Chemistry of the cyclopentoxy and cyclohexoxy radicals at subambient temperatures

Article abstract of DOI:10.1021/jp0002648

The Cl-atom initiated oxidation mechanisms of both cyclopentane and cyclohexane have been studied as a function of temperature using an environmental chamber/FTIR technique. The oxidation of cyclohexane leads to the formation of the cyclohexoxy radical, the chemistry of which is characterized by a competition between ring-opening (R5) and reaction with O₂ (R6) to form cyclohexanone. The yield of cyclohexanone is shown to increase with decreasing temperature, and a rate coefficient ratio k₆/k₅ = (1.3 ± 0.3) × 10^-27 exp(5550 ± 1100/T) cm³ molecule^-1 is obtained. The energy barrier to ring-opening is estimated to be 11.5 ± 2.2 kcal/mol. The dominant fate of the cyclopentoxy radical, formed in the Cl-atom initiated oxidation of cyclopentane, is ring-opening under all conditions studied here (230-300 K, 50-500 Torr O₂), with only a minor contribution from the O₂ reaction at the lowest temperatures studied. The barrier to ring-opening for the cyclopentoxy radical is probably less than 10 kcal/mol.

Full text of DOI:10.1021/jp0002648

5072
J. Phys. Chem. A 2000, 104, 5072-5079
Chemistry of the Cyclopentoxy and Cyclohexoxy Radicals at Subambient Temperatures
John J. Orlando,* Laura T. Iraci,† and Geoffrey S. Tyndall
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
ReceiVed: January 20, 2000; In Final Form: March 14, 2000
The Cl-atom initiated oxidation mechanisms of both cyclopentane and cyclohexane have been studied as a
function of temperature using an environmental chamber/FTIR technique. The oxidation of cyclohexane leads
to the formation of the cyclohexoxy radical, the chemistry of which is characterized by a competition between
ring-opening (R5) and reaction with O2 (R6) to form cyclohexanone. The yield of cyclohexanone is shown
to increase with decreasing temperature, and a rate coefficient ratio k6/k5 ) (1.3 ( 0.3) × 10-27 exp(5550 (
1100/T) cm3 molecule-1 is obtained. The energy barrier to ring-opening is estimated to be 11.5 ( 2.2 kcal/
mol. The dominant fate of the cyclopentoxy radical, formed in the Cl-atom initiated oxidation of cyclopentane,
is ring-opening under all conditions studied here (230-300 K, 50-500 Torr O2), with only a minor contribution
from the O2 reaction at the lowest temperatures studied. The barrier to ring-opening for the cyclopentoxy
radical is probably less than 10 kcal/mol.
Introduction
between ring-opening and reaction with O2 for these species
allows for the determination of the Arrhenius parameters for
the ring opening reactions. As part of this work, yields of organic
nitrates as a function of temperature in the oxidation of both
cyclopentane and cyclohexane are estimated.
The alkanes, including the cycloalkanes, are released into the
atmosphere from a variety of anthropogenic sources.1,2 Their
oxidation leads to the formation of secondary pollutants (e.g.,
ozone, aldehydes, ketones, aerosols), and thus contributes to
the degradation of air quality in and around urban regions.
Numerous laboratory investigations of the oxidation rates and
mechanisms of the atmospheric oxidation of the cycloalkanes
have been carried out.3-12 Like other hydrocarbon species, the
atmospheric oxidation of the cycloalkanes is initiated by their
reaction with OH which, in the presence of O2, leads to the
production of cycloalkylperoxy radicals. In and near urban areas,
the chemistry of these peroxy species will be dominated by their
reactions with NO, leading in part to the formation of organic
nitrates,3,4 and in part to the formation of cycloalkoxy radicals.
The subsequent chemistry of the cycloalkoxy radicals, which
determines the stable products of the oxidation, may involve
reaction with O2 to form the corresponding cycloalkanone,
decomposition by ring opening, or isomerization by 1,5-
hydrogen shifts.3,9,11,12 The behavior of the cyclohexoxy radical
has been shown to be quite different from that of the cyclo-
pentoxy or cycloheptoxy radicals.3,6-9,11 In the case of cyclo-
hexoxy, reaction with O2 (to form cyclohexanone) is competitive
with ring-opening under ambient conditions (cyclohexanone
yield 25-35%).3,6-9,11 However, in the case of the cyclopentoxy
or cycloheptoxy species, increased ring strain leads almost
exclusively to ring-opening and hence to very small yields
(<5%)3,6-8 of the cycloalkanone. Products obtained following
ring-opening have been identified in the case of cyclohexane,11
and include a series of multi-functional aldehydes, alcohols, and
nitrates. Although not yet observed, similar species are expected
to be obtained in high yield from the oxidation of cyclopentane
and cycloheptane.
