FT-IR product studies of the Cl-initiated oxidation of CH3Cl in the presence of NO

Article abstract of DOI:10.1021/jp984523t

The Cl-atom initiated oxidation of CH₃Cl has been studied at 296 K using two different FTIR/environmental chamber systems. In the presence of NO, the carbon-bearing products observed are HCOCl, HCHO, and CO, with yields in 700 Torr of air of (56 ± 10), (32 ± 6), and (12 ± 5)%, respectively. This product distribution is different from previous studies conducted in the absence of NO, in which a nearly 100% yield of HCOCl was obtained. The different product distribution observed in the presence of NO is attributed to the formation and subsequent decomposition of chemically activated CH₂ClO₂ radicals, formed in the exothermic reaction of CH₂ClO₂ with NO.

Full text of DOI:10.1021/jp984523t

J. Phys. Chem. A 1999, 103, 3963-3968
3963
FT-IR Product Studies of the Cl-Initiated Oxidation of CH3Cl in the Presence of NO
Merete Bilde†
Risø National Laboratory, Roskilde, DK-4000, Denmark
John J. Orlando* and Geoffrey S. Tyndall
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
Timothy J. Wallington, Michael D. Hurley, and E. W. Kaiser
Ford Motor Company, Dearborn, Michigan 48121-2053
ReceiVed: NoVember 23, 1998; In Final Form: February 16, 1999
3
The Cl-atom initiated oxidation of CH Cl has been studied at 296 K using two different FTIR/environmental
chamber systems. In the presence of NO, the carbon-bearing products observed are HCOCl, HCHO, and CO,
with yields in 700 Torr of air of (56 ( 10), (32 ( 6), and (12 ( 5)%, respectively. This product distribution
is different from previous studies conducted in the absence of NO, in which a nearly 100% yield of HCOCl
was obtained. The different product distribution observed in the presence of NO is attributed to the formation
and subsequent decomposition of chemically activated CH
of CH ClO with NO.
2
ClO radicals, formed in the exothermic reaction
2
2
Introduction
In general, the self-reactions of peroxy radicals are close to
thermoneutral, and the resulting alkoxy radical has little or no
8
The deleterious effect of chlorine chemistry on stratospheric
ozone levels is now well documented. The sources of chlorine
to the stratosphere include the man-made chlorofluorocarbons
and naturally occurring species, the most abundant of which is
methyl chloride. The atmospheric oxidation of methyl chloride
excitation. However, because reactions of peroxy radicals with
1
8
NO are exothermic and occur via the formation of a ROONO
complex that is sufficiently long-lived to allow for energy
randomization, the alkoxy radicals produced in these reactions
9
-12
can possess internal excitation.
This internal excitation has
2
-6
proceeds via formation of the chloromethoxy radical, CH2ClO.
been shown to be comparable to, or greater than, the barrier to
various unimolecular processes (e.g., C-C bond rupture,
elimination of a halogen atom, or HCl elimination), allowing
the activated radicals to decompose on time scales that are short,
compared to collisional deactivation, and thus leading to product
distributions different from those obtained from thermalized
radicals.
In all previous studies2-6 of CH3Cl oxidation, CH2ClO radicals
were produced via Cl-initiated oxidation of CH3Cl in the absence
of NO, i.e., via reactions 1-4
Cl + hν f 2 Cl
(1)
(2)
(3)
(4)
2
Cl + CH Cl f CH Cl + HCl
3
2
In light of our new understanding of the potential role of
chemical activation in the atmospheric chemistry of alkoxy
radicals, particularly for cases in which the activation barrier
for decomposition of the alkoxy species is low, we have re-
examined the atmospheric fate of CH2ClO radicals. In this study,
the products of the Cl-initiated oxidation of CH3Cl in the
presence of NO are investigated. It will be shown that the
product yields obtained in the presence of NO are significantly
different from those obtained in the absence of NO,2
suggesting that an excited alkoxy radical (CH2ClO*) is formed
in the reaction of CH2ClO2 with NO
CH Cl + O f CH ClO
2
2
2
2
CH ClO + CH ClO f CH ClO + CH ClO + O
2
2
2
2
2
2
2
The conclusion of the two most recent studies5,6 was that three-
center elimination of HCl via reaction 5 and reaction with O2
via reaction 6 are competing fates of the CH2ClO radicals
-
23
generated from reaction 4, with k6/k5 ) 5.6 × 10
exp(3300/
,3,5,6
3
-1
T) cm molecule .
