Product channels of the HCCO + NO reaction

Article abstract of DOI:10.1021/jp040503h

The product branching ratio of the HCCO + NO reaction was investigated using the laser photolysis/infrared absorption technique. Ethyl ethynyl ether (C₂H₅OCCH) was used as the HCCO radical precursor. Transient infrared detection of CO, CO₂, and HCNO products was used to determine the following branching ratios at 296 K: ??(CO+HCNO) = 0.78 ?± 0.04 and ??(CO₂+HCN) = 0.22 ?± 0.04. These values are in good agreement with some recent ab initio calculations. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp040503h

J. Phys. Chem. B 2005, 109, 8363-8366
8363
Product Channels of the HCCO + NO Reaction†
Justin P. Meyer and John F. Hershberger*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity, Fargo, North Dakota 58105
ReceiVed: July 22, 2004; In Final Form: NoVember 20, 2005
The product branching ratio of the HCCO + NO reaction was investigated using the laser photolysis/infrared
2 5
absorption technique. Ethyl ethynyl ether (C H OCCH) was used as the HCCO radical precursor. Transient
infrared detection of CO, CO
2
, and HCNO products was used to determine the following branching ratios at
+HCN) ) 0.22 ( 0.04. These values are in good agreement
296 K: φ(CO+HCNO) ) 0.78 ( 0.04 and φ(CO
2
with some recent ab initio calculations.
Introduction
decreasing to about 0.32 at 2000 K.13 Kinetic modeling of flow
reactor experiments at 1100-1400 K were originally best fit
The spectroscopy1 and kinetics4-9 of the ketenyl radical
-4
14
by a branching ratio of φ1a ) 0.65, but subsequent calculations
on the OH+HCNO reaction suggested that a lower value of
(
HCCO) is of interest, partly because of the role this species
plays in combustion chemistry. HCCO is formed in flames
1
7
φ1a may better fit the model. Other more direct experiments
as well as subsequent ab initio calculations have contradicted
the early results, showing that channel (1a) is in fact the minor
channel. Boullart et al., in their mass spectrometry study,
reported the following product branching ratios at 700 K: φ1a
primarily by the oxidation of acetylene.1 It has been observed
0,11
4
in laboratory studies by infrared absorption and laser-induced
fluorescence spectroscopy.1,2 Several kinetic studies involving
HCCO have appeared recently, with total rate constant measure-
4-7,9
5
5
8
ments reported for reactions with NO,
C2H2. The reaction with NO is of particular interest because
of the role it plays in NO-reburning mechanisms
reduction of NOx emissions from fossil-fuel combustion pro-
NO2, O2, O, and
6
5
) 0.23 ( 0.09, φ1b ) 0.77 ( 0.09. Eickhoff and Temps
12-17
obtained similar results at 300 K (φ1a ) 0.28 ( 0.10, φb )
for the
1
8
0
.64 ( 0.12) in an FTIR study. An infrared laser study in our
laboratory yielded an even lower branching ratio of φ1a ) 0.12
cesses. Several product channels are possible:
1
9
(
0.04. Two recent computational studies have improved on
0
the earlier potential energy surface, predicting φ1a ) 0.22 at
00 K and φ1a ) 0.27 at 300 K. The primary difference
HCCO + NO f CO + (HCN)
∆H ) -527.2 kJ/mol
2
20
21
3
(1a)
between the recent and earlier calculations is that the recent
calculations more correctly describe the isomerization barrier
0
f CO + (HCNO) ∆H ) -200.8 kJ/mol
2
0,21
between cis- and trans-OCC(H)NO minima,
which the
(1b)
7
earlier calculation had assumed to be a low barrier free rotation.
