Kinetics of the NCN radical

Article abstract of DOI:10.1021/jp0208844

The kinetics of the NCN + NO reaction has been studied using laser-induced fluorescence spectroscopy over the temperature range 298-573 K to measure total rate constants and time-resolved infrared diode laser spectroscopy as a probe for possible products. The reaction has a rate constant of k(NCN + NO) = (2.88 ± 0.2) × 10^-13 cm³ molecule^-1 s^-1 at 298 K and low (～3 Torr) total pressure. The rate constant displays both a temperature as well as a pressure dependence, indicating that adduct formation is a major product channel. Only very small amounts of other products were detected. Upper limits for the low-pressure rate constants of NCN + O₂, C₂H₄, and NO₂ were also obtained.

Full text of DOI:10.1021/jp0208844

J. Phys. Chem. A 2002, 106, 11093-11097
11093
Kinetics of the NCN Radical
Randall E. Baren and John F. Hershberger*
Department of Chemistry, North Dakota State UniVersity, Fargo, North Dakota 58105
ReceiVed: April 3, 2002; In Final Form: August 12, 2002
The kinetics of the NCN + NO reaction has been studied using laser-induced fluorescence spectroscopy over
the temperature range 298-573 K to measure total rate constants and time-resolved infrared diode laser
spectroscopy as a probe for possible products. The reaction has a rate constant of k(NCN + NO) ) (2.88 (
-
13
3
-1 -1
0
.2) × 10 cm molecule
s
at 298 K and low (∼3 Torr) total pressure. The rate constant displays both
a temperature as well as a pressure dependence, indicating that adduct formation is a major product channel.
Only very small amounts of other products were detected. Upper limits for the low-pressure rate constants of
2 2 4 2
NCN + O , C H , and NO were also obtained.
Introduction
laser photolysis of diazomethane in the presence of cyanogens
C2N2).
(
Nitrogen oxides are of great interest because of their toxic
effects as atmospheric pollutants generated from the combustion
of fossil fuels. The mechanisms and rate parameters for reactions
involving nitrogen compounds have been extensively investi-
CH N + C N + hν (193 nm) f
2
2
2
2
NCN + other products (3)
1
gated in order to understand combustion generated air pollution.
This method has been used previously for the production of
NCN in early spectroscopic studies.
14,24
One of several routes for the production of NO is the “prompt
Laser-induced fluores-
2
3
3
-
NO,” or Fenimore mechanism. This process has long been
cence near 329 nm via the well characterized A Πu r X Σg
1
7,25,26
1
,3-10
transition
is used to measure the total rate constant of the
thought to principally involve the spin-forbidden reaction
NCN + NO reaction as a function of temperature and pressure.
This reaction has several possible product channels:
2
4
CH( Π) + N f HCN + N( S)
(1)
2
0
r
NCN + NO f N O + CN
∆H ) -25.8 kJ/mol
2
followed by oxidation of HCN and N. Recently, the work by
(4a)
1
1,12
Lin et al.
channel:
has suggested an alternative spin-allowed product
0
r
f N + NCO
∆H ) -411.6 kJ/mol
2
(4b)
2
3
-
0
r
CH( Π) + N f H + NCN( Σ )
(2)
f N + CNO
∆H ) -110.9 kJ/mol
2
g
2
(4c)
0
r
The chemistry of the cyanonitrene (NCN) diradical is
therefore of interest in the modeling of hydrocarbon flames.
This species has been the subject of several spectroscopic
studies. In early work, Jennings and Linnett observed an
emission at 329 nm upon the introduction of hydrocarbons and
f CO + N3
∆H ) -239.3 kJ/mol
(
4d)
0
r
f CNN + NO
f NCNNO
∆H ) +126.4 kJ/mol
(4e)
(4f)
13
14
nitrogen into a flame. Herzberg and Travis monitored the
UV absorption spectrum of NCN upon flash photolysis of
diazomethane. Similar work by Kroto15 and Jacox used
cyanogen azide as a photolytic precursor. Smith et al. proposed
NCN as an intermediate in the combustion of nitramine
propellant molecules.17 Ultraviolet emission studies have also
suggested that NCN is present, and is a source of CN radicals,
in the comet Brorosen-Metcalf.18 Additionally, NCN has been
suggested as a critical growth species in the synthesis of carbon
Thermochemical information was taken from standard JANAF
16
27
0
tables, except for NCO, where a value of ∆Hf ) 131.38 kJ/
28
0
mol was used, CNO, where a value of ∆Hf ) 432.19 kJ/
29,30
mol was used,
NCNNO adduct, for which thermochemical
information is unknown, and for both NCN and CNN, for which
Hf ) 452.71 and 579.06 kJ/mol were used, respectively.
