 cm
length Pyrex tube at ∼20 L min-1 directly into the API mass
spectrometer source. The operation of the API-MS in the MS
(scanning) and MS/MS [with collision activated dissociation
(CAD)] modes has been described previously.16 Use of the MS/
MS mode with CAD allows the “product ion” or “precursor
ion” spectrum of a given ion peak observed in the MS scanning
mode to be obtained.16
The OH radical scavengers investigated were 1,2-, 1,3-, and
2,3-butanediol, for which the rate constants for their reactions
with OH radicals and the formation yields of the corresponding
hydroxyketone have been measured.13 Another potential scav-
enger, 4-hydroxy-3-hexanone, had as an impurity ∼0.7% of 3,4-
hexanedione, its OH radical reaction product. Hence in the
presence of ∼1.5 × 1015 molecule cm-3 of 4-hydroxy-3-
hexanone, the level required for efficient scavenging of OH
radicals, formation of 3,4-hexanedione during the O3 reactions
would have been relatively small compared to its initial
concentration. As in our previous study using 2-butanol as the
OH radical scavenger,11 in preliminary experiments we inves-
tigated formation of the hydroxycarbonyls from the diols in the
presence of O3, with the diol and O3 concentrations being ∼1.7
The majority of the data obtained used the positive ion mode,
in which protonated water hydrates (H3O+ (H2O)n) generated
by the corona discharge in the chamber diluent air were
responsible for the protonation of analytes.16,17 Ions are drawn
by an electric potential from the ion source through the sampling
orifice into the mass-analyzing first quadrupole or third quad-
rupole. In these experiments the API-MS instrument was
operated under conditions that favored the formation of dimer
ions in the ion source region.16 Neutral molecules and particles
are prevented from entering the orifice by a flow of high-purity
nitrogen (“curtain gas”), and as a result of the declustering action
of the curtain gas on the hydrated ions, the ions that are mass
analyzed are mainly protonated molecular ions ([M + H]+) and
their protonated homo- and hetero-dimers.16
× 1015 molecule cm-3 and (4-5) × 1012 molecule cm-3
,
respectively. On the basis of the concentrations of the hydroxy-
carbonyls initially present and the amounts formed in the
presence of O3, we decided to use 2,3-butanediol as the OH
radical scavenger, with formation of 3-hydroxy-2-butanone in
89 ( 9% yield from the OH radical-initiated reaction.13 To test
the use of 2,3-butanediol as an radical scavenger to measure
OH radical yields, OH radical formation yields from the
reactions of O3 with propene (and propene-d6), R-pinene, and
2,3-dimethyl-2-butene were also measured.
Experiments were carried out with cyclohexene + cyclohex-
ane, cyclohexene-d10 + cyclohexane, cyclohexene + cyclohex-
ane-d12, and cyclohexene-d10 + cyclohexane-d12. The initial
reactant concentrations (in molecule cm-3 units) were: cyclo-
hexene or cyclohexene-d10, ∼2.4 × 1013; cyclohexane, ∼2.4 ×
1015; or cyclohexane-d12, ∼3.6 × 1015; and two additions of
50 cm3 volume of O3 in O2 diluent were made to the chamber.
The water vapor concentration was ∼3.4 × 1016 molecule cm-3
(∼5% relative humidity). In additional experiments, ∼2.4 ×
1013 molecule cm-3 of butanal was added to the cyclohexene
(or cyclohexene-d10)/cyclohexane/air mixture (at ∼5% relative
humidity) prior to reaction to investigate whether the butanal
intercepted the Criegee intermediate.
The initial reactant concentrations (molecule cm-3 units) were
cyclohexene or cyclohexene-d10, (2.20-2.47) × 1013; 2,3-
butanediol, 1.7 × 1015; or cyclohexane (used as the OH radical
scavenger in certain of the experiments to measure the yields
of pentanal or pentanal-d10), 4.9 × 1015; and four additions of
50 cm3 volume O3/O2 aliquots were made to the chamber during
an experiment, with each O3/O2 addition corresponding to an
initial concentration of O3 in the chamber of ∼5 × 1012 molecule
cm-3. The water vapor concentration was ∼3.4 × 1016 molecule
cm-3 (∼5% relative humidity). The concentrations of cyclo-
hexene and cyclohexene-d10 and of selected products were
measured during the experiments by GC-FID. For the analyses
of cyclohexene and cyclohexene-d10, gas samples were collected
from the chamber into 100 cm3 volume all-glass gastight
syringes and transferred via a 1 cm3 gas sampling loop onto a
30 m DB-5 megabore column held at -25 °C and then
FT-IR Analyses. Experiments were carried out in which
cyclohexene and cyclohexene-d10 were reacted with O3, both
in the presence and absence of cyclohexane as an OH radical
scavenger, with analyses by in situ FT-IR spectroscopy. The
initial concentrations (in units of molecule cm-3) were cyclo-

Gas-Phase Reaction of O3 with Cyclohexene
J. Phys. Chem. A, Vol. 107, No. 13, 2003 2249
hexene or cyclohexene-d10, (4.77-4.92) × 1014; O3, 1.47 ×
1014; and in certain experiments, cyclohexane, 9.5 × 1016. The
reactants were mixed for 3 min using two magnetically coupled
Teflon-coated fans, including an ∼0.5 min injection time for
O3. FT-IR spectra were recorded every 2.5 min with a path
length of 62.9 m and a full width at half-maximum resolution
of 0.7 cm-1
.