Although either isomer can form HCNO + CO by carbon-
carbon bond fission, only the cis isomer can rearrange into a
four-membered ring intermediate which decomposes to HCN
0
f NCO + HCO
∆H ) -71.1 kJ/mol
(1c)
The thermochemistry shown is for HCN and HCNO, but several
other isomers of these species represent possible, albeit unlikely
product channels as well. Several reports of the total rate
constant of this reaction have appeared. Unfried et al. used
+
CO2. This barrier therefore represents an important bottleneck
in the formation of products (1a). Both of the recent calculations
predict a very gradual decrease in φ1a with increasing T, to ∼0.1
at ∼2500 K.
-
1
infrared absorption near 2023 cm to detect HCCO, and
All the previous experimental studies of this reaction have
been complicated by the lack of an ideal precursor for generation
of the HCCO radical. Several of the experiments used the O +
C2H2 reaction to form HCCO + H. This reaction is quite slow,
and also produces CH2 + CO products, requiring the data be
fit to a substantial kinetic model. Our previous report19 used
ketene (CH2CO) as a photolytic precursor at 193 nm. This has
the similar problem that CH2 + CO are the major photolysis
-
11
3
-1 -1
reported k1 ) (3.9 ( 0.5) × 10
cm molecule
s
at 298
4
K. Temps et al. used far-infrared laser magnetic resonance to
-11
3
-1
detect HCCO and obtained (2.2 ( 0.6) × 10 cm molecule
-
1
at 298 K.5 Boullart et al. used discharge flow-mass
s
-
10
spectrometry to obtain k1 ) (1.0 ( 0.3) × 10
exp[-350 (
3
-1 -1
1
6
50)/T] cm molecule
s
over the temperature range 290-
6
70 K. In the most extensive kinetic study reported to date,
Carl et al. used a laser-induced fluorescence technique to obtain
22
-
11
3
-1
products, and H + HCCO is only a minor channel. As a result,
k1 ) (1.6 ( 0.2) × 10
exp(340 ( 30 K/T) cm s
-
1
9
any attempt to detect the HCNO product suffers from secondary
molecule over the temperature range 297-802 K.
2
3
chemistry, primarily CH2 + NO f H + HCNO.
Several previous studies have also investigated the product
branching in this reaction. In an early prediction, Miller et al.
used statistical theories and a QCISD potential surface of
In the experiments reported here, we use ethyl ethynyl ether,
C2H5OCCH, as a photolytic precursor. The 193 nm photodis-
sociation dynamics of this molecule has been recently studied,24
and this molecule was found to be a relatively clean source of
HCCO (and C2H5) radicals. Since the ethyl radical does not
react quickly with NO, this represents a nearly ideal source of
7
Nguyen et al. to predict that CO2+HCN as the major channel
φ1a ) 0.81) at 300 K with a moderate temperature dependence,
(
†
Part of the special issue “George W. Flynn Festschrift”.
1
0.1021/jp040503h CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/18/2005

8364 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Meyer and Hershberger
HCCO for our experiments. This molecule has been used in
one previous kinetic study, a recent report on the HCCO + O2
reaction.25
Experimental Section
The experimental procedure is similar to that described in
previous publications.26,27 193 nm photolysis light was provided
by an excimer laser (Lambda Physik, Compex 200). Several
lead salt diode lasers (Laser Components) operating in the 80-
1
10 K temperature range were used to provide tunable infrared
probe laser light. The IR beam was collimated by a lens and
combined with the UV light by means of a dichroic mirror,
and both beams were copropagated through a 1.46 m absorption
cell. After the UV light was removed by a second dichroic
mirror, the infrared beam was then passed into a 1/4m
monochromator and focused onto a 1 mm InSb detector
Figure 1. Transient infrared absorption signals for CO, HCNO, and
(Cincinnati Electronics, ∼1 µs response time). Transient infrared
CO
NO ) 0.6 Torr, PXe ) 1.0 Torr, PSF6 ) 1.0 Torr (CO
signals), PCF4 ) 1.0 Torr (CO signals only).
2
product molecules. Reaction conditions: PC2H5OCCH ) 0.01 Torr,
absorption signals were recorded on a LeCroy 9310A digital
oscilloscope and transferred to a computer for analysis.
