Kinetics of the CN + HCNO reaction

Article abstract of DOI:10.1021/jp0650073

The kinetics of the CN + HCNO reaction were studied using laser-induced fluorescence and infrared diode laser absorption spectroscopy. The total rate constant was measured to be k(T) = (3.95 ± 0.53) × 10^-11 exp[(287.1 ± 44.5)/T] cm³ molec^-1 s^-1, over the temperature range 298-388 K, with a value of k₁ = (1.04 ± 0.1) × 10^-10 cm³ molec^-1 s ^-1 at 298 K. After detection of products and consideration of secondary chemistry, we conclude that NO + HCCN is the only major product channel.

Full text of DOI:10.1021/jp0650073

12184
J. Phys. Chem. A 2006, 110, 12184-12190
Kinetics of the CN + HCNO Reaction
Wenhui Feng and John F. Hershberger*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity, Fargo, North Dakota 58105
ReceiVed: August 3, 2006; In Final Form: September 5, 2006
The kinetics of the CN + HCNO reaction were studied using laser-induced fluorescence and infrared diode
-
11
laser absorption spectroscopy. The total rate constant was measured to be k(T) ) (3.95 ( 0.53) × 10
3
-1 -1
exp[(287.1 ( 44.5)/T] cm molec s , over the temperature range 298-388 K, with a value of k
1
) (1.04
at 298 K. After detection of products and consideration of secondary chemistry,
we conclude that NO + HCCN is the only major product channel.
-10
3
-1 -1
(
0.1) × 10 cm molec
s
1
. Introduction
Thermochemical information has been obtained from standard
3
tables as well as other references for the heats of formation of
Fulminic acid, HCNO, has recently been identified as an
4
5
6
7
8
HCNO and NCO, HCNN, HCCO, NCCO, and CCO.
important intermediate in the NO-reburning process for the
reduction of NOx pollutants from fossil-fuel combustion emis-
sions. As a result, the subsequent chemistry of HCNO is of
2
. Experimental Section
Total rate constants were measured by both laser-induced
1
great interest in the overall NO-reburning mechanism. The first
experimental kinetic measurement on fulminic acid was recently
fluorescence (LIF) and infrared diode laser absorption methods,
as described in a previous publication. CN radicals were created
by 248 nm excimer laser photolysis of ICN:
2
2
reported for the reaction of HCNO with OH radicals. This is
-
11
3
-1
a fast reaction with a rate constant of 3.39 × 10 cm molec
-1
s
and produces primarily CO + H2NO and HCO + HNO
products. CN radicals are formed in the oxidation of HCN by
OH radicals, which is also an important part of NOx formation
mechanisms in both fuel-rich hydrocarbon flames and flames
containing fuel nitrogen. In this paper we present a study of
the kinetics of the reaction of CN radicals with HCNO. The
reaction has numerous possible product channels, of which the
following is only a subset of likely possibilities:
ICN + hν (248 nm) f CN + I
(2)
For the LIF measurements, 248 nm light from an excimer laser
(Lambda Physik, Compex 200) was made collinear with 387.088
nm probe light from a frequency doubled Nd:YAG-pumped dye
laser (Continuum). Both beams entered a 91.4 cm reaction cell.
Laser-induced fluorescence of CN on the B Σ r X Σ
transition passed through a 400-500 nm band-pass filter and a
85 nm long-pass filter, and was detected at right angles from
the laser beams with a PMT mounted against a side window.
Signals were recorded on a gated integrator (with 150 ns delay,
00 ns gate width), digitized, and stored on a computer. Timing
2
+
2 +
CN + HCNO f CO + HCNN
3
298
∆
Hf ) -256.9 kJ/mol (1a)
f C N + OH
2
2
2
298
∆
Hf ) -258.1 kJ/mol (1b)
between excimer and dye pulses was controlled by a computer-
controlled digital delay generator. The pump-probe delay was
varied to obtain the CN concentration vs time profiles.
