Products of the gas-phase reaction of OH radicals with cyclohexane: Reactions of the cyclohexoxy radical

Article abstract of DOI:10.1021/jp971869f

Products of the gas-phase reactions of OH radicals with cyclohexane and cyclohexane-d₁₂ in the presence of NO have been investigated using gas chromatography with flame ionization detection, combined gas chromatography-mass spectrometry, and in situ direct air-sampling atmospheric pressure ionization tandem mass spectrometry (API-MS). Cyclohexanone and cyclohexyl nitrate (and their deuterated analogues) were identified and quantified, with formation yields of cyclohexanone and cyclohexyl nitrate from the cyclohexane reaction of 0.321 ± 0.035 and 0.165 ± 0.021, respectively, and with cyclohexanone-d₁₀ and cyclohexyl nitrate-d₁₁ formation yields from the cyclohexane-d₁₂ reaction of 0.156 ± 0.017 and 0.210 ± 0.025, respectively. The remaining products must arise from the decomposition and/or isomerization reactions of the intermediate cyclohexoxy radical. API-MS analyses of the cyclohexane and cyclohexane-d₁₂ reactions showed the formation of cyclohexanone and cyclohexyl nitrate (and their deuterated analogues), together with ion peaks attributed to HC(O)CH₂CH₂CH₂CH₂CH ₂ONO₂ (formed from NO addition to the HC(O)CH₂CH₂CH₂CH₂CH ₂OO^? radical formed after decomposition of the cyclohexoxy radical) and HC(O)CH₂CH(OH)CH₂CH₂CHO (formed after isomerization of the HC(O)CH₂CH₂CH₂CH₂CH ₂O^? radical). No evidence for isomerization of the cyclohexoxy radical was obtained from the API-MS analyses. The reactions of the cyclohexoxy radical are discussed and the data extended to the reactions of the cyclopentoxy and cycloheptoxy radicals formed from cyclopentane and cycloheptane.

Full text of DOI:10.1021/jp971869f

8042
J. Phys. Chem. A 1997, 101, 8042-8048
Products of the Gas-Phase Reaction of OH Radicals with Cyclohexane: Reactions of the
Cyclohexoxy Radical
Sara M. Aschmann, Andrew A. Chew,† Janet Arey,*,† and Roger Atkinson*,†,‡
Statewide Air Pollution Research Center, UniVersity of California, RiVerside, California 92521
ReceiVed: June 9, 1997; In Final Form: August 4, 1997X
Products of the gas-phase reactions of OH radicals with cyclohexane and cyclohexane-d12 in the presence of
NO have been investigated using gas chromatography with flame ionization detection, combined gas
chromatography-mass spectrometry, and in situ direct air-sampling atmospheric pressure ionization tandem
mass spectrometry (API-MS). Cyclohexanone and cyclohexyl nitrate (and their deuterated analogues) were
identified and quantified, with formation yields of cyclohexanone and cyclohexyl nitrate from the cyclohexane
reaction of 0.321 ( 0.035 and 0.165 ( 0.021, respectively, and with cyclohexanone-d10 and cyclohexyl nitrate-
d11 formation yields from the cyclohexane-d12 reaction of 0.156 ( 0.017 and 0.210 ( 0.025, respectively.
The remaining products must arise from the decomposition and/or isomerization reactions of the intermediate
cyclohexoxy radical. API-MS analyses of the cyclohexane and cyclohexane-d12 reactions showed the formation
of cyclohexanone and cyclohexyl nitrate (and their deuterated analogues), together with ion peaks attributed
to HC(O)CH2CH2CH2CH2CH2ONO2 (formed from NO addition to the HC(O)CH2CH2CH2CH2CH2OO• radical
formed after decomposition of the cyclohexoxy radical) and HC(O)CH2CH(OH)CH2CH2CHO (formed after
isomerization of the HC(O)CH2CH2CH2CH2CH2O• radical). No evidence for isomerization of the cyclohexoxy
radical was obtained from the API-MS analyses. The reactions of the cyclohexoxy radical are discussed and
the data extended to the reactions of the cyclopentoxy and cycloheptoxy radicals formed from cyclopentane
and cycloheptane.