SF6, Xe, and CF4 (Matheson) were purified by repeated
freeze-pump-thaw cycles at 77K. Traces of CO2 was removed
from SF6 by the use of an Ascarite trap. NO (Matheson) was
purified by repeated freeze-pump-thaw cycles at 163 K to
remove NO2 and N2O. C2H5OCCH in hexane samples were
obtained from Aldrich and were purified by several freeze-
pump-thaw cycles followed by pumping away of most of the
hexane. Small quantities of hexane in the reaction mixtures are
not expected to affect the results.
P
2
and HCNO
Authentic samples of HCNO for calibration purposes were
synthesized by vacuum pyrolysis of 3-phenyl-4-oximinoisox-
28,29
azol-5-(4H)-one as described in the literature,
and purified
by vacuum distillation to remove CO2 and phenyl cyanide
byproducts as well as HNCO impurities. Care was taken to
minimize exposure of HCNO to metal to minimize decomposi-
tion. In general, HCNO could be allowed to stand at room
temperature for ∼5 min in our Pyrex absorption cell with
minimal decomposition, but ∼50% decomposition over 5 min
was commonly observed if the sample was in contact with
stainless steel vacuum lines.
Figure 2. Transient infrared absorption signals for HCNO product
molecules as a function of SF buffer gas pressure. Reaction condi-
tions: PC2H5OCCH ) 0.01 Torr, PNO ) 0.3 Torr, PSF6 ) 0.0-1.0 Torr.
6
CO, N2O, CO2, and HCNO product molecules were probed
using the following absorption lines:
transient signals displayed rapid rise times followed by slow
decays. The CO signals have both a fast and a moderately slow
CO (V ) 1 r V ) 0)
P(9), P(10), P(16) at
107.424, 2103.270, 2203.161 cm-
(
∼150 µs) rise. All of these rise times are slower than would
1
2
be predicted by the rate constant for reaction 1. This is
presumably due to the formation of excited vibrational states
of the various product molecules. Since we probe infrared
transitions originating from the vibrational ground state, the
observed rise time is essentially a measure of the rate of
vibrational relaxation of this excited nascent distribution.
1
N O (00°1) r (00°0)
P(32) at 2193.540 cm-
2
CO (00°1) r (00°0)
P(18), P(14) at
2
1
2
334.157, 2337.659 cm-
26,32
R(10) at 2203.851 cm-1
Previous experiments in our laboratory
as well as vibrational
HCNO (0100°0°) r (0000°0°)
3
3-36
energy transfer measurements
have demonstrated that SF6
The HITRAN molecular database was used to locate and
identify the spectral lines of CO, N2O, and CO2 product
is an efficient buffer gas for the relaxation of vibrationally
excited CO or N O to a Boltzmann distribution, but that CF
4
2
2
30
31
molecules. Other published spectral data were used to locate
and identify HCNO lines. The spectral lines used are near the
peak of the rotational Boltzmann distribution, minimizing
sensitivity to small heating effects. For CO2 product molecule
measurements, the infrared laser beam path was purged with
N2 to remove atmospheric CO2.
is a more efficient relaxer of vibrationally excited CO. As a
result, SF buffer gas was used for all CO detection experi-
6
2
ments, whereas CF was used for CO detection. The slow rise
4
component of the CO transient signal is attributed to the rather
slow relaxation of vibrationally excited CO. For HCNO, no
literature information is available regarding vibrational relaxation
rates. Figure 2 shows HCNO transient signals obtained at
Typical experimental conditions were PC H OCCH ) 0.01-
2
5
0
.02 Torr, PSF ) 0.5 Torr, and PNO ) 0-0.6 Torr.
different SF pressures. The rise time of the transients decreases
6
6
with increasing SF6 pressure, as is expected if SF6 is an effective
Results and Discussion
vibrational relaxer. As a result, we used SF6 buffer gas for
HCNO detection experiments, and we are confident that the
rovibrational populations of all of the probed molecules are
Time-resolved transient absorption signals of product mol-
ecules at 296 K are shown in Figure 1. The CO2 and HCNO

Product Channels of the HCCO + NO Reaction
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8365
relaxed to a Boltzmann distribution within ∼150 µs. The slow
∼
1 ms decay present in all of the transient signals is attributed
to diffusion of product molecules out of the probed region of
the reaction cell.