The following IR absorption bands with sharp Q branches
(cm-1) were used for quantitative measurements: cyclohexene,
1140; cyclohexene-d10, 1082; HC(O)OH, 1105; and DC(O)-
OH, 1143. Calibrated reference spectra of DC(O)OH were
obtained by introducing 0.241 g of DC(O)OH into air at
atmospheric pressure in the 5800 L chamber, recording the
spectra at a path length of 5.59 m for 30 min with 5-min
intervals, and correcting the concentrations on the basis of the
measured decay rate of 1.90 × 10-3 min-1. A similar procedure
was used for the calibration for anhydrous HC(O)OH, obtained
from a sample of 90.6% HC(O)OH solution in H2O which was
dried over anhydrous CaSO4.
Chemicals. The chemicals used and their stated purities
were: cyclohexane (HPLC grade), Fisher Scientific; 1,2-
butanediol (99%), 2,3-butanediol (98%), 1,3-butanediol (99+%),
cyclohexane-d12 (99.5 atom % D), glutaraldehyde (50 wt %),
1-hydroxy-2-butanone (95%), 3-hydroxy-2-butanone, O-(2,3,4,5,6-
pentafluorobenzyl)hydroxylamine hydrochloride (98+%), and
pentanal (99%), Aldrich Chemical Co.; cyclohexene (99%),
Chem Samples; cyclohexene-d10 (98 atom % D), Isotec Inc;
4-hydroxy-2-butanone (95+%), TCI America; HC(O)OH (90.6%),
Baker Analyzed Reagent; and DC(O)OH (95wt % in water),
Aldrich/Isotec. O3 in O2 diluent was prepared as needed using
a Welsbach T-408 ozone generator.
Figure 1. Plots of the amounts of 3-hydroxy-2-butanone formed against
the amounts of cyclohexene and cyclohexene-d10 reacted with O3, in
the presence of sufficient 2,3-butanediol to scavenge >95% of the OH
radicals formed. The 3-hydroxy-2-butanone data from the cyclohexene
reaction have been displaced vertically by 2.0 × 1012 molecule cm-3
for clarity.
TABLE 1: Measured OH Radical Formation Yields from
Reactions of O3 with Selected Alkenes at Atmospheric
Pressure Using 2,3-Butanediol as the Radical Scavenger,
Together with Recent Literature Data
OH radical formation yield
Results
alkene
cyclohexene
cyclohexene-d10
this worka
literature
reference
Measurement of OH Radical Yields. In addition to inves-
tigating the formation of OH radicals from the reactions of O3
with cyclohexene and cyclohexene-d10, we also studied the
reactions of O3 with propene, propene-d6, R-pinene and 2,3-
dimethyl-2-butene to investigate the effect of deuteration on the
propene reaction and compare the measured OH radical forma-
tion yields for propene, R-pinene and 2,3-dimethyl-2-butene with
previous literature data.10,18-24 Plots of the amounts of 3-hy-
droxy-2-butanone formed against the amounts of cyclohexene
and cyclohexene-d10 reacted are shown in Figure 1, and the OH
(or OD) radical formation yields obtained for the alkenes studied
are given in Table 1.