Introduction
SCHEME 1
Alkanes, including cycloalkanes, are important constituents
of gasoline fuels, vehicle exhaust emissions, and ambient air in
urban areas.1-6 In the troposphere, alkanes and cycloalkanes
react with the hydroxyl (OH) radical,7-9 with the formation of
alkoxy (RO•) radicals as intermediates in the degradation
reaction sequences.9 For example, in the presence of NO the
reactions leading to the formation of alkoxy radicals are9
SCHEME 2
OH + RH f H2O + R•
(1)
(2)
R• + O2
9
M8 RO2
•
M8 RONO2
(3a)
(3b)
•
RO2 + NO
9
•
RO2 + NO f RO• + NO2
Subsequent reactions of the alkoxy radicals determine the
products formed from the tropospheric degradations of alkanes
and cycloalkanes,9,10 and these involve reaction with O2,
unimolecular decomposition, and isomerization,9,10 as shown
in Schemes 1-3, respectively, for the cyclohexoxy radical
formed from cyclohexane. Although there is now a quantitative
or semiquantitative understanding of the reactions of acyclic
alkoxy radicals formed from alkanes,10 there are only few data
available concerning the tropospheric reactions of alkoxy
radicals formed from the cycloalkanes.11-15
The reaction of the OH radical with cyclohexane in the
presence of NO leads to the formation of cyclohexanone and
† Also Environmental Toxicology Graduate Program.
‡ Also Department of Chemistry, University of California, Riverside, CA
92521.
cyclohexyl nitrate, with reported formation yields of ∼0.23-
0.3511,15 and 0.09-0.16,11,16 respectively. The remaining
products have not been identified or quantified but must arise
from the decomposition and/or isomerization reactions of the
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
S1089-5639(97)01869-0 CCC: $14.00 © 1997 American Chemical Society

Cyclohexoxy Radical Reactions
J. Phys. Chem. A, Vol. 101, No. 43, 1997 8043
SCHEME 3
intermediate cyclohexoxy (C6H11O•) radical. The expected
reactions after the decomposition and isomerization of the
cyclohexoxy radical are shown in Schemes 2 and 3, respectively,
with certain reaction pathways being omitted (for example,
decomposition of the HC(O)CH2CH2CH2CH2CH2O• radical in
Scheme 2) based on estimation of the relative importance of
the reactions of certain alkoxy radicals.10
In this work we have used gas chromatography with flame
ionization detection (GC-FID), combined gas chromatography-
mass spectrometry (GC-MS), and in situ direct air sampling
atmospheric pressure ionization tandem mass spectrometry
(API-MS) to investigate the atmospheric chemistry of the OH
radical-initiated reactions of cyclohexane and cyclohexane-d12
in the presence of NO.
Figure 1. API-MS spectra of irradiated CH3ONO-NO-cyclohexane-
air (A) and CH3ONO-NO-cyclohexane-d12-air (B) mixtures.
The concentrations of cyclohexane and the products cyclo-
hexanone and cyclohexyl nitrate were measured by GC-FID.
For the analysis of cyclohexanone and cyclohexyl nitrate, 100
cm3 volume gas samples were collected from the chamber onto
Tenax-TA solid adsorbent with subsequent thermal desorption
at ∼225 °C onto a 30 m DB-1701 megabore column held at 0
Experimental Section
All experiments were carried out in 7000-7900 L Teflon
chambers, each equipped with two parallel banks of black-lamps
for irradiation and with a Teflon-coated fan to ensure rapid
mixing of reactants during their introduction into the chamber.
Products of the reactions of the OH radical with cyclohexane
and cyclohexane-d12 in the presence of NO, at 298 ( 2 K and
740 Torr total pressure of purified air at ∼5% relative humidity,
were analyzed by GC-FID and by API-MS. Separate sets of
experiments were carried out for each of these analytical
methods.
Hydroxyl radicals were generated by the photolysis of methyl
nitrite (CH3ONO) in air at wavelengths greater than 300 nm,17
and NO was added to the reactant mixtures to suppress the
formation of O3 and of NO3 radicals.17 The initial CH3ONO,
NO, and cyclohexane (or cyclohexane-d12) concentrations were
°C and then temperature programmed to 200 °C at 8 °C min-1
.
For the analysis of cyclohexane, gas samples were collected
from the chamber in a 100 cm3 all-glass, gastight syringe and
introduced via a 1 cm3 volume stainless steel loop and gas-
sampling valve onto a 30 m DB-5 megabore column held at
-25 °C and then temperature programmed at 8 °C min-1
.
Calibrations of the GC-FID response factors for cyclohexane,
cyclohexanone, and cyclohexyl nitrate were carried out as
described previously for similar compounds.18,19 Gas samples
were also collected from the chamber onto Tenax solid adsorbent
for subsequent thermal desorption and analysis by GC-MS, using
a 30 m DB-1701 fused silica column in a Hewlett-Packard (HP)
5890 GC interfaced to an HP 5971 mass selective detector
operating in the scanning mode.