CH ClO + M f HCO + HCl + M
(5)
(6)
2
CH ClO + O f HCOCl + HO
2
CH
2
ClO
2
+ NO f CH
2
ClO* + NO
2
(7)
2
2
Semiempirical calculations4,7 show that HCl elimination will
be favored over both Cl-atom and H-atom elimination, although
the absolute barrier heights calculated are higher than those
experimentally observed.6
and that this excited species plays an important role in the
atmospheric chemistry of CH3Cl.
Experimental Section
Experiments were conducted in environmental chambers at
Ford and NCAR, each of which is equipped with FT-IR
*
To whom correspondence should be addressed.
Present address: Department of Chemical Engineering, Carnegie Mellon
†
1
3,14
University, Pittsburgh, PA 15213.
spectrometers as described elsewhere.
CH3Cl oxidation is
1
0.1021/jp984523t CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/22/1999

3964 J. Phys. Chem. A, Vol. 103, No. 20, 1999
Bilde et al.
10- cm3 molecule-1 s . As discussed above, Cl-atom
chemistry was most important for experiments conducted at
NCAR, and corrections were made as follows, i.e., ignoring
the small effects of reaction 13:
11
-1 8
initiated both by Cl-atoms (reaction 2) and by OH, which is
generated via reaction of HO2 radicals with NO.
OH + CH Cl f CH Cl + H O
(8)
3
2
2
The FT-IR system at Ford was interfaced to a 140-L Pyrex
reactor. Radicals were generated by the UV irradiation (using
-0.5k12/k2
[
HCHO]corr ) [HCHO]obs {1 - ∆[CH Cl]/[CH Cl]}
3 3
2
2 black lamps which surround the cell) of mixtures of CH3Cl,
[
CO]corr ) [CO]obs - ([HCHO]corr - [HCHO]obs)
Cl2, O2, and NO in 700 Torr total pressure of N2 at 296 K.
Reactant loss and product formation were monitored by FT-IR
spectroscopy, with an analyzing path-length of 28 m and a
resolution of 0.25 cm . Infrared spectra were derived from 32
co-added spectra.
The apparatus at NCAR consisted of a 47-L stainless steel
reactor fitted with a quartz window at one end to allow
photolysis by means of a filtered xenon arc lamp. Experiments
involved the photolysis of CH3Cl (Matheson), Cl2 (Matheson,
UHP), NO (Linde), O2 (U.S. Welding, UHP), and N2 (liquid
N2 boil-off) and were conducted at 296 K. Typical photolysis
times were 3 min. A Bomem DA 3.01 FT-IR spectrometer was
interfaced to a Hanst-type optical arrangement mounted within
the reaction cell, for in situ IR spectroscopic analysis of the
gas mixture composition. Reactant loss and product formation
were monitored by FT-IR absorption spectroscopy, with an
optical path length of 32.6 m and a spectral resolution of 1.0
For experiments conducted at Ford, conversions of CH3Cl were
sufficiently small that correction to measured HCHO yields were
not required.
Finally, conversion of HCO to HCOCl via reaction with Cl2
was accounted for. Initial yield calculations were based on the
assumption of total conversion of HCO to CO via reaction 14,
-
1
-12
3
-1 -1 8
k14 ) 5.5 × 10
cm molecule s .
HCO + O f HO + CO
(14)
2
2
5
However, with k /k ) 1.15, small amounts of HCO were
converted to HCOCl via reaction 15 in experiments conducted
at low [O2].
1
5
14
HCO + Cl
f HCOCl + Cl
(15)
2
-
1
cm . Infrared spectra were derived from 50 co-added inter-
ferograms.