0.54 ( 0.08 0.54 ( 0.13 Fenske et al.10
0.50 ( 0.07
propene
0.40 ( 0.06 0.35 ( 0.07 Paulson et al.18
0.32 ( 0.08 Rickard et al.19
+0.03
Neeb and
0.34
-0.06
Moortgat20
0.33 ( 0.07 Fenske et al.21
0.37 ( 0.08 Fenske et al.21
propene-d6
R-pinene
0.27 ( 0.04
0.86 ( 0.13 0.76 ( 0.11 Chew and
Atkinson11
0.70 ( 0.17 Paulson et al.22
0.83 ( 0.21 Rickard et al.19
0.91 ( 0.23 Siese et al.23
GC-FID and FT-IR Analyses. In agreement with the
previous studies of Hatakeyama et al.,5 Grosjean et al.,25 and
Kalberer et al.,7 pentanal was observed from the O3 reaction
with cyclohexene. Plots of the amounts of pentanal and pentanal-
2,3-dimethyl-2-butene 1.07 ( 0.16 0.80 ( 0.12 Chew and
Atkinson11
0.89 ( 0.22 Rickard et al.19
0.99 ( 0.18 Fenske et al.21
1.00 ( 0.25 Siese et al.23
0.91 ( 0.14 Orzechowska
and Paulson24
d10 formed against the amounts of cyclohexene and cyclohexene-
d10 reacted are shown in Figure 2. Least-squares analyses of
these data lead to formation yields of pentanal from cyclohexene
and pentanal-d10 from cyclohexene-d10 of 0.236 ( 0.018 and
0.164 ( 0.014, respectively, where the indicated errors are two
least-squares standard deviations combined with estimated
uncertainties in the GC-FID response factors for cyclohexene
and pentanal (or their deuterated analogues) of (5% each.
In situ FT-IR analyses of reacted O3/cyclohexene/cyclohex-
ane/air and O3/cyclohexene-d10/cyclohexane/air mixtures showed
total consumption of the O3 introduced within 4 min of mixing
in all cases. In the cyclohexene reaction in the absence of added
cyclohexane, 1.47 × 1014 molecule cm-3 of O3 consumed 2.18
× 1014 molecule cm-3 of cyclohexene, with a 2.5% yield of
HC(O)OH which gradually increased to 3.2% after a period of
a Indicated errors are two least squares standard deviations combined
with estimated uncertainties in the GC-FID response factors for the
alkene and 3-hydroxy-2-butanone of (5% each.
34 min. HCHO was not detected as a product. In the cyclo-
hexene reaction in the presence of sufficient cyclohexane to
scavenge >95% of the OH radicals formed, 1.47 × 1014
molecule cm-3 of O3 consumed 1.62 × 1014 molecule cm-3 of
cyclohexene, with a 3.5% yield of HC(O)OH which increased
to 4.4% after 17 min. In the cyclohexene-d10 reaction in the
absence of an OH radical scavenger, 1.47 × 1014 molecule cm-3
of O3 consumed 2.19 × 1014 molecule cm-3 of cyclohexene-
d10 and showed a 0.8% yield of DC(O)OH which increased to

2250 J. Phys. Chem. A, Vol. 107, No. 13, 2003
Aschmann et al.
Figure 2. Plots of the amounts of pentanal and pentanal-d10 formed
against the amounts of cyclohexene and cyclohexene-d10, respectively,
reacted with O3, in the presence of sufficient cyclohexane or 2,3-
butanediol to scavenge >95% of the OH radicals formed. The pentanal
data from the cyclohexene reaction have been displaced vertically by
1.0 × 1012 molecule cm-3 for clarity.
Figure 3. API-MS spectra (using H3O+(H2O)n as the reagent ion) of
reacted O3/cyclohexene/cyclohexane/air and O3/cyclohexene-d10/cy-
clohexane/air mixtures. The ion peaks arising from the reaction of OH
radicals with cyclohexane are noted by asterisks. For the identities of
the major ion peaks see Table 2.
1.0% after 10 min. DC(O)OD, which has a sharp Q-branch
absorption at 1171 cm-1 (the equivalent of the 1143 cm-1
absorption of DC(O)OH26) was not observed; it is expected that
the labile D atom of DC(O)OD rapidly undergoes D/H exchange
with the water vapor present in the chamber.27 In the cyclo-
hexene-d10 reaction with added cyclohexane, 1.47 × 1014
molecule cm-3 of O3 consumed 1.80 × 1014 molecule cm-3 of
cyclohexene-d10, and resulted in a 1.6% yield of DC(O)OH
which increased to 2.2% after 10 min.
API-MS Analyses. API-MS spectra were obtained from the
reactions of O3 with cyclohexene and cyclohexene-d10, each in
the presence of sufficient cyclohexane and cyclohexane-d12 to
scavenge >91% (added cyclohexane) or >86% (added cyclo-
hexane-d12) of the OH radicals formed from the O3 reactions.
The use of cyclohexane and cyclohexane-d12 as OH radical
scavengers allowed the products formed from the O3 reactions
with cyclohexene and cyclohexene-d10 to be differentiated from
those formed from the reactions of OH radicals with cyclohex-
ane and cyclohexane-d12. Thus, Figure 3 shows API-MS spectra
from reacted O3/cyclohexene/cyclohexane/air and O3/cyclohex-
ene-d10/cyclohexane/air mixtures using protonated water clusters
as the reagent ion, with the ion peaks arising from the reaction
of OH radicals with cyclohexane being noted by asterisks.