Experimental Section
All experiments were conducted in a 2-m long, 47-liter
stainless steel chamber, which is interfaced to a BOMEM
DA3.01 Fourier Transform infrared spectrometer.13,14 The
chamber is equipped with Hanst-type multipass optics which
provided an IR analysis beam path length of 32.6 m. Chilled
ethanol (from a NESLAB Model EX-250HT cooler) was
circulated around the cell to control the temperature, which was
constant to (1.0 K, as monitored by eight thermocouples along
the length of the cell.
Most experiments involved the photolysis of mixtures of
Cl2 (Linde, 0.1-1 Torr), cyclopentane or cyclohexane (Aldrich,
12-120 mTorr), NO (Linde, 0-25 mTorr), O2 (U.S. Welding,
UHP, 10-700 Torr), and N2 (boil-off from LN2 dewar, U.S.
Welding, 0-700 Torr) at a total pressure of 700-750 Torr.
Minor components of the gas mixtures were flushed into the
cell with N2 from smaller calibrated volumes. A cw Xe-arc
lamp, filtered to provide radiation between 235 and 400 nm,
was used for photolysis of the gas mixtures, and the temporal
profiles of the parent cycloalkane and its oxidation products
were monitored by FTIR spectroscopy. The FTIR spectra were
recorded over the range of 800-3900 cm-1 at a spectral
resolution of 1 cm-1, and were obtained from the co-addition
of 100-250 scans. Quantification of the cycloalkanes and
cycloalkanones was done by comparison with standard spectra
(obtained in the presence of 1 atm air) of known quantities of
these species.
In this study, the behavior of both the cyclohexoxy and
cyclopentoxy radicals are examined at temperatures below
ambient. A quantitative determination of the competition
Results and Discussion
Cyclohexane. The first set of experiments involved the
irradiation of mixtures of Cl2, cyclohexane, NO, O2, and N2 at
a total pressure of 700 Torr, with the O2 pressure varied from
150 to 600 Torr. Major reactions are expected to be as follows:
* To whom correspondence should be addressed.
† Now at SRI International, 331 Ravenswood Ave., Menlo Park,
California 94025.
10.1021/jp0002648 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/02/2000

Chemistry of Cyclic Radicals
Cl2 + hν f Cl + Cl
J. Phys. Chem. A, Vol. 104, No. 21, 2000 5073
(1)
(2)
Cl + c-C6H12 (+ O2) f c-C6H11O2 + HCl
c-C6H11O2 + NO f c-C6H11ONO2
f c-C6H11O + NO2
(3a)
(3b)
c-C6H11O2 + NO2 T c-C6H11OONO2 (4, -4)
c-C6H11O f ring-opening products (5)
c-C6H11O + O2 f c-C6H10O (cyclohexanone) + HO2 (6)
HO2 + NO f OH + NO2
(7)
(8)
OH + c-C6H12 (+O2) f c-C6H11O2 + H2O
Although isomerization of the c-C6H11O radical via a 1,5-H
shift is feasible, this reaction pathway has been shown to be of
negligible importance, likely due to the geometric constraints
imposed by the ring system.11 Products obtained from the
oxidation of cyclohexane included cyclohexanone and formic
acid (identified by comparison to standard spectra), as well as
cyclohexyl nitrate (c-C6H11ONO2) and cyclohexyl peroxynitrate
(c-C6H11OONO2), which were identified by virtue of charac-
teristic absorption features (1280 and 1666 cm-1 for the nitrate,
and 1300 cm-1 for the peroxynitrate).12
Before the competition between (R5) and (R6) can be
properly assessed, the presence of the peroxynitrate and nitrate
species needs to be properly accounted for in the analysis. Yields
of the peroxynitrate, obtained using the absorption cross sections
of Platz et al.,12 were found to be dependent on the initial [NO]
and on the extent of reaction, but typically increased from zero
early in an experiment to about 10% toward the end of an
experiment. This is expected, since the [NO2]/[NO] ratio
increases with time, increasing the importance of (R4) relative
to (R3). All yields of cyclohexyl nitrate and cyclohexanone
reported below have been corrected for the formation of the
cyclohexyl peroxynitrate (that is, they are determined relative
to the difference between the cyclohexane consumed and the
peroxynitrate formed).