CH3Cl, and its oxidation products HCHO, HCOCl, and CO,
were monitored using their characteristic features over the range
The observed product yields were quantified with a precision
of (10%. Possible systematic errors associated with the
references may contribute an additional 10% to the uncertainty,
leading to a total uncertainty of (15% on the product yields.
Uncertainties reported in this paper are two standard deviations
unless otherwise stated, and standard error propagation methods
are used to calculate combined uncertainties.
-
1
1
000-2300 cm . Reference spectra of HCHO and CO were
acquired and calibrated by expanding a known pressure of
reference material into the reactor from a calibrated volume.
CO quantification was done in the 2060-2220 cm region,
while HCHO was quantified using the carbonyl stretch centered
at 1745 cm . HCOCl was identified by means of its charac-
teristic IR features at 1305 and 1784 cm . Quantification of
the HCOCl yield was achieved using a previously reported cross
-
1
Results and Discussion
-
1
To investigate the potential formation of chemically activated
alkoxy radicals in reaction 7, Cl2/CH3Cl/O2/N2/NO mixtures in
00 Torr total pressure were irradiated. In the first series of
-
1
7
-
1
-18
2
-1 5,6
section, σ(1793 cm ) ) 1.63 × 10
cm molecule .
experiments mixtures of 11.5 Torr (Ford) or 0.11 Torr (NCAR)
of CH3Cl, 100 mTorr of Cl2, 10 mTorr of NO, and 4-400 Torr
of O2 in 700 Torr total pressure of N2 were subjected to UV
irradiation. Three carbon-containing products were identified:
HCOCl, HCHO, and CO. In all of the experiments the loss of
CH3Cl was too small (<3%) to be quantified accurately, and
the loss of CH3Cl was thus assumed equal to the sum of the
concentrations of the carbon-containing products: ∆[CH3Cl]
Although [CH3Cl] was kept high to minimize secondary
chemistry, minor corrections to measured product yields needed
to be made in some cases. First, stoichiometric conversion of
HCOCl to CO on the stainless steel walls of the NCAR
6
chamber, which occurred with a first-order rate coefficient of
-
4
-1
khet ) 3 × 10
s , was accounted for as previously
6
documented. Conversion of HCOCl to CO via reaction with
-
13
3
-1 -1 15,16
Cl-atoms, k9 ) 7.6 × 10
cm molecule s ,
or with
)
∆[HCOCl] + ∆[CO] + ∆[HCHO]. Figure 1 shows the yield
-13
3
-1 -1 16
OH, k10 ≈ 1 × 10 cm molecule s , was also considered
of HCOCl and the combined yield of CO and HCHO versus
the O2 partial pressure. The dotted lines are included to aid visual
inspection of the data trend. The solid lines show the expected
yields of CO and HCOCl based upon experiments performed
Cl + HCOCl f ClCO + HCl
(9)
OH + HCOCl f ClCO + H O
(10)
2
5
,6
in the absence of NO, and are given by Y(HCOCl) ) k6[O2]/
ClCO f Cl + CO
(11)
(
k5 + k6[O2]) and Y(CO) ) 1 - Y(HCOCl) with k6/k5 ) 3.9 ×
-
18
3
-1 6
10
cm molecule . Clearly, there is a difference in product
Under conditions used at Ford (very high [CH3Cl] and thus small
fractional conversion), reactions 9 and 10 were insignificant,
yields from oxidation of CH3Cl in the presence and absence of
NO. At the highest concentrations of O2, the HCOCl yield is
expected to exceed 98%. However, in the presence of NO, only
a 60% yield of HCOCl was observed at high [O2], whereas the
combined yield of CO and HCHO was 40% under these
conditions. This significant change in the product yields is a
strong indication that a fraction of the alkoxy radicals formed
in reaction 7 are chemically activated and undergo prompt
decomposition to form products other than those expected from
reactions 5 and 6 alone. Furthermore, the observation of HCHO
suggests that a new reaction channel is opened up, which is
inaccessible to the thermalized CH2ClO radicals.
and no correction was necessary. Computer simulations of the
conditions used at NCAR (using ACUCHEM)17 showed that
the chemistry was largely controlled by Cl-atom reactions, and
corrections were made for reaction 9 as previously described.