0
31
∆
Experimental Section
1
9
nitride (C3N4). Surprisingly, there have been no reported
measurements for the gas phase kinetics of reactions of NCN,
although several early reports have appeared in organic chem-
istry literature on insertion reactions of nitrenes with hydro-
carbons.2
The experimental apparatus using an LIF technique for
kinetics measurements has been described previously. Only a
brief description will be given here. The photolysis laser light
was provided by an excimer laser (Lambda Physik COMPEX
00) operating at 193 or 248 nm and a repetition rate of 4 Hz.
32
0-23
2
This work reports the kinetic study of the reaction of NCN
with NO. The NCN radical is produced by the 193 nm excimer
Typical photolysis pulse energies were 2-6 mJ. The 329.008
nm probe light (∼0.5 mJ/pulse) was produced by frequency
1
0.1021/jp0208844 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/23/2002

11094 J. Phys. Chem. A, Vol. 106, No. 46, 2002
Baren and Hershberger
doubling 658.016 nm light from a dye laser (Continuum ND-
6000) pumped by the second harmonic of a Nd:YAG laser
(Continuum Surelite ΙΙ-10). The probe laser was operated at a
repetition rate of 8 Hz. The photolysis and probe beams were
made collinear using a dichroic mirror and copropagated down
a Pyrex reaction cell. Fluorescence was detected 90° from the
laser beams using an R508 photomultiplier tube. A 320 nm long-
pass filter was used to eliminate the detection of photolysis light.
Spectroscopic studies have indicated that most of the emission
occurs at the 329 nm excitation wavelength, making it difficult
to filter out scattered probe light.17 Fortunately, the LIF emission
is sufficiently strong that scattered probe light does not severely
affect the signals. The unamplified PMT signal was recorded
by a boxcar integrator (Stanford Instruments model 250) with
a 10-60 ns delay and a 400 ns gate width then averaged on a
personal computer. A digital delay generator (Stanford DG535)
was used to vary the delay between excimer and dye laser pulses
in order to produce an NCN concentration vs time profile. The
static cell was resistively heated to collect data at elevated
temperatures up to 573 K. Gases introduced into the reaction
cell were allowed to mix for 10 min to ensure complete mixing
and thermal equilibration.
Figure 1. NCN laser-induced fluorescence signals as a function of
time. Reaction conditions: PCH2N2 ) 0.05 Torr, PC2N2) 0.2 Torr. 193
nm photolysis.
and PHe ) 0-600 Torr (NCN + NO). Pressure dependent
studies, for the reaction of NCN + NO, used He as an inert gas
to collect kinetic data from 50 to 600 Torr total pressure. LIF
experiments were conducted under pseudofirst-order conditions
with [NO], [C2H4], [NO2] or [O2] . [NCN]. Typical conditions
The infrared absorption apparatus for product molecule
detection of N2O and CO2 has been described in previous
publications.33-35
193 nm photolysis light was made collinear
with probe light from a tunable lead-salt diode laser (Laser
Photonics) and copropagated down a 1.46 m absorption cell.
After removal of the UV light, the infrared beam passed through
a 1/4 m monochromator and was focused onto a 1 mm InSb
detector (Cincinnati Electronics, ∼1 µs response time). Transient
infrared absorption signals were recorded on a digital oscil-
loscope (LeCroy 9310A) and transferred to a computer for
analysis. N2O and CO2 product yields were probed by using
the following absorption lines:
for the infrared experiments were PCH N ) 0.10 Torr, PC N )
2
2
2 2
0
.20 Torr, PSF ) 1.0 Torr, and PNO ) 0-1.0 Torr.
6
Results
Previous spectroscopic studies have identified several bands
3
3
-
17
in the LIF spectrum of the A Πu r X Σg transition of NCN.
We choose the (0-0) band at 329.008 nm for NCN detection.