Attempts to detect N2O product molecules in this reaction
system were unsuccessful. If channel (1c) is active, one expects
N2O formation in high yield via the secondary reaction:
NCO + NO f N O + CO
(2a)
(2b)
2
f CO + N2
2
Previous experiments have shown that φ2a ) 0.44 at 298 K.25
Our failure to detect N2O indicates that channel (1c) does not
contribute significantly to the title reaction. This is a more
sensitive test for channel (1c) than direct NCO detection by
infrared spectroscopy37 because N2O has much greater infrared
linestrengths. In addition, N2O formation could also originate
from reactions of CH radicals formed by photolysis of HCCO
radicals:
Figure 3. Beers law plot of the calibration of HCNO static infrared
-
1
absorption signal, using the R(10) line at 2203.851 cm
.
significant amounts of CO are observed (approximately one-
third to one-half of the CO signal when NO reagent is included
in the reaction mixture). This background CO could originate
from several sources: it is possible that C2H5OCCH has a minor
photolysis channel for CO formation, although no such channel
was reported in the photolysis study of this precursor.24 In the
absence of NO reagent, photolytically produced HCCO may
react with trace amounts of oxygen impurities, HCCO may react
with C2H5OCCH, or HCCO may react with itself (or other
radicals), producing some CO. It is also possible that multiple
photon effects (such as HCCO photolysis) may contribute to
the CO formation. We note that similar (although smaller) CO
background signals were observed in a recent study of the
C H OCCH + hν (193 nm) f C H + HCCO
2
5
2
5
HCCO + hν (193 nm) f CH + CO
CH + NO f HCN + O
f NCO + H
f NH + CO
NCO + NO f N O + CO
2
f CO + N2
2
2
5
Our failure to detect N2O therefore also indicates that these
multiple photon effects are not significant in this experiment,
and that significant quantities of CH radicals are not formed.
This is an important point, because the presence of CH would
substantially complicate the secondary chemistry in these
experiments.
Some transient signals were collected with the inclusion of
Xe buffer gas in addition to the CF4 or SF6 buffer gas. This
experiment was to test for effects due to possible excited
electronic states of HCCO, which are likely to be relaxed by
xenon. The transient signals were identical with and without
xenon, suggesting that excited electronic states do not affect
our results.
HCCO + O2 reaction. It is not clear which of these sources
of background CO are most important. An excimer laser power
dependence study showed only a slight nonlinearity in the CO
yield vs excimer pulse energy, suggesting that multiple photon
effects are minor. The lack of observed N2O production
described above supports this conclusion. In principle, some of
these background sources (such as radical-radical chemistry
or trace O2 reactions) should result in a longer rise time than
would be expected from CO originating from the title reaction.
Unfortunately, our CO absorption signals (see Figure 1) always
display a long rise time component because the relaxation of
vibrationally excited CO (mainly by CF4 buffer gas) is quite
slow. Our experimental result is that the CO transient collected
in the absence of NO reagent has a slighly longer rise time in
the slow rise component than that collected with NO reagent,
but the difference is insufficient to allow us to effectively
discriminate against the background. We suspect that reactions
of HCCO (with itself, precursor, trace O2, etc.) are responsible
for most of the background. If this is true, such reactions are
virtually entirely suppressed when excess NO reagent is
included, and therefore this background signal should not be
subtracted from the signal with NO. Under this assumption, the
CO and CO2 signals upon photolysis of a C2H5OCCH/NO/buffer
gas mixture are predominantly due to the title reactions, with
little or no contribution from secondary chemistry. The relative
yields of these two products is therefore a direct measurement
of the branching ratio. Using this approach, we determine φ1a
) 0.23 ( 0.03 and φ1b ) 0.77 ( 0.03.