For the API-MS analyses, a 7000 L Teflon chamber was
interfaced to a PE SCIEX API III MS/MS direct air-sampling
atmospheric pressure ionization tandem mass spectrometer (API-
MS). The chamber contents were sampled through a 25 mm
diameter × 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
elsewhere.20-22 Use of the MS/MS mode with CAD allows
the “daughter ion” or “parent ion” spectrum of a given ion peak
2.4 × 1014, 2.4 × 1014, and (2.27-2.45) × 1013 molecule cm-3
,
respectively, for the experiments with GC-FID analyses, and
4.8 × 1013 molecule cm-3 of each reactant for the experiments
with API-MS analyses. Irradiations were carried out at 20%
of the maximum light intensity (corresponding to an NO2
photolysis rate of ∼2.5 × 10-3 s-1) for 15-45 min for the
experiments with analyses by GC-FID and at the maximum light
intensity for 2-3 min (cyclohexane) and 5-7.5 min (cyclo-
hexane-d12) for the experiments with in situ API-MS analyses.

8044 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Aschmann et al.
TABLE 1: Potential Products Formed from the OH Radical-Initiated Reaction of Cyclohexane and Cyclohexane-d12 in the
Presence of NO (see Schemes 1-3) and Experimental Observations from in Situ API-MS and API-MS/MS Analyses
productsa
API-MS data
other evidence
Reaction 3a
[M1 + H] ) 146 (v weak)
cyclohexyl nitrate M1 (MW 145)
GC-FID quantification. MS/MS of 146 u
ion peak (Figure 2) shows loss of NO2 and
NO2+ fragment.
cyclohexyl nitrate-d11 (MW 156)
[M1 + H] ) 157 (v weak)
GC-FID quantification. MS/MS of 157 u
ion peak shows loss of NO2 and NO2
+
fragment.
Reaction Scheme 1
[M2 + H] ) 99
[M2 + M2 + H] ) 197
[M2 + M5 + H] ) 229
[M2 + M5 + H - H2O] ) 211b
[M2 + H] ) 109
[M2 + M2 + H] ) 217
[M2 + M5 + H] ) 248
[M2 + M5 + H - HDO] ) 230
cyclohexanone M2 (MW 98)
GC-FID quantification. MS/MS of 197,
211, and 229 u ion peaks, with parent of 99
observed at 197 and 211 u.
cyclohexanone-d10 (MW 108)
GC-FID quantification. MS/MS of 230 and
248 u ion peaks.
Reaction Scheme 2
[M3 + H] ) 162 (v weak)
[M3 + M5 + H] ) 292
HC(O)CH2CH2CH2CH2CH2ONO2 M3 (MW 161)
DC(O)CD2CD2CD2CD2CD2ONO2 (MW 172)
HC(O)CH2CH2CH2CH2CHO M4 (MW 114)
DC(O)CD2CD2CD2CD2CDO (MW 124)
MS/MS of 162 u ion peak shows loss of
NO2 and NO2+ fragment. MS/MS of 292 u
ion peak shows loss of NO2.
MS/MS of 312 u ion peak shows loss of
NO2. MS/MS of 281 u ion peak.
[M3 + H] ) 173 (weak)
[M3 + M5 + H] ) 312
[M3 + M2 + H] ) 281
[M4 + H] ) 115 (v weak)
[M4 + M2 + H] ) 213
MS/MS of 115 u ion peak (see Figure 4).
Parent of 115 u was 213 u [M4 + M2 + H] )
213. MS/MS of 213 u ion peak.
MS/MS of 125 u ion peak (see Figure 4)
with HDO loss (and not H2O loss). Parent
of 125 u was 233 u. MS/MS of 233 u ion
peak.
[M4 + H] ) 125
[M4 + M2 + H] ) 233
HC(O)CH2CH(ONO2)CH2CH2CH2OH (MW 177)
DC(O)CD2CD(ONO2)CD2CD2CD2OHd (MW 187)
HC(O)CH2CH(OH)CH2CH2CHO M5 (MW 130)
No evidence for formation.
No evidence for formation.
MS/MS of 131 u ion peak (see Figure 3).
MS/MS of 211 and 229 u ion peaks.
MS/MS of 243 and 261 u ion peaks.
[M5 + H] ) 131 (v weak)
[M5 + H - H2O] ) 113b
[M5 + M5 + H] ) 261
[M5 + M5 + H - H2O] ) 243b
[M5 + M2 + H] ) 229
[M5 + M2 + H - H2O] ) 211b
[M5 + H] ) 140c (v weak)
[M5 + H - H2O] ) 122b,c
[M5 + M2 + H] ) 248
DC(O)CD2CD(OH)CD2CD2CDO (MW 139)
MS/MS of 140 u ion peak (see Figure 3).
MS/MS of 230 and 248 u ion peaks.
[M5 + M2 + H - H2O] ) 230c
Reaction Scheme 3
expect strong 97 u peak (not seen)
4-hydroxycyclohexanone (MW 114)
No evidence for formation (see text).