Conversion of HCHO to CO, via reaction with either OH or
Cl-atoms, was also considered
6
Cl + HCHO f HCO + HCl
(12)
(13)
OH + HCHO f HCO + H O
2
with k12 ) 7.3 × 10-11 cm molecule s-1 and k13 ) 1.0 ×
3
-1

Oxidation of CH3Cl in the Presence of NO
J. Phys. Chem. A, Vol. 103, No. 20, 1999 3965
The remainder of this paper will be concerned with the
determination of the relative importance of reactions 16 and
1
7a-c in the chemistry of the CH2ClO* radical formed in
reaction 7. The following notation will be used throughout: R
is the fraction of alkoxy radicals produced with insufficient
energy to overcome any barrier to unimolecular reaction and
that is thus collisionally thermalized, reaction 16; â is the
fraction of activated alkoxy radicals that eliminates a hydrogen
atom to form HCOCl, reaction 17a; γ1 is the fraction of activated
alkoxy radicals that eliminates HCl to form HCO (reaction 17b);
and γ2 is the fraction that eliminates Cl atoms to form HCHO
(reaction 17c).
There are three potential sources of HCOCl in the system:
hydrogen atom elimination from the excited alkoxy radical
reaction 17a), reaction of the thermalized alkoxy radical with
(
O2 (reaction 6), and reaction of the thermalized alkoxy radical
with NO, via reaction 18a
CH ClO + NO f HCOCl + HNO
(18a)
(18b)
(18c)
2
Figure 1. Product yields obtained in the Cl-atom-initiated oxidation
CH ClO + NO f HCHO + NOCl
2
of CH
3
Cl (0.11 Torr, NCAR; 11.5 Torr, Ford) in the presence of NO
partial pressure: diamonds,
(10 mTorr), as a function of the O
2
CH ClO + NO + M f CH ClONO + M
2 2
combined yield of CO and HCHO; circles, yield of HCOCl, obtained
at NCAR (open symbols) or Ford (filled symbols). The data were
corrected for the effects of secondary chemistry, as described in the
text. The dotted lines are included to aid visual inspection of the data
trend. The solid lines are the expected product yields in the absence of
NO.
In Figure 1, the yield of HCOCl converges toward 60% at the
highest concentrations of O2. This suggests that essentially all
of the thermalized alkoxy radicals are converted to HCOCl via
reaction with O2 and that reaction 18 is unimportant under these
conditions. Thus, the yield of HCOCl in the limit of high O2
provides a value of R + â of roughly 60%.
On the basis of our previous studies of CF3CFHO* 9 and
HOCH2CH2O* 12 radicals, those radicals produced with an
energy that is insufficient to overcome any barrier to unimo-
lecular reaction are expected to be thermalized via collisions
with molecules of the bath gas
Also, at high [O ], where reaction 18 is of negligible
2
importance, channel 17c is the only source of HCHO. CO is
formed via reaction 17b followed by reaction 14 or via
destruction of HCHO, reactions 12-14. Thus, from the com-
bined yield of CO and HCHO at high [O2], we obtain γ1 + γ2
≈ 40%. That is, approximately 40% of the alkoxy radicals
generated in reaction 7 decompose promptly to generate products
not observed in previous studies of methyl chloride oxidation
carried out under similar conditions, but in the absence of NO.
To determine the individual values of γ and γ , the effects
CH ClO* + M f CH ClO + M
(16)
2
2
whereas those produced above barriers to decomposition are
expected to decompose on a subnanosecond time scale (i.e., on
a time scale that does not allow for collisional deactivation or
for bimolecular chemical reaction to occur). Possible channels
for the decomposition of CH2ClO* radicals include
1
2
of reactions 12 and 13, which convert HCHO to CO, must be
minimized. Thus, the yields of CO and HCHO were analyzed
in the experiments using 11.5 Torr of CH3Cl, and 10 mTorr of
NO. In these experiments, Cl and OH react almost exclusively
with CH3Cl and the occurrence of reactions 12 and 13 is of
minor importance. Figure 2 shows the yields of CO and HCHO
versus the O2 concentration. At the highest [O2], 100-400 Torr,
where the effects of reaction 18 are also minimized, the product
yields converge toward 32% for CO and 12% for HCHO. Since
there is no (<2%) HCHO formation from the thermalized alkoxy
CH ClO* f H + HCOCl
(17a)
(17b)
(17c)
(17d)
2
CH ClO* f HCl + HCO
2
CH ClO* f Cl + HCHO
2
CH ClO* f H + ClCO
2
2
5
,6
Using a calculated enthalpy of formation for the thermalized
radicals and increasing [O2] from 100 to 400 Torr with
constant [NO] has no discernible impact on the product yields,
it seems reasonable to interpret the yield of 12% HCHO as the
fraction γ2 of the excited alkoxy radicals that undergo Cl-atom
elimination. Similarly, the yield of CO at high concentrations
of O2 can be ascribed to the HCl elimination from the excited
alkoxy radical, γ1 ≈ 32%.