An experiment was run to determine the thermal stability of
the cyclohexyl peroxynitrate species at room temperature. In
this experiment, a mixture of Cl2 (140 mTorr), cyclohexane (17
mTorr), NO (10 mTorr), O2 (400 Torr), and N2 (300 Torr) was
irradiated until NO was no longer detectable in the IR spectra
([NO] < 0.2 mTorr), and measurable amounts (1.5 mTorr) of
the peroxynitrate species had built up. The photolyzed mixture
was then allowed to sit in the dark, and the concentration of
the cyclohexyl peroxynitrate was monitored. Its temporal profile
was characterized by an early rapid decay (of order 10-3 s-1),
Figure 1. Plot of the apparent nitrate yield as a function of O2 partial
pressure in the photolysis of Cl2/cyclohexane/NO/O2/N2 mixtures at
700 Torr total pressure. Open circles, 273 K; Filled circles, 296 K.
loss rate of the cyclohexyl peroxynitrate was determined by a
competition between (R9) and (R4). Using data for the rate
coefficient of (R9) from Rowley et al.,6,7 the rate coefficient
for (R{-4}) was adjusted in the model to best match the
observed data. A first-order rate coefficient of 7 ( 2 s-1 for
thermal decomposition of the peroxynitrate species is obtained,
in reasonable agreement with measurements made by Zabel and
co-workers (5 s-1).16
Concentrations of cyclohexyl nitrate were obtained by as-
suming a peak absorption cross section at 1284 cm-1 of 1.8 ×
10-18 cm2 molecule-1, by analogy to methyl nitrate.17
A
determination of the cyclohexyl nitrate yield is complicated by
the likelihood of the formation of other organic nitrate species
from the reaction of NO with various peroxy radicals formed
after ring-opening,12 which is most prevalent at low O2 partial
pressures. A plot of the apparent nitrate yield as a function of
[O2] at 296 K is shown in Figure 1. As seen from the figure,
the apparent nitrate yield decreases slightly with increasing [O2]
until a constant value is reached at 400 Torr. The cyclohexyl
nitrate yield, obtained from an average of all data at [O2] g
400 Torr, is 15 ( 4%. This value is in excellent agreement
with previously reported cyclohexyl nitrate yields (9.0 ( 4.4%,3
16.0 ( 1.5%,4 16.5 ( 2.1%,11 16 ( 4%12). The increase in
apparent nitrate yield at low [O2] indicates that additional
nitrates are being produced from peroxy radicals formed after
ring-opening (a series of straight-chain, partially oxygenated C6
species).
A similar procedure was used to obtain the cyclohexyl nitrate
yield at temperatures of 273 and 261 K. For the 273 K data,
experiments with O2 partial pressures in excess of 60 Torr were
used in the analysis (see Figure 1), while at 261 K a series of
experiments was conducted at a partial pressure of 500 Torr
O2 to cleanly evaluate the cyclohexyl nitrate yield. Values
obtained were 21 ( 6%, and 18 ( 7%. Thus, the cyclohexyl
nitrate yield shows a fairly weak dependence on temperature
(albeit over a small temperature range, and with a substantial
uncertainty).
followed by a slow steady first-order decay, k ) 7 × 10-5 s-1
.
This reaction system is characterized by thermal decomposition
and reformation of the cyclohexyl peroxynitrate, reactions
(4, -4), and reaction of the cyclohexyl peroxy radical with NO
(R3) and with itself, reaction 9:
c-C6H11O2 + c-C6H11O2 f 2c-C6H11O + O2
f c-C6H10O +
(9a)
Following a proper accounting of the nitrate and peroxynitrate
formation, examination of the competition between (R5) and
(R6) can be done from an analysis of the dependence of the
cyclohexanone yield on [O2]. From the reaction scheme above,
it can be seen that the yield of cyclohexanone (Y) is given by:
c-C6H11OH + O2 (9b)
Simulations (using Acuchem)15 of the system showed that the
initial rapid decay of the cyclohexyl peroxynitrate was due to
consumption of the residual NO by the peroxy radicals. Once
the NO was completely titrated away, the observed first-order
Y ) R k6[O2]/{k5 + k6[O2]}
(A)

5074 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Orlando et al.
Figure 4. Formic acid yield as a function of O2 partial pressure in the
photolysis of Cl2/cyclohexane/NO/O2/N2 mixtures at 700 Torr total
pressure. Filled circles, 296 K; Open circles, 273 K.
Figure 2. Plots of Yield/(R-Yield) vs O2 partial pressure in the
photolysis of Cl2/cyclohexane/NO/O2/N2 mixtures at 700 Torr total
pressure, where Yield refers to the yield of cyclohexanone and (1-R)
is the cyclohexyl nitrate yield (see text for details). Filled circles, 296
K; Open triangles, 288 K; Filled triangles, 280 K; Open circles, 273
K.
owing to the strong temperature dependence to the endothermic
decomposition (R5). A nonweighted linear least-squares fit to
the data yields k6/k5 ) (1.3 ( 0.3) × 10-27 exp(5550 ( 1100/
T) cm3 molecule-1, where the uncertainty in the A-factor ratio
reflects the uncertainty in the room-temperature value. While
measurements of k6 are not presently available, its value can
be estimated to be 1.5 × 10-14 exp(-200/T) cm3 molecule-1
s-1 18
,
which leads to k5 ) (1.2 ( 0.3) × 1013 exp (-5750 (
1100/T) s-1. Thus, we estimate that ring-opening of the
cyclohexoxy radical, (R5), occurs with a barrier of 11.5 ( 2.2
kcal/mol.