f NO + HCCN
2
9 8
∆
Hf ) -32.4 kJ/mol (1c)
f (NCO) + HCN
Reaction products as well as CN reactants were detected by
infrared diode laser absorption spectroscopy. Several lead salts
diode lasers (Laser Components) operating in the 80-110 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 excimer light by means of a dichroic mirror, and
both beams were co-propagated though a 1.43 m absorption
cell. After the UV light was removed by a second dichroic
mirror, the infrared beam was then passed into a 1/4 m
monochromator and focused onto a 1 mm InSb detector
2
98
∆
Hf ) -343.5 kJ/mol (1d)
f HCCO + N2
298
∆
Hf ) -428.9 kJ/mol (1e)
f HCO + NCN
298
∆
Hf ) -89.6 kJ/mol (1f)
f NH + NCCO
(
Cincinnati Electronics, ∼1 µs response time). Transient infrared
298
∆
Hf ) -19.4 kJ/mol (1g)
absorption signals were recorded on a LeCroy 9310A digital
oscilloscope and transferred to a computer for analysis. To
account for small probe laser thermal deflection affects, signals
were collected with the diode laser slightly detuned off the
absorption lines, and such transients were subtracted from the
on-resonant transients. This is only a minor correction (∼
f N + H + CCO
2
298
∆
Hf ) -7.2 kJ/mol (1h)
f H + CO + NCN
298
∆
Hf ) -122.5 kJ/mol (1i)
2
-10%).
f N + CO + CH
2
HCNO samples were synthesized as previously described
298
2,9,10,11
∆
Hf ) -122.5 kJ/mol (1j)
by flash vacuum pyrolysis of 3-phenyl-4-oximino-
isoxazol-5(4H)-one. The purity of the HCNO samples was
characterized by FT-IR spectroscopy and was typically 95%
pure or better, with only small CO2 and HNCO impurities.
f HCN + CO + N
298
∆
Hf ) -108.6 kJ/mol (1k)
1
0.1021/jp0650073 CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/13/2006

Kinetics of the CN + HCNO Reaction
J. Phys. Chem. A, Vol. 110, No. 44, 2006 12185
Because HCNO has poor long-term stability, samples were kept
at 77 K except when filling the reaction cell. In general, HCNO
could be allowed to stand at room temperature for ∼5 min in
our Pyrex absorption cell with minimal decomposition, but
significant decomposition over ∼5 min was commonly observed
if the sample was in contact with stainless vacuum lines. As a
result, the vacuum system was modified to minimize (but not
entirely eliminate) exposure of HCNO to metal.
ICN (Aldrich) was purified by vacuum sublimation to remove
dissolved air. SF6 and CF4 (Matheson) were purified by repeated
freeze-pump-thaw cycles at 77 K and by passing through an
Ascarite II column to remove traces of CO2. NO (Matheson)
1
5
18
and N O (Isotec) were purified by repeated freeze-pump-
15
18
18
thaw cycles at 153 K to remove NO2 and N2O. N O and O2
Isotec) were additionally purified by passing through an
Ascarite II column in order to remove trace amounts of
O C O and O C O.
The following molecules were probed using infrared diode
(
1
6
12 18
18 12 18
Figure 1. Diode infrared absorption signal of CN as a function of
time. Upper trace: 0.0 Torr HCNO; lower trace, 0.0253 Torr HCNO.
Other reaction conditions: P(ICN) ) 0.10 Torr, P(SF ) ) 1.0 Torr.
6
laser absorption spectroscopy:
R(7) at 2071.409 cm-
1
CN (V )1 r V ) 0)
0
0
0
0
R(10) at 2203.851 cm-1
R(11) at 2186.639 cm-
P(23) at 2202.744 cm-1
HCNO (0100 0 ) r (0000 0 )
1
CO (V )1 r V ) 0)
0
0
N O (00 1) r (00 0)
2
R(10.5) at 1912.072 cm-
1
NO (V )1 r V ) 0)
HNO (100) r (000)
P(6) at 2667.785 cm-
1
HCCO (ν , k ) 0) (V )1 r V ) 0)
2
R(12) at 2031.593 cm-
1
1
6
8
12 18
0
0
P(13) at 2322.089 cm-1
O C O (00 1) r (00 0)
1
12 18
0
0
R(12) at 2322.568 cm-1
O C O (00 1) r (00 0)
Figure 2. Natural log of the CN laser-induced fluorescence signal as
a function of time. Diamonds, 0.0 Torr HCNO; triangles, 0.0123 Torr
HCNO; hollow diamonds, 0.0197 Torr HCNO. Other reaction condi-
The HITRAN molecular database was used to locate and
identify the spectral lines of NO, CO, N2O, and CO2 product
tions: P(ICN) ) 0.10 Torr, P(SF ) ) 1.0 Torr.