-
1 6
alkoxy radical of ∆Hf (CH2ClO) ) -2.4 kcal mol , the
reaction enthalpies for decomposition of the thermalized radicals
8
via reactions 17a-d are as follows: ∆Hr (17a) ) 10.2 kcal
-
1
-1
mol , ∆Hr (17b) ) -9.7 kcal mol , ∆Hr (17c) ) 5.3 kcal
-
1
-1
mol , and ∆Hr (17d) ) -2.6 kcal mol . Again, on the basis
of comparison with previous studies,9 it is reasonable that at
least a fraction of the nascent alkoxy radicals will possess
sufficient energy to allow Cl-atom or possibly H-atom elimina-
tion to occur. The energy barrier to HCl elimination from the
,12
So far, CO and HCHO have been identified as products of
the prompt decomposition of chemically activated CH2ClO
radicals, with a total yield of 40-45%. It is now left to address
the origin of the observed HCOCl product. This issue is
complicated by the multiple possible sources of this com-
pound: H-atom elimination from chemically activated CH2ClO
(reaction 17a) and the reactions of thermalized CH2ClO radicals
with either O2 (reaction 6) or NO (reaction 18a).
-
1 6
thermalized alkoxy radical is 9 kcal mol
and is thus also
-
1
accessible. H2 elimination is 7 kcal mol less favorable than
HCl elimination; thus, the barrier to this process is likely to be
prohibitively high, and its occurrence will not be further
considered.

3966 J. Phys. Chem. A, Vol. 103, No. 20, 1999
Bilde et al.
Figure 2. Yields of HCHO (2) and CO (9) following irradiation of
1.5 Torr of CH Cl, 100 mTorr of Cl , 10 mTorr of NO, 4-400 Torr
of O in 700 Torr total pressure of N plotted versus the partial pressure
of O
1
3
2
2
2
2
.
To assess the relative contributions of these sources, experi-
ments were performed using low O2 partial pressures to increase
the relative importance of loss of thermalized CH2ClO radicals
via HCl elimination, reaction 5. However, under these condi-
tions, the observed product yields are also influenced by the
reaction of the thermalized CH2ClO with NO. Thus, to
determine the effects of reaction 18, experiments were conducted
at a fixed low O2 pressure (5 Torr), with the initial NO
concentration varied over a wide range (5-110 mTorr). Experi-
ments were conducted both at Ford (with an initial [CH3Cl] of
Figure 3. Product yields of (A) HCHO, (B) HCOCl, and (C) CO versus
the concentration of NO following UV irradiation of mixtures of 5
Torr of O
2
, 1-11 Torr of CH
3
Cl, ∼100 mTorr of Cl
2
and 5-110 mTorr
of NO at 700 Torr total pressure. Experimental data were obtained at
NCAR (open squares, triangles, and circles) or Ford (filled squares,
triangles, and circles). Solid lines: model results with k18c ) 0, k18a
)
-12
3
-1 -1
-12
3
-1
7
s
× 10 cm molecule
s
, and k18b ) 9 × 10 cm molecule
-
1
1
1.5 Torr) and at NCAR (initial [CH3Cl] of 1.1 Torr). In all of
, R ) 56%, â ) 0%, γ
1
) 12%, and γ ) 32%; see text for details.