It is clear from the above experiments that ring-opening is a
significant fate of the cyclohexoxy radical at room temperature,
and becomes of little or no importance at the lower temperatures
of the upper troposphere. Some previously identified ring-
opening products,11 including HC(O)(CH2)4CH2ONO2 and
HC(O)CH2CH(OH)CH2CH2CHO, could not be uniquely identi-
fied in our study, though carbonyl-type absorptions near 1740
cm-1 are clearly evident in the product spectra. Formic acid
was also observed in small yield in most, but not all product
spectra. Closer examination of the data revealed that the formic
acid yields increased with decreasing O2 partial pressures,
indicating that it may be originating from chemistry occurring
after ring-opening (see Figure 4). For example, at 296 K, formic
acid yields varied from about 5-6% at 100-200 Torr O2, to
e2% at 500-600 Torr O2. At 273 K, yields of HCOOH were
about 3% for O2 greater than 100 Torr, and increased sharply
(though with considerable scatter) at lower O2. At 261 K, in
the presence of 500 Torr O2, (where ring-opening should not
occur to any extent), formic acid yields were immeasurably
small (<0.8%). Recent studies in our laboratory19 indicate that
formic acid may be obtained from the reaction of R-CHOH
radicals (where R is any relatively large organic fragment) with
O2:20
Figure 3. Rate coefficient ratio k6/k5 plotted in Arrhenius form.
where (1 - R) is defined as the cyclohexyl nitrate yield from
(R3), k3a/(k3a + k3b), which was assumed to increase from 16%
at 296 K to 20% at 273 K. Rearrangement of (A) yields the
following relationship:
Y/(R - Y) ) k6[O2]/k5
(B)
Thus, the slope of plots of Y/(R -Y) as a function of the O2
partial pressure at a given temperature yields values for the rate
constant ratio, k6/k5. Data obtained at temperatures ranging from
273 to 296 K are shown in Figure 2. The rate coefficient ratio
obtained at 296 K is k6/k5 ) (1.5 ( 0.4) × 10-19 cm3
molecule-1, in very good agreement with the value recently
reported by Platz et al.,12 (1.2 ( 0.2) × 10-19 cm3 molecule-1
,
and with the estimate of Rowley et al.,6 2 × 10-19 cm3
molecule-1. The cyclohexanone yield in 1 atm air obtained from
our data (calculated from eq A) is then 36 ( 6%, which agrees
well with previous studies performed in air in which cyclo-
hexanone yields of 23 ( 13%,3 32 ( 4%,11 and 35 ( 4%9
were reported.
R-CHOH + O2 f R-CHO + HO2
f RCH(OH)O2
(10a)
(10b)
(11)
RCH(OH)O2 + NO f RCH(OH)O + NO2
RCH(OH)O f R + HCOOH
(12)
Rate coefficient ratios obtained at the four temperatures
studied are plotted in Arrhenius form in Figure 3. As expected,
the ratio k6/k5 increases sharply with decreasing temperature,
As shown by Aschmann et al.,11 such a radical is formed from
ring-opening, see Reaction Scheme 1. Formic acid was also

Chemistry of Cyclic Radicals
J. Phys. Chem. A, Vol. 104, No. 21, 2000 5075
SCHEME 1: Chemistry Occurring Subsequent to Ring-Opening of the Cyclohexoxy Radical, Showing a Possible Route
to Formic Acid Formation; Formation of Organic Nitrates and Peroxynitrates Omitted for Clarity
among the products reported by Takagi et al.,3 though their
yields are somewhat larger than ours. Takagi et al.3 also report
rather large (40% by mol) yields of formaldehyde, which we
did not observe in our experiments (yield < 10%), and acetylene
(5% yield). While the acetylene is too weak an absorber to be
excluded as a minor product in our experiments, it is extremely
unlikely to be a product of cyclohexane oxidation. It is apparent
that acetylene (as well as CH2O and to a certain extent HCOOH)
were artifacts in the Takagi et al.3 experiments.