6
1
2
molecules. Other published spectral data were used to locate
1
3,14
15
16
17
and identify CN,
HNO, HCNO, and HCCO lines. The
tion with iodine atom, reaction with trace of O2, and diffusion
out of the probed region of the reaction cell. Upon the addition
of HCNO reagent, a very large increase in CN decay rate is
observed. Decay curves such as those shown in Figures 1 and
2 were fit to a single-exponential decay function. Figure 3 shows
the resulting decay rates as a function of HCNO pressure,
obtained at several different temperatures using LIF detection.
A linear dependence is observed, as is expected under the
pseudo-first-order kinetics conditions, in which [HCNO]0 .
[CN]0. As per a standard kinetic treatment, the slope of this
plot is the desired bimolecular rate constant k1.
spectral lines used are near the peak of the rotational Boltzmann
distribution, minimizing sensitivity to small heating effects.
Typical experimental conditions were P(HCNO) ) 0.2 Torr,
P(ICN) ) 0.1 Torr, P(SF6) or P(CF4) ) 1.00 Torr, P(NO or
1
5
18
N O) ) 0.0-2.00 Torr, and excimer laser pulse energies of
2
5
mJ (fluence of ∼18 mJ/cm ). CF4 buffer gas was used for
CO detection, while SF6 buffer gas was used for detection of
all other product molecules. The choice of buffer gas was
motivated by the desire to relax any nascent vibrationally excited
product molecules to a Boltzmann distribution.18,19
3
. Results
Figure 4 shows an Arrhenius plot for the CN + HCNO
reaction. Measurements were limited to a rather narrow tem-
perature range of 298-388 K because HCNO decomposition
in the reaction cell becomes unacceptably rapid at high
3
.1. Total Rate Constants. Measurement of total rate
constants of the title reaction at room temperature were
performed using both infrared absorption and laser-induced
fluorescence, yielding identical results. LIF was used exclusively
for experiments at elevated temperatures. Figure 1 shows
transient infrared absorption signals of the CN radical, obtained
with and without HCNO reagent. Figure 2 shows a plot of CN
radical LIF signal as a function of excimer-probe delay time.
As both figures show, in the absence of HCNO reagent, a ∼15
2
temperatures. Nevertheless, the temperature range is sufficient
to obtain reasonably precise Arrhenius parameters. We obtain
the following rate constants (error bars represent one standard
deviation):
k (T) ) (3.95 ( 0.53) ×
1
-
1
10 exp [(287.1 ( 44.5)/T] cm molec s-1
-11
3
-1
ms decay rate over the time range 10-200 µs is observed.
This decay is attributed to removal of CN radicals by pathways
other than the title reaction, including self-reaction, recombina-
-
10
cm molec s-1
3
-1
k1(298) ) (1.04 ( 0.1) × 10

12186 J. Phys. Chem. A, Vol. 110, No. 44, 2006
Feng and Hershberger
Figure 3. Pseudo-first-older decay rate constant of the CN radical as
a function of HCNO pressure. Reaction conditions: P(ICN) ) 0.10
Torr, P(SF ) ) 1.0 Torr, P(HCNO) ) variable.
6
Figure 5. Transient signals of (a) CO and (b) NO. Reaction
conditions: P(ICN) ) 0 Torr (lower traces), P(ICN) ) 0.10 Torr (upper
traces), P(HCNO) ) 0.20 Torr, P(CF
4
) ) 1.0 Torr (for CO transient
only), P(SF ) ) 1.0 Torr (for NO transient only).
6
ICN/HCNO/CF4 mixtures; however, nearly identical signals
were observed upon photolysis of HCNO/CF4 mixtures, as
shown in Figure 5a. This indicates that most of the CO originates
from background sources such as HCNO photolysis and
resulting secondary chemistry, and it indicates that direct
formation of CO in channels 1a, 1i, 1j, and 1k are not major
product channels. We estimate a total upper limit of æ1a + æ1i
Figure 4. Arrhenius plot for the CN + HCNO reaction.