Analysis of the API-MS spectra from the various combinations
of reactions showed the products listed in Table 2 from the
reactions of O3 with cyclohexene and cyclohexene-d10.
ion peaks at 115 and 125 u in the cyclohexene and cyclohexene-
d10 reactions, respectively, are attributed to adipaldehyde (see
Table 2).
An API-MS/MS CAD “product ion” spectra of the protonated
molecular weight 130 and 140 products from the cyclohexene
and cyclohexene-d10 reactions, respectively, are shown in Figure
4. The CAD spectra reveal that these are analogous products,
with the 131 u ion peak from the cyclohexene reaction showing
losses of H2O, 2H2O, H2O+CO, and 2H2O+CO, while the 141
u ion peak from the cyclohexene-d10 reaction shows losses of
HDO+CO and HDO+D2O+CO. These fragmentation patterns
suggest the presence of three oxygens, and the presence of 10
deuteriums in the cyclohexene-d10 reaction product is consistent
with a secondary ozonide.
Figure 5 shows that the 133 u ion peak from the cyclohexene
reaction has losses of H2O, 34 (H2O2) and H2O+CO, while the
140 u ion peak from the cyclohexene-d10 reaction has losses of
H2O, H2O2, and H2O/HDO+CO. Particularly interesting (and
allowing identification of the 140 u ion peak in the cyclohexene-
d10 reaction as the analogue to the 133 u ion peak in the
cyclohexene reaction) is the loss of a 34 mass unit fragment
(H2O2) in both cases. Loss of H2O2 is indicative of the presence
of an OOH group and, because OD (and OOD) groups undergo
rapid D/H exchange,28,29 these losses of H2O and H2O2 are
indicative of products containing OOH and OOD groups.
Therefore, the products of molecular weight 132 and 139 from
the cyclohexene and cyclohexene-d10 reactions, respectively,
have formula of C5H7O2(OOH) and C5D7O2(OOH), respectively.
Addition of butanal to the reactant mixtures resulted in
changes in the post-reaction API-MS spectra. However, at least
qualitatively these changes were due to the formation of
heterodimers of protonated butanal and the protonated butanal
dimer with reaction products. Thus, for example, in a reacted
The most intense ion peaks from pentanal and pentanal-d10
were those at [M+H+H2O]+ at 105 and 115 u, respectively,
and an API-MS/MS CAD “product ion” spectrum of the weak
87 u ion peak from the cyclohexene reaction was identical to
that of an authentic standard of pentanal. An API-MS/MS CAD
“product ion” spectrum of the 101 u ion peak in the cyclohexene
reaction in the presence of cyclohexane-d12 (the reaction of O3
with cyclohexene in the presence of cyclohexane leads to a 101
u ion peak from cyclohexanol formed from cyclohexane9) was
identical to that of an authentic standard of glutaraldehyde. The

TABLE 2: Mass Spectral Evidence for Products Formed from the Gas-Phase Reactions of O3 with Cyclohexene and Cyclohexene-d10 in the Presence of an OH Radical Scavenger
O3 + cyclohexene O3 + cyclohexene-d10
APIa (see Figure 3) APIc(see Figure 3)
[M+H]+ ) 87 [M+H]+ ) 97
product (MW)
pentanal (86)
CH3CH2CH2CH2CHO
SPME oximesb
product (MW)
pentanal-d10 (96)
CD3CD2CD2CD2CDO
SPME oximesd
[M+H]+ ) 282e
[M+H]+ ) 292
[M+H+H2O]+ ) 105 (2 isomers)
[M+H+H2O]+ ) 115 [M+H]+ (with H/D exchange) = 293
glutaraldehyde (100)
HC(O)(CH2)3CHO
[M+H]+ ) 101f
[M+H]+ ) 115
[M+H]+ ) 491e
glutaraldehyde-d8 (108)
DC(O)CD2CD2CD2CDO
[M+H]+ ) 109