The following control experiments were performed to ensure
that the fluorescence signals were indeed from NCN alone: The
193 nm photolysis of diazomethane alone produced very little
LIF signal. This is consistent with previous reports that this
precursor alone produces only small amounts of NCN.14
Photolysis of C2N2 alone produced no signal, indicating the CN
radicals are not detected at the probe wavelength used. Upon
CO ((00°0),J)12) f CO ((00°1),J)13)
2
2
R(12) line at 2358.728 cm-1
N O ((00°0),J)21) f N O ((00°1) J)20)
2
2
P(21) line at 2204.715 cm-
1
1
93 nm photolysis of CH2N2/C2N2 mixtures, large LIF signals
with ∼200 ns lifetimes were observed at 329.008 nm. When
the dye laser was detuned off of this spectral line, no LIF signal
was observed.
NCN + NO Reaction. Figure 1 shows typical boxcar
integrated signal amplitudes as a function of pump-probe delay
time between excimer and dye laser pulses. The NCN signals
displayed rise times on the order of ∼15 µs followed by slower
decay times of ∼200 µs. The rise is attributed to formation of
NCN through photolysis and subsequent reaction of CH2N2/
C2N2 mixtures. It is possible that NCN is formed vibrationally
The HITRAN molecular infrared database36 was used to locate
and identify the spectral absorption lines of N2O and CO2. When
detecting CO2 as a product, the infrared laser beam path was
purged with N2 to remove atmospheric CO2.
SF6 (Matheson) was purified by repeated freeze-pump-thaw
cycles at 77 K. NO (Matheson) was purified by vacuum
distillation at 77 and 163 K. NO2 (Matheson) was purified by
vacuum distillation at 77 and 220 K. O2 (Matheson) was used
without further purification. C2H4 was purified by several
freeze-pump-thaw cycles at 77 K. Cyanogen (C2N2) was
synthesized by the reaction of copper sulfate with aqueous
sodium cyanide37 and purified by freeze-pump-thaw cycles
at 77 K as well as vacuum distillation at 223 K to remove CO2.
Diazomethane (CH2N2) was synthesized by the reaction of
MNNG (1-methyl-3-nitro-1-nitrosoguanidine, C2H5N5O3) with
aqueous sodium hydroxide38 and purified by vacuum distillation
at 223 K to remove air and CO2. Traces of CO2 were removed
from SF6, C2N2 and CH2N2 by the use of an ascarite trap.
excited. SF6 is known to be an efficient relaxer of vibrational
39,40
excitation of several triatomic molecules.
The rise and decay
rates of the LIF signals were not affected by the addition of
SF6 buffer gas; however, the signal magnitudes were noticeably
smaller, presumably due to fluorescence quenching effects. The
unchanged temporal response suggests that NCN is either
produced vibrationally cold or vibrational relaxation occurs
within ∼15 µs even in the absence of buffer gas. Most of the
LIF experiments reported here were performed without SF6
buffer gas.
Typical reaction conditions for the LIF experiments were
PCH N ) 0.05-0.10 Torr, PC N ) 0.20 Torr, PNO ) 0-6.0
When nitric oxide was added to the reaction mixture, an
increase in the NCN decay rate was observed. In the presence
of added reactant, pseudo-first-order conditions apply and a
2
2
2 2
Torr (NCN + NO), PC H 0-0.60 Torr (NCN + C2H4), PNO )
2
4
2
0
-2.0 Torr (NCN + NO2), PO ) 0-5.0 Torr (NCN + O2),
2

Kinetics of the NCN Radical
J. Phys. Chem. A, Vol. 106, No. 46, 2002 11095
Figure 4. Arrhenius plot of the NCN + NO reaction at pressures of
Figure 2. Pseudofirst-order decay rates of NCN LIF signals as a
∼2 Torr (b), 300 Torr (]), and 600 Torr (0) total pressure. CH
2 2
N
function of NO pressure. CH
those in Figure 1.
N
2 2
and C
2
N
2
pressures are the same as
2 2
and C N pressures are the same as in Figures 1 and 2. PHe ) 0, 300,
and 600 Torr, respectively.
shown in Figure 2). Figure 3 shows a plot of k′ vs NO pressure
when 100 Torr of He was included in the reaction mixture. This
plot shows a much greater slope than in Figure 2, indicating a
pressure dependence in k4. Furthermore, the curvature seen in
Figure 2 is not apparent in Figure 3, because the total pressure
under the conditions of Figure 3 is very nearly constant, varying
only from 200.05 to 200.85 Torr.