The negative activation energy observed in this reaction is
typical of radical-radical reaction kinetics, and it suggests that
the reaction proceeds by complex formation, followed by
possible rearrangement and decomposition to form products.
A direct hydrogen abstraction mechanism would be expected
to have a significant positive activation energy barrier, and it is
apparently not a significant contribution in this reaction.
We did not investigate any possible pressure dependence of
the rate constant, but the large value obtained at the low
pressures used in these experiments indicates that any pressure
dependence must be very slight.
+
æ1j + æ1k < 0.12.
As shown in Figure 5b, a large transient signal for nitric oxide,
NO, was detected upon 248 nm photolysis of ICN/HCNO/SF6
mixture, while much smaller amounts were produced when the
ICN precursor was omitted. We attribute this to nitric oxide
formation by channel 1c.
In principle, NCO formed in channel 1d or via secondary
chemistry could be probed directly by infrared absorption.
Quantification is rather difficult, however, because of rather low
infrared absorption coefficients and the reactivity of this species.
A preferred procedure was to include nitric oxide in the reaction
mixture, in order to convert NCO into stable products with high
IR absorbance:
3.2. Product Yields. Infrared diode laser absorption was used
to attempt detection of NO, N2O, CO, CO2, HNO, and HCCO
products. Some transient signals are shown in Figures 5-7. All
detection experiments were conducted at 298 K. A key point is
that many of the same products produced by the title reaction
may also be produced through 248 nm photolysis of HCNO
and subsequent secondary chemistry. Possible chemistry in-
2
volved was discussed in our previous publication. To obtain
NCO + NO f N O + CO
(3a)
(3b)
2
product yields from the title reaction, product yields obtained
from HCNO/buffer gas mixtures were subtracted from those
obtained with the full ICN/HCNO buffer gas mixtures. This
approach assumes that the secondary chemistry is not greatly
affected by the presence or absence of the ICN precursor.
Although this assumption is probably not perfect, we believe
that it is sufficient for the purposes of this study, i.e., the
identification of the major product channels of the title reaction.
Transient signals for CO were observed upon photolysis of
f N + CO
2
2
Branching ratios (at 296 K) of æ3a ) 0.44 and æ3b ) 0.56 have
1
9
been reported. Large transient signals of N2O and CO were
detected upon 248 nm photolysis of ICN/HCNO/NO/buffer gas
mixture and HCNO/NO/buffer gas mixture (SF6 for N2O
detection and CF4 for CO detection). Figure 6a shows the N2O
signals. There are significant differences between the signals

Kinetics of the CN + HCNO Reaction
J. Phys. Chem. A, Vol. 110, No. 44, 2006 12187
1
6
12 18
18 12 18
Figure 6. Transient signals of (a) N
of NO. Reaction conditions: P(ICN) ) 0 Torr (lower traces), P(ICN)
0.10 Torr (upper traces), P(HCNO) ) 0.20 Torr, P(NO) ) 2.0 Torr,
P(SF ) ) 1.0 Torr.
2
O and (b) HNO in the presence
Figure 7. Transient signals of (a) O C O and (b) O C O in the
presence of 15N O. Reaction conditions: P(ICN) ) 0 Torr (lower
traces), P(ICN) ) 0.10 Torr (upper traces), P(HCNO) ) 0.20 Torr,
18
)
15
18
P( N O) ) 2.0 Torr, P(SF
6
) ) 1.0 Torr.
6
obtained with and without ICN, although a substantial back-
ground from HCNO photolysis is present. We attribute this
additional N2O and CO formation to production of NCO
followed by reaction 3a. As we will show below, however, most
or all of this NCO is not a direct product from channel 1d but
is produced by secondary chemistry.
No transient signals for HCCO was observed upon 248 nm
photolysis of ICN/HCNO/SF6, indicating that channel 1e is not
significant. We are unable to reliably estimate an upper limit
because the infrared absorption coefficients of HCCO are
unknown. Preliminary measurements in our laboratory indicate
that the HCCO + HCNO reaction is quite slow, so if HCCO is
formed in significant quantities, we would expect to be able to
detect it.