[M+H]+ ) 125
[M+H]+ (with H/D exchange) = 500
[M+H]+ (with 2 H/D exchanges) = 501
(dioxime)
adipaldehyde (114)
HC(O)(CH2)4CHO
[M+H]+ ) 505
(dioxime)
adipaldehyde-d10 (124)
DC(O)(CD2)4CDO
[M+H]+ (with H/D exchange) = 516
[M+H]+ (with 2 H/D exchanges) = 517
secondary ozonide (130)
C6H10O3
[M+H]+ ) 131g
secondary ozonide (140)
C6D10O3
[M+H]+ ) 141g
[M+H+H2O]+ ) 149
[M+H+H2O]+ ) 159
C6H10O3 (130)h
[M+H]+ ) 326
C6D10O3 (140)
[M+H]+ ) 336
[M+H-H2CO2]+ ) 280
[M+H]+ (with H/D exchange) = 337
[M+H-HDCO2]+ ) 289
peracid (132)
C5H7O2(OOH)
[M+H]+ ) 133i
peracid (139)
[M+H]+ ) 140i
C5D7O2(OOH)j
hydroxydicarbonyl (130)
[M+H]+ ) 326
[M+H-H2O]+ ) 308
(monooxime)
hydroxydicarbonyl (139)
[M+H]+ ) 335
HC(O)(CH2)3CH(OH)CHOk
[M+H]+ (with H/D exchange) = 336
[M+H-H2O]+ ) 317
(monooxime)
[M+H]+ ) 521
[M+H-H2O]+ ) 503
(dioxime)
[M+H]+ (with H/D exchange) = 531
[531-H2O]+ ) 513
[531-HDO]+ ) 512
(dioxime)
hydroxycarbonyl or oxo-acid (116)
[M+H]+ ) 312
hydroxycarbonyl or oxo-acid (123)
[M+H]+ ) 319
[M+H-H2O]+ ) 294
[M+H]+ (with H/D exchange) = 320
[319-H2O/HDO]+ and [320-HDO]+ ) 300 and 301
a The high mass peaks in Figure 3 (top) are those of homo- and heterodimers. For example, 261 ) 130+130+H, 215 ) 114+100+H, 231 ) 130+100+H, and 169 ) 100+86+H-H2O. Note that the
major products from the cyclohexane scavenger reaction with the OH radical are cyclohexanone (MW 98) and cyclohexanol (MW 100) and these may participate in the formation of heterodimers. b Forming
an oxime derivative adds 195 mass units to the molecular weight; a diderivative adds 390 units. c The high mass peaks in Figure 3 (bottom) are those of homo- and heterodimers. For example, 225 )
124+100+H, 241 ) 140+100+H, and 179 ) 100+96+H-H2O. Note that the major products from the cyclohexane scavenger reaction with the OH radical are cyclohexanone (MW 98) and cyclohexanol
(MW 100) and these participate in the formation of heterodimers. d D/H exchange occurs in ion source. Forming an oxime derivative adds 195 mass units to the molecular weight; a diderivative adds 390
units. e GC retention times and mass spectra verified with standard compounds. f See text for discussion of reaction with cyclohexane-d12 which allowed unambiguous identification of glutaraldehyde. g See
Figure 4 for CAD spectra. h Possibly the secondary ozonide which was sampled by the SPME, with decomposition on the fiber to form an oxo-acid which was then derivatized and analyzed as its oxime (see
text). i See Figure 5 for CAD spectra. j OD/OH exchange occurs, presumably with water in the chamber. k Tentative identification (see Scheme 3).

2252 J. Phys. Chem. A, Vol. 107, No. 13, 2003
Aschmann et al.
Figure 5. API-MS/MS CAD “product ion” spectra (using H3O+(H2O)n
as the reagent ion) of the 133 and 140 u ion peaks observed in the
reactions of O3 with cyclohexene and cyclohexene-d10 reactions,
respectively. Cyclohexane was present to scavenge OH radicals. Note
the presence of fragment ions at 99 and 106 u from the cyclohexene
and cyclohexene-d10 reactions, respectively, indicating a loss of H2O2
in both cases.
Figure 4. API-MS/MS CAD “product ion” spectra (using H3O+(H2O)n
as the reagent ion) of the 131 and 141 u ion peaks observed in the
reactions of O3 with cyclohexene and cyclohexene-d10 reactions,
respectively. Cyclohexane was present to scavenge OH radicals.