Kinetic experiments for the reaction of NCN + NO were
conducted over the temperature range from 298 to 573 K. Figure
4
illustrates an Arrhenius plot for the measured rate constant,
k4, over the temperature range from 298 to 573 K with error
bars representing one standard deviation. Experiments were also
performed to examine the dependence of this reaction on total
pressure. Figure 5 shows the NCN + NO rate constant measured
at 298, 373, 473 and 573 K as a function of added He. The rate
3
constants total pressures of ∼3, 300, and 600 Torr (in cm
-
1
-1
molecule s , 1 σ error bars), respectively, are represented
Figure 3. Pseudofirst-order decay rates of NCN LIF signals as a
function of NO pressure, in the presence of added He buffer gas.
by
Reaction conditions: PCH N ) 0.05 Torr, PC2N ) 0.2 Torr, PNO
)
2
2
2
k4 ) (1.706 ( 0.39) × 10-15e[(1545.2(85.4)/T]
(∼3 Torr)
(300 Torr)
(600 Torr)
0
.0-0.8 Torr, PHe ) 100.0 Torr, T ) 298 K.
k4 ) (8.605(5.33) × 10 15e[(1983.2(284.5)/T]
-
standard kinetic analysis results in
(-k't)
k4 ) (8.663(4.56) × 10 15e[(1272.6(212.1)/T]
-
[
NCN] ) [NCN]0e
k′ ) k [NO] + kD
The values of k4 at 3 Torr were obtained by estimating the slope
of the tangent of the curves in Figure 2 at PNO ) 3.0 Torr.
When 100-600 Torr of helium was included in the reagent
mixture, no curvature was observed in the plots of pseudofirst-
order rate constant vs [NO], because under these conditions the
total pressure was essentially constant. Least squares best fit
lines of the k′ vs [NO] data were therefore used to determine
k4.
4
where k′ is the observed pseudofirst-order rate constant, k4 is
the desired bimolecular rate constant for the NCN + NO
reaction, and kD represents the decay rate in the absence of added
nitric oxide. Contributions to kD include self-reaction, reaction
with one or more precursor molecules, and diffusion of detected
species out of the probed volume. Figure 2 shows the pseudo-
first-order decays, k′, as a function of NO pressure at several
different temperatures. In the absence of added NO, very small
values of k′ were observed, indicating that the concentration of
transient species was quite low, and that radical-radical
chemistry other than the title reaction did not significantly affect
the results. The slope of these plots represents the desired
bimolecular rate constant k4. As is apparent in Figure 2, the
slope is not constant, indicating significant deviation from
second order kinetics, especially in the 298 K data. This is
attributed to a dependence of the bimolecular rate constant on
total pressure (which is not held constant in the experiments
Product detection in reaction 4 was measured at 298 K using
infrared absorption spectroscopy. Very small amounts of N2O
and CO2 molecules were detected, as shown in the transient
absorption signals of Figure 6. The CO2 signal is noisy, partly
because of the presence of a small amount of static CO2 impurity
in the CH2N2 samples (frequency jitter in the infrared laser
causes larger amplitude noise in the signal when a significant
static absorption line is present). The rise times observed are
consistent with production of products in excited vibrational
states, followed by relaxation into the ground vibrational state
4
0,41
in the presence of SF6 buffer gas.
Absorption signals were

11096 J. Phys. Chem. A, Vol. 106, No. 46, 2002
Baren and Hershberger
channel 4a would not quickly react with NO at low pressures).
Unfortunately, other complications make the reliability of this
1
approach questionable. For example, CH2 formed in the CH2N2
photolysis could react with NO, possibly forming NCO as well.
1
The products of CH2 + NO are not well characterized.
Furthermore, product channels such as 4c, N2 + CNO, or 4d,
CO + N3, could also result in secondary chemistry. N3 + NO
presumably produces N2 + N2O, and the products of the CNO
+
NO reaction are not quantitatively known. In general, our
results suggest that both channels 4a and 4b are active to some
minor extent, but a quantitative determination of the branching
ratio will require further experiments, probably involving
different precursors for NCN formation. The observation of
significant pressure dependence in the total rate constants
suggests that adduct formation, channel 4f, is a major and
possibly dominant product channel. No attempts were made to
detect NCNNO, for which no spectroscopic information is
known.
Figure 5. Pressure dependence of the NCN + NO bimolecular rate
constant at temperatures of 298 K (+), 373 K (]), 473 K (0), and 573
K (4).