TABLE 1: Product Yields of Reaction CN + HCNO
without
ICNa
with
rel.
yield
ICNa
differencea,b
c
product
NO
5.71
9.22
21.52
5.82
19.0
13.3
1.84
2.77
3.01
1
COd
11.06
24.29
8.83
0.139
0.209
0.226
CO
Od
N
2
a
In units of 1012 molecule cm-3. b Difference obtained by subtracting
column 3 (yield with ICN) by column 2 (yield without ICN).
c
d
Difference yields (column 4) normalized to [NO] ) 1.00. Yields
obtained in the presence of 2.0 Torr of NO.
products observed in the transient experiments are observable
in the FT-IR, but no C2N2 was detected. Furthermore, any OH
produced in channel 1b would be expected to react with HCNO,
HCO products were not directly probed, but an HNO transient
signal was detected upon 248 nm photolysis of an ICN/HCNO/
NO/SF6 mixture. HNO is presumably formed by the reaction
2
producing large amounts of CO . Our lack of evidence for CO
production (described above) suggests that channel 1b is not a
HCO + NO f HNO + CO
(4)
major product channel. We estimate an upper limit of æ1b <
0
.20.
However, this signal is almost same as that obtained from 248
nm photolysis of HCNO/NO/SF6 mixture. Because the HNO
signal is the same with and without ICN, we conclude that most
of the HCO originated from HCNO photolysis and/or subsequent
secondary chemistry and not from channel 1f of the title reaction.
C2N2 is not detectable in our diode laser spectroscopy because
of the lack of available laser diodes in their significant spectral
region. As a result, we cannot probe the importance of channel
Transient signal amplitudes (peak-peak) of NO, N2O, and
CO were converted into absolute concentration using HITRAN
line-strengths as described in a previous publication.19 Table 1
shows a typical dataset of product molecule yields. Shown are
concentrations obtained both with and without the ICN precur-
sor, and the difference between these two measurements. The
right-hand column of Table 1 shows the resulting relative
product yield.
1
b by transient infrared spectroscopy. We have, however,
Table 1 shows that large amounts of N2O and CO molecules
are produced from photolysis of ICN/HCNO/NO/SF6, which
can be attributed to NCO formation as mentioned above. In
obtained static FT-IR spectra of the reaction mixture before and
after extensive photolysis (1000 Excimer laser pulse). The stable

12188 J. Phys. Chem. A, Vol. 110, No. 44, 2006
Feng and Hershberger
1
8
18
principle, other sources of these products could exist, such as
CO formation from the reaction of HCCO with NO. If this
reaction were important, however, we would expect an excess
of CO. Our observation of nearly identical [CO] and [N2O]
yields is fairly strong evidence that reaction 3 is primarily
responsible for the formation of these products. Employing the
known branching ratio of reaction 3, we can estimate the yield
of NCO to be [N2O]/0.44. This results in an [NO]:[NCO] ratio
of 1:0.51. Similarly, one can use CO yields to reach the same
conclusion (since the CO and N2O yields are approximately
equal). Based on these data, one is tempted to conclude channel
HCCN + N O f HCN + NC O
(7a)
(8a)
1
8
18
18
18
NC O + N O f OC O + N
2
1
8
f C O + N O
(8b)
2
The ratio of 18OC O to OC O is therefore a sensitive test of
the origin of the NCO radicals, with the caveat that we must
still measure the difference in yields obtained with and without
the ICN precursor, in order to subtract the yields of HCNO
photolysis products.
18
16
18
The results of the isotopic labeling experiment are shown in
Figure 7 and especially Table 2. Large yields of both OC O
and OC O are observed upon photolysis of an ICN/HCNO/
N O/SF6 mixture; however, almost all of the OC O isotope
signal is background, probably due to NCO produced by HCNO
1
d is a significant product channel. However, given our
18
18
observation of large amounts of NO products attributed to
reaction 1c, we must also consider secondary chemistry of the
HCCN molecule that is also produced. Under ICN/NO/HCNO/
buffer gas conditions, the following reaction is expected to be
significant:
16
18
15 18
16
18
18
18
photolysis. Virtually none of the OC O signal is background.
After subtracting the photolysis background, we obtain a ratio
1
8
12 18
16 12 18
of [ C C O]/[ O C O] ) 6.01. As a result, we conclude
that most of the NCO (other than HCNO photolysis background)
is formed by secondary chemistry such as reaction 5a, and that
direct formation of NCO by the title reaction in channel 1d is
not significant. A similar conclusion can be reached from a
comparison of C O and C O yields, but this experiment was
less conclusive than the CO2 experiment because of large
photolysis backgrounds.