Finally, we note that chemical activation has recently been
shown to be important in the chemistry of certain alkoxy
radicals, when formed from the exothermic reaction of the
corresponding peroxy radicals with NO.21-24 In these cases, a
measurable fraction of the alkoxy radicals are born with an
energy in excess of the barrier to their decomposition, and thus
decompose on a time scale that is short compared with
collisions. In cases where chemical activation is significant,
plots of the inverse yield of the O2 reaction product (in this
case cyclohexanone) versus the inverse [O2] show intercepts
significantly greater than unity.21 As noted by Platz et al.,12
Figure 5. Plot of the inverse of the cyclohexanone yields versus
the inverse O2 partial pressure in the photolysis of Cl2/cyclohexane/
NO/O2/N2 mixtures at 700 Torr total pressure and 296 K.
and confirmed by the work carried out here (see Figure 5),
there is apparently no large chemical activation effect in the
case of cyclohexoxy radicals. Chemical activation has been
shown to be important in cases where the barrier to decom-
position is of order 9 kcal/mol or less, somewhat less than the
11-12 kcal/mol barrier determined herein for cyclohexoxy
decomposition. Note that the average amount of energy imparted
to the alkoxy fragment is typically about 10 kcal/mol (although
with a wide distribution),22 somewhat below the measured
decomposition barrier. In addition, the large size of the cyclo-
hexyl system may act against the occurrence of decomposition
due to chemical activation. First, the peroxynitrite complex
initially formed in (R3) is likely fairly long-lived and is thus
likely to suffer some collisional deactivation before decomposi-
tion to products. Furthermore, the RRKM lifetime of the hot
alkoxy species is likely to be considerably longer than previously
studied systems, and the likelihood of collisional deactivation
is again enhanced.
Cyclopentane. Initial experiments involved the photolysis
of mixtures of Cl2 and cyclopentane in O2/N2 mixtures, with
the O2 partial pressure varied from 50 to 700 Torr, at a total
pressure of 700 Torr. Major reactions are expected to be as
follows, where RO2 are organic peroxy radicals formed after
ring opening:

5076 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Cl2 + hν f Cl + Cl
Orlando et al.
Similar experiments were conducted at lower temperatures
(260 and 238 K). The yield of cyclopentanone increased
substantially with decreasing temperature (25% at 260 K, and
43% at 238 K), indicating an increase in the branching ratios
of the molecular channels of (R14b) and/or (R16b), but was
still found to be independent of O2 at a given temperature.
However, firm conclusions on the competition between (R17)
and (R18) are difficult to make owing to the preponderance of
the formation of molecular products in the peroxy radical self-
reactions at these low temperatures. Glutaric dialdehyde was
observed at low temperature, but was found to be lost in the
chamber on a time scale of a couple of minutes, presumably
via heterogeneous processes.7
To further investigate the competition between (R17) and
(R18) at low temperature, similar experiments to those just
described were conducted in the presence of NO (initial
concentrations of 16-24 mTorr). In these experiments, reactions
of the cyclopentylperoxy radicals (and other organic peroxy
species) will be dominated by their reaction with NO, e.g.:
(1)
Cl + c-C5H10(+O2) f c-C5H9O2 + HCl
(13)
c-C5H9O2 + c-C5H9O2 f c-C5H9O + c-C5H9O + O2
(14a)
f c-C5H8O + c-C5H9OH + O2
(14b)
c-C5H9O2 + HO2 f c-C5H9OOH + O2
(15)
c-C5H9O2 + RO2 f c-C5H9O + RO + O2 (16a)
f molecular products
(incl. cyclopentanone) (16b)
c-C5H9O + O2 f c-C5H8O (cyclopentanone) + HO2 (17)
c-C5H9O + M f CH2(CH2)3CHO + M
(18)
As in the work of Rowley et al.,7 products identified in
experiments conducted at room temperature were cyclopen-
tanone and glutaric dialdehyde. Cyclopentanol, observed in the
work of Rowley et al.,7 could not be detected in our experiments
due to its weak absorption features, but can be assumed to have
a yield similar to that of cyclopentanone. The cyclopentanone
yield was found to be 11 ( 3%, independent of the O2 partial
pressure. Although our yield is somewhat lower than the value
of 18 ( 1% reported by Rowley et al.,7 the lack of an O2
dependence to the cyclopentanone yield nevertheless indicates
that its formation is predominantly from (R14b) and (R16b),
and that reaction with O2 (R17) is not important for the
cyclopentoxy radical. That is, at room temperature, the decom-
position of the c-C5H9O radical (R18) dominates its chemistry,
even in the presence of 700 Torr O2.
Absorption cross sections for glutaric dialdehyde are not
available, and thus its absolute yield cannot be obtained.