2
the experiments the conversion of CH3Cl was <0.2% and
HCOCl, CO, and HCHO were again the only observed carbon-
containing products. Figure 3 shows the corrected yields of
HCHO, HCOCl, and CO versus the partial pressure of NO.
Experimental data acquired using the setup at NCAR are shown
as open symbols, and data acquired at Ford are shown as filled
symbols. Clearly, the product yields depend on the partial
pressure of NO relative to that of O2. Since the excited alkoxy
radicals are expected to be too short-lived to undergo chemical
reactions, it can be concluded that the NO dependency is due
to the occurrence of reaction 18. Furthermore, the constant yields
reached at the higher NO partial pressure (0.05-0.12 Torr)
indicate that the thermalized CH2ClO radicals are reacting
almost exclusively with NO under these conditions.
There are a number of conclusions that can be drawn from
the data in Figure 3. First, the HCHO yield is seen to increase
from 12% at low NO to about 40% at higher NO, offering clear
evidence for the importance of reaction 18b in the system. A
more thorough discussion of the impact of reaction 18 is given
below. The CO yield at high [NO] reaches a constant value of
about 32%. Since there are no apparent sources of CO from
the reaction of CH2ClO with NO, this CO must be coming from
the decomposition of chemically activated CH2ClO via process
Thus, an upper limit to â (reaction 17a) of 25% can be
established. This result can be combined with the estimate that
R + â ) 60% (inferred from the HCOCl yield at high [O2],
see above). Hence, the value of R (the fraction of CH2ClO
radicals that is thermalized) must fall in the range 35-60%.
Further conclusions regarding the values of R and â can only
be obtained with a prior knowledge of the overall rate and
mechanism of reaction 18, information which is not currently
available. However, with reasonable assumptions about this
reaction, some computer simulations of the experiments shown
in Figure 3 were conducted, as is now described. First, 32% of
the alkoxy radicals were assumed to decompose promptly via
HCl elimination, γ1, while 12% were assumed to decompose
-
1
via Cl-atom elimination, γ2. Values of k5 ) 2600 s and k6 )
-
14
3
-1 -1
1 × 10 cm molecule
s
were assumed, in agreement with
6
the measured ratio of these rate coefficients and consistent with
typical rate coefficients for reaction of alkoxy radicals with O2.8
Since reaction of CH3O with NO to form CH3ONO occurs with
-
11
3
-1 -1
a rate coefficient of about 2.5 × 10
cm molecule
s
at
8
700 Torr, the same rate coefficient was assumed in the model
for reaction 18c.
Two model scenarios were then considered, one in which
H-atom elimination was assumed to be important (R ) 34%, â
) 22%), and one in which this process was assumed to be
negligible (â ) 0, R ) 56%). In the first scenario, the rate
coefficient for reaction 18b was adjusted until the proper trend
in HCHO production with the initial [NO] was obtained. Best
agreement between the model and the experiments was obtained
(
3
17b). This result is wholly consistent with the value of γ1 )
2%, inferred from the yield of CO in the presence of high O2
concentrations (see above).
The HCOCl yield is relatively independent of [NO] in this
set of experiments, with a value of 25% at high [NO]. Because
all thermalized CH2ClO radicals react with NO under these
conditions, the only sources of HCOCl are reactions 17a or 18a.
-
11
3
-1 -1
with k18b ) (3.5 ( 1.0) × 10
cm molecule s . The

Oxidation of CH3Cl in the Presence of NO
J. Phys. Chem. A, Vol. 103, No. 20, 1999 3967
modeled trends in the data matched those observed fairly well.
The drawback to this model scenario is that it seems improbable
that as many as 20-25% of the nascent CH2ClO radicals have
internal energy that is above what is likely to be a fairly large
barrier to H-atom elimination (probably in excess of 15 kcal/
mol).
In the second model scenario, values of â ) 0% and R )
6% were assumed. This model can also be made to fit the
5
data quite well if significant amounts of HCOCl are formed in
the reaction of CH2ClO with NO, reaction 18a. Again assuming
Figure 4. Fate of the excited CH
reaction of CH ClO with NO in 700 Torr total pressure of O
96 K: R is the fraction of excited alkoxy radicals which are
thermalized by reaction with a third body; â is the fraction which
eliminates a hydrogen atom; γ and γ are the fractions which eliminate
HCl, and Cl, respectively. Most likely, the value of â is close to zero,
and that of R is near 60%; see text for details.