The isotopically labeled N O experiment above provides
fairly strong evidence for production of HCCN products in the
title reaction, although we cannot directly detect HCCN in these
experiments. Further evidence for HCCN formation is provided
HCCN + NO f HCN + (NCO)
298
∆
Hf ) -311.0 kJ/mol (5a)
f HCCO + N2
1
8
16
298
∆
Hf ) -397.2 kJ/mol (5b)
f OH + NCCN
1
8
298
∆
Hf ) -225.7 kJ/mol (5c)
The reaction has been studied by Curl et al.20 Several other
product channels may be written as well. Curl et al. reported
detection of HCN and HCNO products. They attribute the HCN
products to channel 5a. HCNO is unlikely be formed directly
in reaction 5, as this would be endothermic (it is the reverse of
our channel 1c). Curl et al. proposed several possible secondary
routes to HCNO formation. An additional route that they did
not consider was channel 5b followed by the reaction of HCCO
18
by the following experiment: If isotopically labeled O2 is
included (i.e., an ICN/ O2/HCNO/SF6 mixture is photolyzed),
the following reaction is expected to occur:
18
1
8
18
HCCN + O f C O + HCN
(9)
2
2
20
The reaction of HCCN with O2 has been studied previously,
-
12
cm molec s-1
3
-1
+
NO. Although quantitative branching ratios were not reported
with a reported rate constant of 1.8 × 10
in their study, it does appear very likely that reaction 5a is a
major product channel. As a result, we cannot rule out the
possibility that some or all of the NCO produced in our
experiments originated from reaction 5a rather than 1d.
and with CO + HCN (or HNC) the only products identified.
2
1
8
If O and HCNO pressures are comparable, some of the CN
2
1
8
18
18
radicals will react with O2 to make primarily NC O + O,
and reactions of NC O with HCNO may form CO2; however,
these products are expected to be of the mixed isotope
1
8
To determine the origin of NCO, we performed experiments
1
6
12 18
with isotopically labeled 15N O (only the oxygen labeling is
18
O C O:
relevant to this discussion). If NCO is produced directly in
1
8
18
18
reaction 1d, 1 OC O and C O would be produced upon 248
6
18
16
CN + O f NC O + O
(10)
(11)
2
15
18
nm photolysis of an ICN/HCNO/ N O/buffer mixture:
1
8
16
16 12 18
NC O + HCN O f O C O + HCNN
CN + HCNO f NCO + HCN
(1d)
(6a)
(6b)
-
11
cm molec s-1
3
-1
The rate of reaction 10 is k10 ) 2.4 × 10
2
2
18
16
18
at 298 K, while no literature reports exist for k11. Note that
CO2 + HCNN is only one of many possible product channels
of reaction 11, which will be the subject of an upcoming paper.
NCO + N O f OC O + N
2
16
18
2
f C O + N
O
12
The result of this experiment is that we detected 6.84 × 10
3
18 12 18
molec/cm of O C O upon photolysis of an HCNO/ICN/
The isotopic labeling in the products of channel 6a is definite.
The labeling in 6b is based on the assumption that this reaction
proceeds via a strait-chain O-N-N-C-O intermediate, fol-
lowed by N-C bond fission to form the products. Some rather
difficult rearrangements would be necessary to put the O atom
on the CO product. This assumption is supported by ab initio
studies on this reaction.21 On the other hand, if NCO was
produced from the secondary reaction 5a, 18OC O and C O
TABLE 2: 16O C O and O C O Yields in Secondary
12 18
18 12 18
15
18
Reaction of CN + HCNO with N O
without
ICNa
with
rel.
yield
18
ICNa
differencea,b
c
product
16 12 18
O C O
3.129
0.726
3.495
2.919
0.365
2.193
0.166
1
18
12 18
O C O
18
18
a
In units of 1012 molecule cm-3. b Difference obtained by subtracting
column 3 by column 2. Difference yields (column 4) normalized to
molecules would be produced upon 248 nm photolysis of an
c
1
5
18
18 12 18
ICN/HCNO/ N O/buffer mixture:
[ O C O] ) 1.00.