No evidence for formation (see text).
4-hydroxycyclohexanone-d9d (MW 123)
a Mi has been used to refer to the products of both the cyclohexane and cyclohexane-d12 reactions. b Strong [M + H - H2O]+ fragment ion peaks
are expected for hydroxy-containing compounds. c Expect loss of mainly H2O and not HDO or D2O. d After expected rapid -OD to -OH exchange.
observed in the MS scanning mode to be obtained.20-22 The
positive ion mode was used for all API-MS and API-MS/MS
analyses, with protonated water hydrates (H3O+(H2O)n) (n ≈
3-6 at 298 K and ∼5% relative humidity23) generated by the
corona discharge in the chamber diluent gas being responsible
for the protonation of analytes,
The chemicals used, and their stated purities, were the
following: cyclohexane (high-purity solvent grade), American
Burdick and Jackson; cyclohexane-d12 (99.5% atom D) and
cyclohexanone (99.8%), Aldrich Chemical Co.; cyclohexyl
nitrate, Fluorochem, Inc.; NO (g99.0%), Matheson Gas Prod-
ucts. Methyl nitrite was prepared and stored as described
previously.17
H3O+(H2O)n + M f MH+(H2O)m + (n - m + 1)H2O (4)
Results and Discussion
where M is the neutral analyte of interest. 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. For these experiments, the API-MS instrument was
operated under conditions that favored the formation of dimer
ions in the ion source region.24 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 were mass-
analyzed were mainly protonated molecular ions ([M + H]+)
and their protonated homo- and heterodimers.20-22,24
API-MS/MS Analyses. A series of CH3ONO-NO-cyclo-
hexane-air and CH3ONO-NO-cyclohexane-d12-air irradia-
tions were carried out with API-MS analyses. The API-MS
spectra of the irradiated mixtures are shown in parts A and B
of Figure 1 for the cyclohexane and cyclohexane-d12 reactions,
respectively. Based on Schemes 1-3 and the formation of
cyclohexyl nitrate from the reaction of the cyclohexyl peroxy
radical with NO (reaction 3a), seven potential products, their
molecular weights, and the corresponding deuterated species
expected from the reaction of cyclohexane-d12 are listed in Table

Cyclohexoxy Radical Reactions
J. Phys. Chem. A, Vol. 101, No. 43, 1997 8045
Figure 2. API-MS/MS daughter ion spectrum of the 146 u ion peak
attributed to the [M + H]+ ion of cyclohexyl nitrate.
1. Evidence for the formation of five of these seven potential
products in the form of API-MS data of molecular ions,
dominant fragment ions, and the presence of dimers, including
heterodimers formed in the API-MS under the experimental
conditions employed, is summarized in Table 1 together with
additional confirmatory data (including MS/MS data). No
evidence was found for the formation of 4-hydroxycyclohex-
anone ( Scheme 3) or for the organic nitrate HC(O)CH2CH-
(ONO2)CH2CH2CH2OH, which could be formed from reaction
of the HC(O)CH2CH(OO•)CH2CH2CH2OH radical with NO
(Scheme 2).
Cyclohexyl nitrate and cyclohexyl nitrate-d11 were observed
and quantified by GC-FID, as discussed below. Only relatively
weak [M + H]+ ions at masses 146 and 157 u for cyclohexyl
nitrate and cyclohexyl nitrate-d11, respectively, were observed
in the API-MS spectra of the reaction products (Figure 1). An
additional even-mass [M + H]+ molecular ion at 162 u was
attributed to HC(O)CH2CH2CH2CH2CH2ONO2. Although the
molecular ions of these nitrates were weak, their identities were
confirmed through API-MS/MS daughter ion spectra, which
showed losses of 46 mass units (NO2) and the presence of a
strong 46 u fragment ion ([NO2]+) [see, for example, Figure 2
for the API-MS/MS daughter ion spectrum of cyclohexyl
nitrate]. A heterodimer of HC(O)CH2CH2CH2CH2CH2ONO2
with the hydroxycarbonyl product, HC(O)CH2CH(OH)CH2CH2-
CHO, was more readily observed in the API-MS spectrum
shown in Figure 1A, and an API-MS/MS daughter ion spectrum
of this dimer ion peak was consistent with the product
assignments. As noted in Table 1, the corresponding deuterated
nitrate was also observed, and the 312 u ion peak in Figure 1B
is the heterodimer of DC(O)CD2CD2CD2CD2CD2ONO2 and
DC(O)CD2CD(OH)CD2CD2CDO (Table 1).