Absorption signals for CO, CO2, and N2O products were
3
0
converted to number densities using tabulated line strengths
and equations described previously.26 For HCNO, no literature
information on infrared line strengths is available. Instead, we
calibrated HCNO signals by directly measuring the static
infrared absorption of authentic samples of HCNO synthesized
in our laboratory. Figure 3 shows a plot of this calibration. This
measurement was somewhat complicated by the tendency of
HCNO to quickly decompose, especially when in contact with
metal (stainless steel) vacuum lines. To minimize decomposition,
the HCNO sample was kept frozen except when needed to fill
the absorption cell. The static absorption signal was measured
within approximately ∼0.5 min of filling of the cell, and contact
with metal was minimized. Each point in Figure 3 represents a
new fill of the absorption cell.
An important test for background sources of the detected
molecules is to check whether the transient signals are observed
in the absence of nitric oxide reagent, i.e., upon photolysis of
a C2H5OCCH/buffer gas mixture. We find negligible amounts
of CO2 and HCNO formation under these conditions, but
An alternative approach to determining the branching ratio
of the title reaction is to compare the HCNO and CO2 yields.
This avoids the uncertainties inherent in the above discussion
of background sources of CO. The key here is that unlike most
previous literature on the HCCO + NO reaction, our HCCO

8366 J. Phys. Chem. B, Vol. 109, No. 17, 2005
Meyer and Hershberger
TABLE 1: Product Branching Ratio of the HCCO + NO
References and Notes
Reactiona
(
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CO
2
+ HCN
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method
ab initio
DF/MS (700 K)
modeling (1100 K)
FP/FTIR
FP/IR
ab initio
ab initio
FP/IR
ref
(
0
0
0
0
0
0
0
0
.81
.23 ( 0.09
.65
.28 ( 0.10
.12 ( 0.04
.22
.27
.22 ( 0.04
0.19
0.77 ( 0.09
0.35
0.64 ( 0.12
0.88 ( 0.04
0.78
0.73
0.78 ( 0.04
13
6
14
18
19
20
21
1
(
(
1
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a
8036.
All values at 296 K unless otherwise indicated. Abbreviations:
(
7) Nguyen, M. T.; Boullart, W.; Peeters, J. J. Phys. Chem. 1994, 98,
DF/MS ) discharge flow/mass spectrometry. FP/IR ) flash photolysis/
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8
030.
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3
583.
(
formation method does not produce CH2 radicals, and therefore
the CH2 + NO f HCNO + H secondary reaction does not
contribute to the HCNO yield. Comparison of HCNO and CO2
yields is therefore a direct measurement of the branching ratio.
The disadvantage of this approach is the requirement to calibrate
the HCNO signals, as described above. Using this method, we
obtain φ1a ) 0.21 ( 0.04 and φ1b ) 0.79 ( 0.04, nearly identical
to the value obtained using CO. In other words, our calibration
of the HCNO signals yields HCNO yields very similar to the
CO yields obtained, as is expected if secondary chemistry does
not significantly affect the yields of these molecules.
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(
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On the basis of both sets of data, we recommend the value
φ1a ) 0.22 ( 0.04 and φ1b ) 0.78 ( 0.04 for the branching
ratio at 296 K. Table 1 shows this and other literature values
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the two most recent computational studies of this reaction, which
(
(
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0
21
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(
19
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6
identical to the values obtained by Boullart et al. at 700 K,
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precursor for HCCO formation. Detection of CO, CO2, and
HCNO product yields the following branching ratio at 296 K:
φ1a ) 0.22 ( 0.04 and φ1b ) 0.78 ( 0.04. These values are in
good agreement with other recent experimental and computa-
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1
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Acknowledgment. This work was supported by the Division
of Chemical Sciences, Office of Basic Energy Sciences of the
Department of Energy, Grant DE-FG03-96ER14645.
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