Kinetics of the CN + HCNO Reaction
J. Phys. Chem. A, Vol. 110, No. 44, 2006 12189
1
8
12
3
18 12 18
O2/SF6 mixture while only 1.3 × 10 molec/cm of O C O
of the data sets of Figure 1. A simple test for this effect is to
measure the CN decay twice in rapid succession on the same
gas fill. If the CN reaction with stable products was a significant
problem, we would expect the second measurement to yield a
different value for the pseudo-first-order CN decay rate. In fact,
we observed no change in the CN decay rate from two
successive measurements on the same gas fill.
In addition to the product channels listed, one can write
numerous other possibilities, for example involving formation
of CNO rather than NCO, formation of HNC rather than HCN,
etc. We cannot completely rule out CNO formation, except to
note that it likely would be formed with enough internal energy
to isomerize to the more stable NCO species. No evidence for
HNC formation was observed in FTIR spectra taken. One can
also write channels such as CN + HCNO f CN + HNCO,
which are exothermic because HNCO is the more stable isomer.
Although we did not investigate this possibility in detail, we
note that if such a channel were important, we would not observe
the rapid CN disappearance apparent in our data.
1
8
as background was detected upon photolysis of HCNO/ O2/
SF6 mixture. Although we do not attempt to obtain quantitative
branching ratio information from these yields, our observation
of significant amounts of 18O C O is certainly consistent with
12 18
HCCN formation followed by reaction 9.
Two other possible product channels of the title reaction
deserve mention. If channel (1g) was significant, we would
expect to observe more N2O yield than CO yield with 248 nm
photolysis of ICN/HCNO/NO/buffer gas, because the secondary
reaction of NH with NO will produce additional N2O.
NH + NO f N O + H
(12a)
(12b)
2
f OH + N2
N2O is a major product of reaction 12, with æ12a ) 0.77.23
Conversely, if channel 1h were significant, we would expect to
observe more CO than N2O upon photolysis of ICN/HCNO/
2
4
NO/buffer gas, because of the following reaction sequence:
No ab initio studies of the potential energy surface of this
reaction have been reported. We can therefore only speculate
regarding details of the reaction mechanism. Our observation
of channel 1c as the only major product channel suggests a rather
simple mechanism in which CN attack at the carbon atom of
HCNO forms an HC(CN)NO complex, which then directly
dissociates by C-N bond fission to form HCCN + NO. A
complete potential surface would show other structures and
probably numerous possible rearrangements; however, our
experimental results suggest that other pathways are higher in
energy and are not significant.
CCO + NO f NCO + CO
(13)
(3a)
(3b)
NCO + NO f N O + CO
2
f CO + N2
2
In fact, we only measure approximately equal yields of N2O
and CO products upon 248 nm photolysis of ICN/HCNO/NO/
buffer gas. This is consistent with our assumption that N2O and
CO originate from reaction 3.
As a result of the above discussion, we conclude that 1c is
the major product channel of the title reaction.
5
. Conclusion
4
. Discussion
The kinetics and product of the CN + HCNO reaction were
studied using laser-induced fluorescence and IR diode laser
Our results represent the first study of the title reaction. This
absorption spectroscopy. The reaction is very fast, with k1 )
is a very fast reaction with slight temperature dependence. An
extrapolation of our measurement to the temperature range T
-
10
3
-1 -1
(
1.04 ( 0.10) × 10
cm molec
s
at 298 K, and has a
slight, negative temperature dependence. The major product
)
1200-1500 K relevant in NO-reburning suggests a rate
channel is NO + HCCN.
constant approximately half of our 298 K value, although clearly
such an extrapolation is not warranted by the narrow temperature
range of our data.
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.
Several experimental artifacts can cause systematic errors in
a pseudo-first-order kinetics experiment. The first, decomposi-
tion of HCNO sample during the experiment, was minimized
by completing a single CN LIF decay measurement in about 5
min (typically, this allows 4 min for filling the cell and allowing
the reagents to mix and 1 min for the LIF data collection). In
this amount of time, less than 10% decomposition occurs. A
related issue is the possible reaction of CN radicals with
decomposition products, photoproducts, or reaction products
from the title and/or secondary reactions. By using an estimated
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