However, based on an approximate peak absorption cross section
for the carbonyl band of 5 × 10-19 cm2 molecule-1 (by analogy
to other carbonyl species studied in our reaction chamber), a
yield of order 20% can be estimated. Regardless of the exact
yield of the glutaric dialdehyde, the important fact is that its
yield was found to be independent of O2. Since it is likely to
originate from the chemistry of the peroxy radical formed after
ring-opening, (R18),
c-C5H9O2 + NO f c-C5H9O + NO2
(23a)
thus eliminating the possibility of direct cyclopentanone produc-
tion from peroxy radical chemistry. However, as was the case
in the cyclohexane experiments, added complications arise from
the formation of organic nitrates (including cyclopentyl nitrate)
and at higher conversions, when the [NO2]/[NO] ratio increases,
organic peroxynitrates:
c-C5H9O2 + NO + M f c-C5H9ONO2 + M (23b)
c-C5H9O2 + NO2 + M f c-C5H9O2NO2 + M (24)
Organic nitrates were indeed found in these experiments, as
evidenced by the observation of absorption features at 1280 and
1665 cm-1. Assuming a 296 K peak absorption cross section
at 1280 cm-1 of 1.8 × 10-18 cm2 molecule-1, again by analogy
to CH3ONO2,17 the overall nitrate yield is estimated to be 12 (
5% at 296 K, similar to the previous estimate of Takagi et al.,
13%. This value provides an upper limit to the cyclopentyl
nitrate yield from (R23b), since there is a likelihood of nitrate
production from peroxy radicals formed after ring-opening.
Takagi et al.,3 using GC analysis, assigned a yield of only 5%
to the cyclopentyl nitrate, which is considerably lower than the
cyclohexyl nitrate yield of 15% discussed above. Assuming a
10% increase in the peak absorption cross section of the organic
nitrate species at 1284 cm-1 due to the lower temperature, a
total nitrate yield of 21 ( 5% is estimated at 230 K.
At higher cyclopentane conversions, absorption features
centered at 1295 and 1715 cm-1 appeared, consistent with the
formation of cyclopentyl peroxynitrate. To assess the contribu-
tion of the cyclopentyl peroxynitrate to the chemistry, a
calibrated reference spectrum was generated from the photolysis
of Cl2 (400 mTorr) in the presence of cyclopentane (75 mTorr)
and NO2 (20 mTorr), conditions which should lead to near-
100% formation of the peroxynitrate. Absorption cross sections
of 1.3 × 10-18 cm2 molecule-1 at 1295 cm-1, and 2.0 × 10-18
cm2 molecule-1 at 1715 cm-1 were determined, consistent with
values for other peroxynitrates, including cyclohexyl peroxy-
nitrate.12 For peroxynitrate concentration estimates below ambi-
ent temperature, the 1295 cm-1 peak absorption cross section
was assumed to vary inversely with temperature, and to reach
a value of 1.5 × 10-18 cm2 molecule-1 at 230 K.
CH2(CH2)3CHO + O2 f O2CH2(CH2)3CHO (19)
2O2CH2(CH2)3CHO f 2OCH2(CH2)3CHO + O2 (20a)
f HC(O)(CH2)3CHO +
HOCH2(CH2)3CHO + O2 (20b)
OCH2(CH2)3 CHO + O2 f HC(O)(CH2)3CHO + HO2 (21)
OCH2(CH2)3CHO + M f isomerization, decomposition
(22)
the lack of an O2 dependence to its yield is again indicative of
the unimportance of (R17) in the chemistry. Furthermore, the
independence of the glutaric dialdehyde yield on O2 indicates
that (R22) is dominant over (R21) under these conditions (as
expected),9,25 and that the majority of the dialdehyde originates
from (R20b). Yields of CO, CO2, and CH2O were all less than
5%, indicating that they are not significant products of chemistry
occurring after ring-opening. Thus, it is likely that a very
complex set of multi-functional products arises following ring-
opening, as a result of the peroxy radical self-reactions (such
as R20b) and of isomerization of the C5 oxy radical (R22).
Experiments involving the photolysis of Cl2/c-C5H10/NO/
O2/N2 mixtures were conducted at temperatures of 296 K to
230 K, with the O2 partial pressure varied from 100 to 500 Torr

Chemistry of Cyclic Radicals
J. Phys. Chem. A, Vol. 104, No. 21, 2000 5077
SCHEME 2: Chemistry Occurring Subsequent to Ring-Opening of the Cyclopentoxy Radical; Formation of Organic
Nitrates and Peroxynitrates Omitted for Clarity
at all temperatures studied. The competition between (R17) and
(R18) was determined from the ratio of the observed cyclo-
pentanone concentration to the amount of cyclopentoxy radical
formed, which is in turn defined as the amount of cyclopentane
consumed less the estimated cyclopentyl nitrate (assumed to
be 5% at 296 K, and 8% at 230 K, based on the Takagi et al.3
data) and cyclopentyl peroxy nitrate produced (as described
above). While the actual values assumed for the cyclopentyl
nitrate yields make little difference on the conclusions that
follow regarding the competition between (R17) and (R18), we
do note that these cyclopentylnitrate yields do seem lower than
expected, based on previous studies on n-pentane,25 n-hexane,25
and cyclohexane (see earlier).