Cyclohexanone and cyclohexanone-d12 were quantified by
GC-FID, and API-MS/MS daughter and parent ion spectra
confirmed their protonated molecular ions at 99 and 109 u,
respectively, as well as the presence of numerous heterodimers
(see Table 1). The hydroxydicarbonyl HC(O)CH2CH(OH)CH2-
CH2CHO (Scheme 2) showed a weak molecular ion [M + H]+
at 131 u and a strong [M + H - H2O]+ fragment ion at 113 u
in the API-MS spectrum and also showed heterodimer formation
(Table 1). API-MS/MS daughter ion spectra of the 131 u ion
peak (Figure 3A) showed a strong fragment ion at 113 u [M +
H - H2O]+, consistent with the strong 113 u ion peak observed
in the API-MS spectrum. The corresponding deuterated product
from the cyclohexane-d12 reaction has an [M + H]+ ion at 140
Figure 3. API-MS/MS daughter ion spectra of the 131 u (A) and 140
u (B) ion peaks attributed to the [M + H]+ ions of HC(O)CH2CH-
(OH)CH2CH2CHO and DC(O)CD2CD(OH)CD2CD2CDO (after -OD
to -OH exchange), respectively.
u due to rapid -OD to -OH exchange (Figure 3B). API-MS/
MS daughter ion spectra of the 140 u ion peak (Figure 3B)
showed a strong fragment ion at 122 u (-H2O) and a weaker
fragment at 121 u (-HDO), consistent with the strong 122 u
ion peak observed in the API-MS analyses (loss of mainly H2O
and not HDO or D2O is expected because of the presence of an
-OH group).
The two potential products HC(O)CH2CH2CH2CH2CHO and
4-hydroxycyclohexanone both have molecular weights of 114
but may be distinguished through the cyclohexane-d12 reaction
owing to the expected rapid -OD to -OH exchange for
hydroxylated products, as previously observed.20,21 There is
evidence for the formation of the dialdehyde product, HC(O)CH2-
CH2CH2CH2CHO, but not for formation of 4-hydroxycyclo-
hexanone. The API-MS/MS daughter ion spectra of the very
weak 115 u protonated molecular ion peak (Figure 4A) showed
fragment ions at 97 (-H2O), 79 (-2H2O), and 69 u (-H2O -
CO). The corresponding deuterated product ion occurred at 125
u, ruling out the formation of 4-hydroxycyclohexanone, which
would have an [M + H]+ ion at 124 u (due to rapid -OD to
-OH exchange). Dimer ions provide confirming evidence (see
Table 1) for this assignment.
GC-FID Analyses. As in previous studies of the reaction
of the OH radical with cyclohexane in the presence of NO,11,15,16
cyclohexanone and cyclohexyl nitrate (and their deuterated
analogues from the cyclohexane-d12 reaction) were observed
by GC-FID analyses and were confirmed by GC-MS analysis.
The measured concentrations of cyclohexanone and cyclohexyl

8046 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Aschmann et al.
Figure 5. Plots of the amounts of cyclohexanone and cyclohexanone-
d10 formed, corrected for reaction with the OH radical (see text), against
the amounts of cyclohexane and cyclohexane-d12 reacted.
Figure 4. API-MS/MS daughter ion spectra of the 115 u (A) and 125
u (B) ion peaks attributed to the [M + H]+ ions of HC(O)CH2CH2-
CH2CH2CHO and DC(O)CD2CD2CD2CD2CDO, respectively.
nitrate from the cyclohexane reaction were corrected to take
into account secondary reactions with the OH radical.25 By use
of rate constants for the reactions of the OH radical with
cyclohexane, cyclohexanone, and cyclohexyl nitrate (in units
of 10-12 cm3 molecule-1 s-1) of 7.49,9 6.39,26 and 3.30,9,16 the
multiplicative correction factors for secondary reactions with
the OH radical, which increase with the rate constant ratio k(OH
+ product)/k(OH + cyclohexane) and with the extent of
reaction, were e1.26 for cyclohexanone and e1.13 for cyclo-
hexyl nitrate. Because no rate constants have been measured
for the reactions of the OH radical with cyclohexanone-d10 or
cyclohexyl nitrate-d11, the rate constant ratios k(OH + cyclo-
hexanone-d10)/k(OH + cyclohexane-d12) and k(OH + cyclo-
hexyl nitrate-d11)/k(OH + cyclohexane-d12) were assumed to
be identical with those for the nondeuterated species. The lower
reactivity of cyclohexane-d12 compared to cyclohexane (by a
factor of ∼2.5 at 298 K27) led to a lower fraction of cyclohexane-
d12 reacting (e23% for cyclohexane-d12 compared to e40%
for cyclohexane), and the correction factors for secondary
reaction in the cyclohexane-d12 system were lower than in the
cyclohexane system, being e1.12 for cyclohexanone-d10 and
e1.06 for cyclohexyl nitrate-d11. Hence, the assumption of
identical k(OH + product)/k(OH + reactant) ratios for the
cyclohexane-d12 and cyclohexane reaction systems is expected
to result in only small additional uncertainties (<10%).