O3/cyclohexene/cyclohexane/butanal/air mixture compared to a
reacted O3/cyclohexene/cyclohexane/air mixture the ion peaks
of the molecular weight 114 and 130 products (which are
believed to involve reactions of the thermalized Criegee
intermediate; see below) and their protonated homo- and
heterodimers at 115, 131, 149, 215, 231, and 261 u (see Table
2) decreased in the presence of butanal and were replaced (at
least in part) by ion peaks at 217 u ([114+butanal+H]+) and
275 u ([130+butanal+butanal+H]+) in the presence of butanal
in the reactant mixture. In particular, no evidence for any
significant formation of the secondary ozonide arising from
reaction of the Criegee intermediate HC(O)CH2CH2CH2CH2-
CHOO (and its deuterated analog) with butanal was observed
(the protonated secondary ozonides being at 203 and 213 u,
respectively).
retention times and mass spectra with those of authentic
standards. Note that two isomers of the oxime from pentanal
were formed and both carbonyls in glutaraldehyde were de-
rivatized, with only a trace of the singly derivatized compound
oxime being observed. The corresponding peaks from the
cyclohexene-d10 reaction eluted from the GC column a few
seconds earlier than the cyclohexene products. The presence of
the apparent molecular ion of glutaraldehyde-d8 at 500 u rather
than the predicted 499 u, with an ion peak also observed at 501
u, must be attributed to H/D exchange in the ion trap source,
which is subject to secondary ion-molecule reactions.30
The SPME analysis confirmed the presence of two carbonyl
groups in the molecular weight 114 compound attributed to
adipaldehyde in the API analyses. A large oxime peak corre-
sponding to a molecular weight 130 compound was also
observed from the cyclohexene reaction, and in contrast to the
oximes of pentanal, glutaraldehyde, and adipaldehyde which
showed almost no fragmentation, the base peak in the spectrum
was due to a loss of H2CO2 from the [M+H]+ molecular ion.
As noted in Table 2, the corresponding peak from the cyclo-
hexene-d10 reaction suggested 10 deuteriums and a base peak
due to loss of HDCO2 from the [M+H]+ ion (or loss of D2CO2
from the [M+D]+ ion). It is possible that this oxime peak is
due to SPME sampling of the secondary ozonide, with
decomposition of the secondary ozonide on the SPME fiber to
an oxo-acid which was then derivatized and analyzed as its
oxime.
GC-MS Analyses of PFBHA Derivatives. In two experi-
ments, PDMS/DVB SPME fibers precoated with PFBHA were
exposed in the chamber to cyclohexene and cyclohexene-d10
reaction products. Oxime derivatives are formed from carbonyl-
containing compounds through the reaction.
C6F5CH2ONH2 + R1C(O)R2 f
C6F5CH2ONdCR1R2 + H2O
Utilizing GC-MS with isobutane chemical ionization, generally
intense protonated molecules [M+H]+ and small adduct ions
at [M+41]+ are observed, where the molecular mass M of the
oxime is 195 mass units greater than the weight of the carbonyl,
or in the case of a dicarbonyl forming a dioxime, 390 mass
units higher. Note that Z and E configurations are sometimes
present and resolved.
Significantly smaller peaks corresponding to two carbonyl-
containing compounds of molecular weight 116 and 130 were
also observed in the SPME analysis. For the molecular weight
130 product, both mono- and dioxime derivatives were observed,
The oximes observed are listed in Table 2. Pentanal and
glutaraldehyde were identified based on matching the GC

Gas-Phase Reaction of O3 with Cyclohexene
J. Phys. Chem. A, Vol. 107, No. 13, 2003 2253
indicating a dicarbonyl. The oximes gave fragments correspond-
ing to a loss of H2O and, in the cyclohexene-d10 reaction, a
loss of H2O and HDO, suggesting31 that this product is a
hydroxydicarbonyl and possibly that shown in Scheme 3 [HC-
(O)CH2CH2CH2CH(OH)CHO]. For the molecular weight 116
carbonyl-containing product only a monooxime derivative was
observed. The oxime gave a fragment corresponding to a loss
of H2O and, in the cyclohexene-d10 reaction, a loss of H2O and
HDO, suggesting that this product is a hydroxycarbonyl31 or
possibly an oxo-acid (without standards no conclusive identi-
fication is possible).
pentanal yield from the O3 reaction with cyclohexene of 26.5
( 3.0% assuming that all of the OH radicals formed reacted
with the cyclohexene and did not lead to pentanal forma-
tion, in agreement with our pentanal yield) and by Kalberer
et al.7 (17.0 ( 9.4%, in the presence of an OH radical
scavenger).
Our yield of HC(O)OH from the reaction of O3 with
cyclohexene of 2.5-3.2% in the absence of an OH radical
scavenger is similar to that of 3.7% calculated from the spectrum
shown by Niki et al.,34 but is significantly lower than the yield
of 12 ( 1% reported by Hatakeyama et al.5 In agreement with
previous studies,5,34 HCHO was not detected as a reaction
product. In the presence of sufficient cyclohexane to scavenge
>95% of the OH radicals formed, the HC(O)OH yield from
the cyclohexene reaction was initially 3.5%, increasing to 4.4%
after 17 min (note that the O3 was consumed after 4 min), while
the yield of DC(O)OH [which, because of rapid OD/OH
exchange, could have been formed as DC(O)OD] from the
cyclohexene-d10 reaction was initially 1.6%, increasing to 2.2%
after 10 min. There therefore appears to be a significant
deuterium isotope effect on the formic acid yield, of a factor of
∼2.0-2.2.