The origin of the CO2, observed in the NOx experiments, but
not in the NOx-free experiments, is not immediately evident.
One possibility, shown as reaction C of Scheme 2, involves
the transfer of the weak aldehydic H-atom of the ‚OCH2(CH2)3-
CHO radical via a seven-membered transition state.
Formic acid was consistently observed in these NOx experi-
ments, with a yield of order 2%, considerably lower than the
7.5% yield reported by Takagi et al.3 and lower than was
observed in the cyclohexane system under conditions where
substantial ring-opening occurred. The production of the formic
acid could occur after Reaction (E) of Scheme 2. The reduced
formic acid yield relative to the cyclohexane system is likely a
result of the occurrence of HCO elimination (Reaction D), a
reaction not available in the cyclohexoxy case. Some formic
acid may also be generated in conjunction with CO2 formation.
As was the case for cyclohexane oxidation, Takagi et al.3
also observed formaldehyde (44% molar yield) and acetylene
(17.5% molar yield) as products of the OH-initiated oxidation
of cyclopentane in the presence of NO. These species were not
observed in our experiments, and are very likely artifacts.
The lack of cyclopentanone formation in these experiments
confirms that reaction with O2 is not an important fate of the
cyclopentoxy radical at room temperature; its chemistry is
clearly dominated by decomposition, which will lead to a
number of multi-functional C4 and C5 straight chain species
in conjunction with the observed CO and CO2. It is possible
that some of the cyclopentoxy ring-opening is occurring as a
result of chemical activation,21-24 but the importance of this
process cannot be determined using the current experimental
methodology.
No cyclopentanone was observed in experiments conducted
at room temperature, with O2 partial pressures of 150 to 700
Torr, indicating a yield of < 0.5%. In similar experiments,
conducted at 300 K in 1 atm air, Takagi et al.3 reported a
cyclopentanone yield of 0.2%, consistent with our measured
upper limit. Other products observed in our experiments were
CO (29% yield) and CO2 (10 ( 5% yield), which likely result
from chemistry of the radical obtained following ring-opening,
CH2(CH2)3CHO, see Reaction Scheme 2. The observation of
significant yields of CO point to the probable importance of
isomerization of the OCH2(CH2)3CHO radical, and eventual
elimination of HCO. In fact, the observation of measurable CO
yields indicates that HCO elimination (reaction D of Scheme
2) is able to compete effectively with isomerization (reaction
E), a process that is anticipated to be rapid (≈107 s-1).26 It is
also interesting to note that CO is observed in the presence of
NOx, but not in its absence. The reason for this observation
may be due to the large yield of molecular products that are
obtained in the consecutive peroxy radical self-reactions that
occur in the NOx-free experiments, which reduces the yield of
each successive oxy radical, and hence the final CO yield.
In experiments conducted at lower temperature (260, 250,
240, and 230 K), cyclopentanone was observed in small yields,
which increased with decreasing temperature and increasing O2
(see Table 1 for a summary). The highest cyclopentanone yield

5078 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Orlando et al.
energies below the estimated26 values, have recently reported
for the decomposition reactions of tert-butoxy28 and ethoxy29
radicals.
TABLE 1: Product Yields (Molar %) Measured in the
Cl-Atom Initiated Oxidation of Cyclopentane, as a Function
of Temperature and O2 Concentration, in the Presence and
a
Absence of NOx
Firm conclusions regarding the energetics of cyclopentoxy
radical ring-opening are more difficult to obtain, owing to the
dominance of the ring-opening over the O2 reaction, and thus
the difficulty in obtaining definitive temperature-dependent
values for the rate coefficient ratio, k17/k18. Further complications
arise from the possible influence of chemical activation effects21-24
in the chemistry of the cyclopentoxy species. Approximate
Arrhenius parameters for the ring-opening reaction can be
estimated from our low-temperature data, however, under the
initial assumption that the degree of decomposition due to
chemical activation is minimal. The data presented in Table 1
indicate that for conditions of high O2 (500 Torr) and low
temperature (230 K), the decomposition reaction rate still
exceeds that of reaction with O2 by about an order of magnitude,
and thus must be of order 1 × 106 s-1. Assuming an A-factor
of about 1 × 1013 s-1, as was obtained for the cyclohexoxy
ring-opening reaction, leads to an energy barrier to ring-opening
for the cyclopentoxy radical of about 7-8 kcal/mol. Assuming
a higher A-factor, 2 × 1014 s-1 as suggested by Atkinson,18
leads to an activation energy of about 9 kcal/mol. Even with a
very high (70%) occurrence of cyclopentoxy decomposition by
chemical activation, these barrier estimates would only increase
by about 1 kcal/mol. Thus, even the highest probable de-
composition barrier implied by the data obtained in our study
(10 kcal/mol) falls somewhat below the 11-12 kcal/mol that
is obtained using the estimation methods outlined above. The
data obtained regarding the thermodynamics of the cyclopentoxy
radical chemistry support the general conclusion alluded to
above, that decomposition barriers and A-factors for alkoxy
radical decomposition reactions are lower than previously
believed.