Figure 6. Plots of the amounts of cyclohexyl nitrate and cyclohexyl
nitrate-d11 formed, corrected for reaction with the OH radical (see text),
against the amounts of cyclohexane and cyclohexane-d12 reacted.
nitrate-d11), respectively. The formation yields obtained from
these data by least-squares analyses are given in Table 2. Our
present formation yield for cyclohexanone from the cyclohexane
reaction (0.321 ( 0.035) agrees within 10% with our previous
measurement of 0.354 ( 0.04215 and is in agreement within
the uncertainties with the yield reported by Takagi et al.11 (Table
2). Our present formation yield of cyclohexyl nitrate, arising
from the reaction of the cyclohexyl peroxy radical with NO, of
0.165 ( 0.021 is in excellent agreement with our previous value
of 0.160 ( 0.01516 and is consistent within the large cited
uncertainties11 with the yield reported by Takagi et al.11 (Table
2).
Our API-MS and API-MS/MS analyses show that isomer-
ization of the cyclohexoxy radical (reaction Scheme 3) must
be of minor or negligible importance, as may be expected
because of the preferred chair conformation of the cyclohexoxy
radical. Rather, the cyclohexoxy radical undergoes reaction with
O2, leading to cyclohexanone, and decomposition (reaction
Schemes 1 and 2), with calculated reaction rates of 2.2 × 104
s-1 for the O2 reaction at 298 K and 740 Torr total pressure of
air and 6.3 × 104 s-1 for decomposition at 298 K10 (Table 3).
The HC(O)CH2CH2CH2CH2CH2O• alkoxy radical formed after
the cyclohexoxy radical decomposition can decompose, isomer-
Plots of the amounts of these products, corrected for reaction
with the OH radical, against the amounts of cyclohexane and
cyclohexane-d12 reacted are shown in Figures 5 (cyclohexanone
and cyclohexanone-d10) and 6 (cyclohexyl nitrate and cyclohexyl

Cyclohexoxy Radical Reactions
J. Phys. Chem. A, Vol. 101, No. 43, 1997 8047
TABLE 2: Products Formed, and Their Formation Yields, from the Gas-Phase Reactions of the OH Radical with Cyclohexane
and Cyclohexane-d12 in the Presence of NO at 298 ( 2 K and 740 Torr Total Pressure of Air
formation yield
reactant
product
this worka
lit
cyclohexane
cyclohexanone
0.321 ( 0.035
0.23 ( 0.13b
0.354 ( 0.042a,c
0.090 ( 0.044b
0.160 ( 0.015d
cyclohexyl nitrate
0.165 ( 0.021
cyclohexane-d12
cyclohexanone-d10
cyclohexyl nitrate-d11
0.156 ( 0.017
0.210 ( 0.025
a Indicated errors are two least-squares standard deviations combined with estimated overall uncertainties in the GC-FID response factors for
cyclohexane, cyclohexanone, and cyclohexyl nitrate of (5% each. b Reference 11. Indicated error is one standard deviation. c Reference 15. d Reference
16. Indicated error is two least-squares standard deviations.
TABLE 3: Calculated Rates for the Decomposition and
Reaction with O2 of Cycloalkoxy Radicals at 298 K and 760
Torr Total Pressure of O2 and Comparison of the Calculated
and Experimentally Measured Cycloketone Formation Yields
from Cycloalkoxy Radicals
anone, and cycloheptanone from the corresponding cycloalkoxy
radical. The ring-strain energies in the cyclopentoxy and
cycloheptoxy radicals lead to significantly lower heats of
reaction for decomposition than for the cyclohexoxy radical
(with calculated values of ∆Hdecomp of -1.1, 6.3, and 1.2 kcal
mol-1 for the cyclopentoxy, cyclohexoxy, and cycloheptoxy
radicals, respectively28,29), resulting in more rapid decomposition
of the cyclopentoxy and cycloheptoxy radicals than for the
cyclohexoxy radical (Table 3). The calculated rate of reaction
of the cyclohexoxy radical with O2 is lower than the rates of
reaction with O2 of the cyclopentoxy and cycloheptoxy radicals
(Table 3). The calculated ratios of the rates of reaction with
O2 versus decomposition for the cyclopentoxy, cyclohexoxy,
and cycloheptoxy radicals are in agreement with the experi-
mental data (Table 3). Thus, reaction with O2 and decomposi-
tion of the cyclohexoxy radical are competitive, while decom-
position dominates for the cyclopentoxy and cycloheptoxy
radicals (Table 3).