The presence of a small hydroxydicarbonyl of molecular
weight 130 (as noted, possibly HC(O)CH2CH2CH2CH(OH)-
CHO) is not inconsistent with the API-MS/MS data shown in
Figures 4 and 5. In Figure 4 (top) the [M+H]+ ) 131 u ion
peak from the cyclohexene reaction may be attributed primarily
to the protonated secondary ozonide plus a small amount of
the protonated hydroxydicarbonyl, while the [M+H]+ ) 141 u
ion peak (Figure 4, bottom) from the cyclohexene-d10 reaction
is solely due to the protonated secondary ozonide-d10. In Figure
5 (top) the [M+H]+ ) 133 u ion peak from the cyclohexene
reaction may be attributed solely to the protonated peracid, while
the [M+H]+ ) 140 u ion peak (Figure 5, bottom) from the
cyclohexene-d10 reaction is due primarily to the protonated
peracid-d7 plus a small amount of the protonated hydroxydi-
carbonyl-d9.
The initial reaction of cyclohexene with O3 forms the primary
ozonide which rapidly decomposes to an energy-rich Criegee
intermediate, which theoretical studies show to be a carbonyl
oxide and which can exist in a syn- or anti-configuration (see,
for example, Fenske et al.35 and Kroll et al.36,37).
Discussion
O3 + cyclo-C6H10 f [C6H10O3]* f
On the basis of our GC-FID, GC-MS, API-MS, and FT-IR
analyses, the gas-phase products observed from the reaction of
O3 with cyclohexene in the presence of an OH radical scavenger
are pentanal (23.6 ( 1.8%), OH radicals (54 ( 8%), formic
acid (3.5% initial yield), glutaraldehyde [HC(O)CH2CH2CH2-
CHO], adipaldehyde [HC(O)CH2CH2CH2CH2CHO], a C6H10O3
product which is attributed to the secondary ozonide, a C5H7O2-
(OOH) product, a hydroxydicarbonyl of molecular weight 130
(possibly HC(O)CH2CH2CH2CH(OH)CHO), and a hydroxy-
carbonyl or possibly oxo-acid of molecular weight 116. As
evident from Table 1, the OH radical formation yield measured
here from the cyclohexene reaction is in excellent agreement
with the value of Fenske et al.10 Furthermore, our present OH
radical formation yields from the reactions of O3 with propene,
R-pinene and 2,3-dimethyl-2-butene are in good agreement with
recent literature data18-24 (Table 1), indicating that 2,3-butane-
diol can be used to determine OH radical yields from the
amounts of 3-hydroxy-2-butanone formed.
[HC(O)CH2CH2CH2CH2CHOO]* (3)
As shown in Scheme 1 (in which identified products are
shown in boxes), this Criegee intermediate can be collisionally
stabilized, decompose to form CO2 plus pentanal (possibly
through the ester channel), and, for the syn-intermediate,
isomerize to a hydroperoxide which then eliminates an OH
radical.1-3,35-37 The recent studies of Kroll et al.36,37 and Fenske
et al.35 show that thermalized Criegee intermediates can also
undergo isomerization with subsequent decomposition to form
an OH radical (plus organic radical coproduct), as also indicated
in Scheme 1 for the syn-intermediate.
The thermalized Criegee intermediate may also react with
water vapor present to form an R-hydroxyhydroperoxide, which
may be thermally stable or decompose to either 6-oxohexanoic
acid [HC(O)CH2CH2CH2CH2C(O)OH] plus H2O or to adipal-
dehyde [HC(O)CH2CH2CH2CH2CHO] plus H2O2 (Scheme
2).38,39 Our product data indicate that decomposition and/or
isomerization of the Criegee intermediate to form pentanal (plus
CO2) and an OH radical plus organic radical coproduct account
for 78 ( 9% of the reaction pathways. Adipaldehyde can be
formed from reaction of the thermalized Criegee intermediate
(presumably the anti-intermediate, with the syn-intermediate
decomposing to an OH radical) with water vapor (Scheme 2).
The product of molecular weight 130 from cyclohexene and
140 from cyclohexene-d10 observed by API-MS (and possibly
by SPME after decomposition) is consistent with this product
being the secondary ozonide formed by recyclization of the
Criegee intermediate (Scheme 1).