with NOx present
no NOx
temp
cyclopentanone cyclopentanone
(K) cyclopentanone CO (100 Torr O2)
(500 Torr O2) CO2
230
240
250
260
300
10
10
11
17
28
7
2.9
2
0.7
<0.5
11
5.5
5
1.7
<0.5
9
9
11
12
11
42
25
10
a Yields were independent of O2, unless otherwise indicated.
measured was 11%, at 230 K and an O2 partial pressure of 500
Torr. The observation of only small yields of cyclopentanone
under these conditions indicates that reaction with O2 (R18) is
of only minor importance in the chemistry of cyclopentoxy
radicals throughout the entire range of conditions present in the
troposphere. Yields of CO were found to decrease with
decreasing temperature (see Table 1), from a value of about
30% at room temperature to values near 10% at 230-240 K.
The decreasing CO yield may result from a decrease in the rate
of the isomerization of the HC(O)CH2CH2CH2CH2O radical
(reaction B of Scheme 2) relative to its reaction with O2 (reaction
A), and/or to an increase in the rate of reaction (E) relative to
that of reaction (D).
Discussion
The data presented above show that, at ambient temperature,
the chemistry of the cyclohexoxy radical is characterized by a
competition between reaction with O2 and ring-opening. Because
of the fairly substantial barrier to ring opening (about 12 kcal/
mol), the importance of this process decreases rapidly with
temperature, such that the O2 reaction will dominate the
atmospheric chemistry of cyclohexoxy at temperatures below
about 270 K. In contrast, the dominant fate of the cyclopentoxy
radical at all temperatures relevant to atmospheric chemistry
will be ring-opening. As first discussed by Takagi et al.,3 the
added ring strain in the C5 system lowers the enthalpy of the
ring-opening, leading to an approximate 100-fold increase in
the rate coefficient for cyclopentoxy ring-opening compared to
that for cyclohexoxy.
Methods for the estimation of the rate coefficients for the
various reactions of alkoxy radicals (reaction with O2, decom-
positions, and isomerizations) have been developed by Atkinson
and co-workers.11,18,26,27 For decomposition reactions, rate
coefficient estimates are obtained by first assuming an A-factor
of 2 × 1014 s-1, multiplied by the reaction degeneracy. The
activation energy (Ed, in kcal/mol) is then estimated from the
following formula: Ed ) (2.4*IP - 8.1) + 0.36∆Hd, where IP
is the ionization potential of the leaving group (in eV), and ∆Hd
is the enthalpy of the decomposition (in kcal/mol). Using this
methodology for the cyclohexoxy radical leads to an Arrhenius
expression for (R5) of k5 ) 4 × 1014 exp(-6730/T) s-1, which
can be compared to our experimentally determined value of
k5 ) 1.2 × 1013 exp (-5750/T) s-1. Over the temperature range
covered by our experiments (260-300 K), the two expressions
agree very well, generating values for k5 that agree to within
20% (approximately the uncertainty in our experimental data).
However, the data obtained herein open up the possibility that
the cyclohexoxy ring-opening process is characterized by a
lower A-factor and lower energy barrier than previously
believed. Similarly low A-factors (≈1013 s-1), with activation
Conclusions
The Cl-atom oxidation of cyclohexane and cyclopentane have
been studied over a range of conditions relevant to atmospheric
chemistry. The chemistry of the cyclohexoxy radical is governed
by a competition between reaction with O2 (R6) and ring-
opening (R5), with k6/k5 ) (1.3 ( 0.3) × 10-27 exp (5550 (
1100/T) cm3 molecule-1. Reaction with O2 will dominate the
chemistry at the temperatures encountered in the middle to upper
troposphere. In contrast, ring-opening is the major fate of the
cyclopentoxy radical under all conditions relevant to the
troposphere. The energy barrier to ring-opening for the cyclo-
pentoxy radicals, < 10 kcal/mol, and possibly for the cyclo-
hexoxy radical as well, are lower than values currently estimated.
Acknowledgment. The National Center for Atmospheric
Research is operated by the University Corporation for Atmo-
spheric Research under the sponsorship of the National Science
Foundation. A portion of this work was supported by the NASA
Upper Atmosphere Research Program. Thanks are due to Frank
Flocke and Alan Fried of NCAR for helpful comments on the
manuscript, and to Karen Henning for assistance in conducting
some of the experiments.
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