The products formed from the reaction of the OH radical with
cyclohexane in the presence of NO are therefore cyclohexyl
nitrate from the reaction of the cyclohexyl peroxy radical with
NO, cyclohexanone from the cyclohexoxy radical reaction with
O2, and HC(O)CH2CH2CH2CH2CH2ONO2, HC(O)CH2CH2CH2-
CH2CHO, and HC(O)CH2CH(OH)CH2CH2CHO formed after
the decomposition of the cyclohexoxy radical. Analogous
products will be formed from cyclopentane and cycloheptane,
except that the formation yields of cyclopentanone and cyclo-
heptanone from cyclopentane and cycloheptane, respectively,
are much lower than that of cyclohexanone from cyclohexane
(Table 3). The products formed and the reaction schemes
involved (Schemes 1 and 2 and analogous schemes for cyclo-
pentane and cycloheptane), including the number of NO to NO2
conversions involved in the product formation, can now be
incorporated into chemical mechanisms for the atmospheric
photooxidation of cycloalkanes and for more accurate calculation
of their ozone-forming potentials. It is also possible that the
multifunctional products HC(O)CH2CH2CH2CH2CH2ONO2,
HC(O)CH2CH2CH2CH2CHO, and HC(O)CH2CH(OH)CH2CH2-
CHO, and their homologues from the cyclopentane, cyclohep-
tane, and other cycloalkane reactions, undergo gas/particle
partitioning leading to secondary organic aerosol formation and/
or are wet- and dry-deposited.
a
reaction rate (s-1
)
cycloketone yield
cycloalkoxy
radical
reaction
with O2
decompn
calcd exptl
cyclopentoxy 5.6 × 104 2.9 × 106 0.019 0.017 ( 0.017b
cyclohexoxy
2.2 × 104 6.3 × 104 0.26
0.25 ( 0.15b
0.42 ( 0.06c
0.38 ( 0.05d
cycloheptoxy 1.3 × 105 1.4 × 106 0.085 0.033 ( 0.009b
a Calculated as discussed by Atkinson.10 b Reference 11. Indicated
error is one standard deviation. c References 15 and 16. d This work.
ize, or react with O2.9,10 Our API-MS and API-MS/MS data
show that the HC(O)CH2CH2CH2CH2CH2O• radical preferen-
tially isomerizes to yield the hydroxydicarbonyl HC(O)CH2-
CH(OH)CH2CH2CHO, with the reaction with O2 leading to
CHO(CH2)4CHO being of minor importance. These observa-
tions are consistent with estimated10 rates of decomposition,
isomerization, and reaction with O2 of the HC(O)CH2CH2CH2-
CH2CH2O• radical at 298 K and 740 Torr total pressure of air,
of 3.5 × 103, 2.6 × 106, and 3.1 × 104 s-1, respectively, with
the necessary thermochemical data being obtained from the
NIST program28 and Kerr.29
The deuterium isotope effect on the cyclohexanone formation
yield (Table 2) is consistent with the intermediate cyclohexoxy
radical reacting with O2 in competition with decomposition via
C-C bond scission, with the O2 reaction involving H- (or D-)
atom abstraction with a deuterium isotope effect and the
decomposition pathway having no significant isotope effect.
However, there is no marked deuterium isotope effect on the
cyclohexyl nitrate formation yield, with the cyclohexyl nitrate-
d11 yield from cyclohexane-d12 being a factor of 1.27 ( 0.23
higher than the cyclohexyl nitrate yield from cyclohexane, where
the indicated error is two least-squares standard deviations. This
observation is consistent with the reaction pathway forming
cyclohexyl nitrate involving addition of NO to the cyclohexyl
peroxy radical (reaction 3a) and with the reaction channels 3a
and 3b both proceeding via an intermediate [C6H11OONO]*
complex.9 The only literature data to compare with our
cyclohexyl nitrate yield data concern the rate constants for the
Acknowledgment. The authors gratefully thank the U.S.
Environmental Protection Agency for supporting this research
through Cooperative Agreement CR-821787-01-0 (Dr. Deborah
J. Luecken, Project Officer) and Assistance Agreement R-825252-
01-0 (Office of Research and Development) and thank the
National Science Foundation (Grant No. ATM-9015361) and
the University of California, Riverside, for funds for the
purchase of the SCIEX API III MS/MS instrument. Although
this research has been supported by the U.S. Environmental
•
•
reactions of CH3O2 and CD3O2 radicals with NO,30,31 with
•
measured rate constant ratios at room temperature of k(CD3O2
•
+ NO)/k(CH3O2 + NO) ) 0.97 ( 0.1730 and 1.15 ( 0.21,31
consistent with our nitrate yield data.