2
ClO* radicals produced in the
-
11
3
-1 -1
that k18c ) 2.5 × 10 cm molecule s , k18a must be 4 ×
2
2
2
/N at
2
-
11
3
-1 -1
1
0
cm molecule s for sufficient HCOCl production to
2
occur. While this scenario again fits the observed data quite
well, it seems unlikely that reactions 18a through c are all
1
2
-
11
3
operative with rate coefficients in excess of 2 × 10
cm
-
1
-1
-10
3
-1 -1
molecule
s
(total k18 ≈ 1 × 10
cm molecule s ),
given that analogous RO + NO reactions are typically in the
-
11
8
A-factors for the Cl-atom elimination channel are likely greater
low 10
range. A further possibility is that CH2ClONO is
1
3
-1 12
than 10 s .)
not formed in reaction 18. This reaction probably occurs through
the formation of a complex
Conclusions and Atmospheric Implications
CH ClO + NO f [CH ClONO]* f HCOCl + HNO (18a)
2
2
It has been shown that the products of the oxidation of CH3-
Cl are different in the presence of NO from what had previously
2,3,5,6
f HCHO + NOCl (18b)
f CH ClONO (18c)
been observed in its absence. Previous studies
showed that
the dominant product obtained under atmospheric conditions,
but in the absence of NO, is HCOCl. However, the chemically
activated CH2ClO radicals formed in the presence of NO lead
to a different product distribution. The observed product yields
when low levels of NO are present (in 700 Torr air, at 296 K)
are: HCOCl (56%), CO and HCl (32%), and HCHO and Cl
2
and it is possible that stabilization of the nitrite does not occur.
A similar mechanism has been postulated for the reaction of
CF3O with NO,
(12%). These product yields will be obtained from the OH- or
Cl-initiated oxidation of CH3Cl in the atmosphere in regions
where the chemistry of the CH2ClO2 radical is dominated by
its reaction with NO, such as throughout most of the continental
boundary layer. Although studies have only been conducted at
CF O + NO f [CF ONO]* f CF O + FNO (19)
3
3
2
for which the yields of CF2O and FNO have been shown to be
8
,18,19
unity.
Under the assumption that k18c ) 0, lower values
2
96 K, these product yields should be relatively insensitive to
-12
-12
3
-1
for k18a and k18b (about 7 × 10 and 9 × 10 cm molecule
temperature, since the prompt decomposition of the chemically
activated CH2ClO should be largely unaffected by temperature,
and the majority of the thermalized CH2ClO radicals will react
with O2 at all temperatures encountered in the atmosphere.
Chemical activation effects in the atmospheric chemistry of
a number of alkoxy radicals have now been demonstrated9
and appear to be a phenomenon general to radicals possessing
activation barriers to decomposition of about 12 kcal/mol or
less.
-1
s ) are required to model the data (see Figure 3), and a much
more reasonable total value for k18 is obtained. Note that the
exothermicities of reactions 18a, 18b, and 19 are all at least 8
kcal/mol greater than that of reaction 20b, offering a possible
reason nitrite formation may not be important in reactions 18
or 19.
-12
CH O + NO f [CH ONO]* f CH ONO
(20a)
(20b)
3
3
3
f HCHO + HNO
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. Thanks are due to Barry Lefer and Alan Fried of
NCAR for helpful comments on this manuscript.
We reiterate that all of the scenarios discussed above are
capable of reasonably explaining the observed data, and firmer
conclusions regarding the actual values of R and â await further
information on the rate and mechanism of reaction 18. In any
event, the above discussion has served to identify and quantify
the various reactions important in the chemistry of activated
CH2ClO* radicals (see Figure 4 for a summary). On the basis
of previous studies,12 the alkoxy radicals are expected to be
born with a range of excess energies. Because the lowest barrier
References and Notes
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4
6
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1
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