The deuterium isotope ratios kH/kD for the OH (or OD) radical
yields from the reactions of O3 with propene and cyclohexene
are 1.48 ( 0.31 and 1.08 ( 0.22, respectively. These are fairly
close to unity (especially for the cyclohexene reaction) and
presumably reflect the fractions of the Criegee intermediates
formed in the syn-configuration (see below) together with the
effect of deuterium substitution on the branching ratios for the
various reaction channels.
Our pentanal yield from cyclohexene (23.6 ( 1.8%) is a factor
of 1.5 higher than the yield reported by Grosjean et al.25 of
15.6 ( 0.4%. A similar discrepancy occurs for the reaction of
O3 with cyclopentene where we previously measured a butanal
yield of 19.5 ( 2.7%,32 compared to the yield of 12.0 (
0.1% reported by Grosjean and Grosjean.33 Pentanal yields
have also been reported by Hatakeyama et al.5 (17.2 (
1.7% relative to the amount of cyclohexene consumed in the
absence of an OH radical scavenger; this corresponds to a
Addition of ∼2.4 × 1013 molecule cm-3 of butanal to the
reactant mixture resulted in no marked changes in the products
formed, as deduced from the API-MS spectra. In particular, no

2254 J. Phys. Chem. A, Vol. 107, No. 13, 2003
Aschmann et al.
Scheme 1.
Scheme 2.
Scheme 3.
secondary ozonide arising from reaction of butanal with the
Criegee intermediate was observed. The kinetic data reported
by Tobias and Ziemann40 for reactions of the CH3(CH2)11CHOO
intermediate with water and formaldehyde shows that formal-
dehyde is factor of 2700 more reactive than water vapor.
Assuming that the relative reactivities of butanal and water vapor
toward the HC(O)CH2CH2CH2CH2CHOO intermediate are the
same as those of formaldehyde and water vapor toward the
CH3(CH2)11CHOO intermediate, then for the water vapor and
butanal concentrations used here (∼3.4 × 1016 and 2.4 × 1013
molecule cm-3, respectively) reaction of the HC(O)CH2CH2-
CH2CH2CHOO intermediate with butanal would be only a factor
of ∼2 faster than with water vapor. Use of higher concentrations
of butanal was precluded because the API-MS spectrum was
then dominated by ions arising from protonated butanal and its
protonated dimer, trimer and tetramer.

Gas-Phase Reaction of O3 with Cyclohexene
J. Phys. Chem. A, Vol. 107, No. 13, 2003 2255
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O2 and the resulting peroxy radical will react with HO2 and
•
organic peroxy (RO2 ) radicals as shown in Scheme 3,2,3 leading
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3), including glutaraldehyde and the hydroxydicarbonyl HC-
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(18O)18OH from the reaction of 18O3 with cyclohexene (in the
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postulated that HC(O)OH arises through the reaction sequence
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•
involving the HC(O)CH2CH2CH2CH2CH(O•)O• and OCCH2-
CH2CH2CH2CH(OH)O• species (i.e, via the ester channel
pathway in Scheme 1). The coproduct to HC(O)OH is then
•
postulated5 to be the OCCH2CH2CH2C•H2 biradical which
•
reacts, as proposed by Jenkin et al.,41 to form the OOC(O)-
CH2CH2CH2CHO species (Scheme 1), then leading to the
molecular weight 132 peracid HOOC(O)CH2CH2CH2CHO. The
corresponding products in the cyclohexene-d10 reaction will then
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C5H7O2(OOH) product shown in Scheme 1 is speculative, but
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would also lead to 5-oxopentanoic acid of molecular weight
116, which may possibly be the product of this molecular weight
observed in the SPME analysis. It should also be noted that the
formic acid formation yield was observed to increase somewhat
with increasing extent of reaction, suggesting in addition to a
“prompt” formation route the existence of a slow secondary
formation pathway for formic acid.
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aerosol-phase products observed by Ziemann.8 In particular,
Ziemann8 observed aerosol-phase products attributed to diacyl
peroxides formed from reactions of peroxyacyl radicals of
structure HC(O)(CH2)nC(O)OO• and HOC(O)(CH2)nC(O)OO•,
where n ) 3 and 4, consistent with our observation of the
molecular weight 132 product suggested to be the peracid
HOOC(O)CH2CH2CH2CHO and formed from the acyl peroxy
radical HC(O)(CH2)3C(O)OO• (Scheme 1).
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Acknowledgment. The authors thank the California Air
Resources Board (CARB) for supporting this research (Contract
No. 99-330), and thank Dr. Paul J. Ziemann for helpful
discussions. While this research has been funded by the CARB,
the results and content of this publication do not necessarily
reflect the views of the CARB.
References and Notes
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