The calculated10 rates of decomposition and reaction with O2
of the cyclopentoxy, cyclohexoxy, and cycloheptoxy radicals
are given in Table 3, together with a comparison of the estimated
and measured formation yields of cyclopentanone, cyclohex-

8048 J. Phys. Chem. A, Vol. 101, No. 43, 1997
Aschmann et al.
(16) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.;
Pitts, J. N., Jr. Int. J. Chem. Kinet. 1984, 16, 1085.
(17) Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Air
Pollut. Control Assoc. 1981, 31, 1090.
Protection Agency, it has not been subjected to Agency review
and, therefore, does not necessarily reflect the views of the
Agency, and no official endorsement should be inferred.
(18) Atkinson, R.; Aschmann, S. M. EnViron. Sci. Technol. 1995, 29,
528.
References and Notes
(19) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. EnViron. Sci.
Technol. 1995, 29, 1674.
(20) Atkinson, R.; Kwok, E. S. C.; Arey, J.; Aschmann, S. M. Faraday
Discuss. 1995, 100, 23.
(1) Lonneman, W. A.; Seila, R. L.; Meeks, S. A. EnViron. Sci. Technol.
1986, 20, 790.
(2) Zweidinger, R. B.; Sigsby, J. E., Jr.; Tejada, S. B.; Stump, F. D.;
Dropkin, D. L.; Ray, W. D.; Duncan, J. W. EnViron. Sci. Technol. 1988,
22, 956.
(21) Kwok, E. S. C.; Arey, J.; Atkinson, R. J. Phys. Chem. 1996, 100,
(3) Lurmann, F. W.; Main, H. H. Analysis of the Ambient VOC Data
Collected in the Southern California Air Quality Study; Sonoma Technology,
Inc.: Santa Rosa, CA, 1992. Final Report to California Air Resources Board
Contract No. A832-130, Sacramento, CA, February 1992; Sonoma Technol-
ogy Inc.: Santa Rosa, CA, 1992.
(4) Hoekman, S. K. EnViron. Sci. Technol. 1992, 26, 1206.
(5) Olson, K. L.; Sinkevitch, R. M.; Sloane, T. M. J. Chromatogr. Sci.
1992, 30, 500.
(6) Harley, R. A.; Cass, G. R. Atmos. EnViron. 1995, 29, 905.
(7) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 1 1989, 1.
(8) National Research Council. Rethinking the Ozone Problem in Urban
and Regional Air Pollution; National Academy Press: Washington, DC,
1991.
(9) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 2 1994, 1.
(10) Atkinson, R. Int. J. Chem. Kinet. 1997, 29, 99.
(11) Takagi, H.; Washida, N.; Bandow, H.; Okuda, M. J. Phys. Chem.
1981, 85, 2701.
214.
(22) Kwok, E. S. C.; Atkinson, R.; Arey, J. EnViron. Sci. Technol. 1996,
30, 1048.
(23) Lau, Y. K.; Ikuta, S.; Kebarle, P. J. Am. Chem. Soc. 1982, 104,
1462.
(24) Kwok, E. S. C.; Arey, J. Manuscript in preparation.
(25) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.;
Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563.
(26) Dagaut, P.; Wallington, T. J.; Liu, R.; Kurylo, M. J. J. Phys. Chem.
1988, 92, 4375.
(27) Droege, A. T.; Tully, F. P. J. Phys. Chem. 1987, 91, 1222.
(28) Stein, S. E. Structures and Properties Database and Estimation
Program, Version 2.0; National Institute of Standards and Technology
Standard Reference Database 25; Chemical Kinetics and Thermodynamics
Division, NIST: Gaithersburg, MD, 1994.
(29) Kerr, J. A. Strengths of Chemical Bonds. In Handbook of Chemistry
and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1993-
94.
(12) McEwen, C. N.; Druliner, J. D. J. Am. Chem. Soc. 1988, 110, 8579.
(13) Rowley, D. M.; Lightfoot, P. D.; Lesclaux, R.; Wallington, T. J. J.
Chem. Soc., Faraday Trans. 1991, 87, 3221.
(14) Rowley, D. M.; Lightfoot, P. D.; Lesclaux, R.; Wallington, T. J. J.
Chem. Soc., Faraday Trans. 1992, 88, 1369.
(30) Masaki, A.; Tsunashima, S.; Washida, N. Chem. Phys. Lett. 1994,
218, 523.
(15) Atkinson, R.; Aschmann, S. M.; Arey, J.; Shorees, B. J. Geophys.
Res. 1992, 97, 6065.
(31) Helleis, F.; Moortgat, G. K.; Crowley, J. N. J. Phys. Chem. 1996,
100, 17846.