NCN + C2H4, O2, NO2 Reactions. Reactions of NCN with
C2H4, O2, and NO2 were also investigated. There was no
significant increase in NCN pseudofirst-order decay rates upon
addition of several Torr of these molecules. This indicates that
these reactions are extremely slow, at least at low total pressures.
Estimated upper limits for the rate constants of reaction of NCN
at 298 K in the 1-3 Torr pressure range are: k(NCN + C2H4)
-
15
3
-1 -1
<
8.5 × 10
cm molecule s , k(NCN + O2) < 1.0 ×
-
14
3
-1 -1 -14
1
0
cm molecule s , and k(NCN + NO2) < 1.5 × 10
3
-1 -1
cm molecule s .
Discussion
This study represents the first report of direct measurements
of the kinetics of the NCN radical. We are aware of only one
previous report of any kinetic information on this species. In a
recent modeling study of NO-reburning processes, several NCN
reactions were included with estimated rate parameters, includ-
-
11
3
-1 -1
ing the estimate of k ) 1.6 × 10
cm molecule
s
for
4
4
the NCN + O2 reaction. This value is several orders of
magnitude greater than our estimate of an upper limit for the
rate constant. Although it is possible that this reaction is faster
at higher pressures and/or temperatures, it is still likely that this
rate constant was overestimated in the modeling study. Further
work on the temperature and pressure dependence of the NCN
Figure 6. Transient infrared absorption signals for N
2 2
O and CO
product molecules. Lines probed: N O (00°1) r (00°0) P(21) at
2
-
1
-1
2
2
204.715 cm and CO (00°1) r (00°0) R(12) at 2358.728 cm .
Each transient was obtained from a single photolysis laser shot. Reaction
conditions: PCH N ) 0.10 Torr, PC2N2 ) 0.20 Torr, PSF6 ) 1.0 Torr,
2
2
and PNO ) 0.20 Torr.
+
O2 reaction is necessary.
The mechanism responsible for the formation of NCN is not
converted to number densities using tabulated line strengths and
33
equations described previously. Typical number densities of
well understood at this time. At least two routes are possible:
1
1
11
3
[
N2O] ≈ 3 × 10 and [CO2] ≈ 1.6 × 10 molecule cm were
obtained for the signals in Figure 6. These concentrations are
1
CH N + hν (193 nm) f CH + N
(6a)
(6b)
2
2
2
2
1
-2 orders of magnitude smaller than typical values obtained
42,43
in similar experiments,
suggesting that the pathways leading
1
CH + C N f NCN + C H
2
2
2
2
2
to these products are of only minor importance, although our
lack of information regarding initial NCN concentrations
prevents a quantitative comparison.
or
This system is complicated by several possible secondary
reactions. If NCO is produced in channel 4b, both N2O and
CO2 would be formed by the reaction
C N + hν (193 nm) f 2CN
(7a)
2
2
CN + CH N f NCN + CH N (7b)
2
2
2
NCO + NO f N O + CO
(5a)
(5b)
2
If reaction 7 is a major route for NCN formation, then in
principle CN radical precursors other than C2N2 should also
work. For example, ICN photolysis at 248 nm produces CN
radicals in high yields. We attempted to detect NCN produced
from 248-nm photolysis of ICN/CH2N2 mixtures. No LIF signals
were detected, suggesting that reaction 7b does not occur, and
that reactions 6a and 6b (or some other, unknown, mechanism)
are the dominant routes to NCN formation.
2
f CO + N2
Since the branching ratios of reaction 5 are known (φ5a ) 0.44
and φ5b ) 0.56 at 298 K),33 it is possible in principle to estimate
the amount of N2O produced in channel 5a if it is assumed that
channel 5b is the only source of CO2. Any excess N2O would
then be attributed to channel 4a (note that CN produced in

Kinetics of the NCN Radical
J. Phys. Chem. A, Vol. 106, No. 46, 2002 11097
No high level ab initio calculations on the title reaction have
been reported. Wang et al., however, reported an experimental
and BAC-MP4 computational study of the reverse reaction
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(
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5
3
31, 269.
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CN + N O f NCNNO f NCN + NO (8)
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2
(
(
(
15) Kroto, H. W. Can. J. Phys. 1967, 45, 1439.
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no direct comparison can be made, but their prediction of a
fairly deep NCNNO well is qualitatively consistent with our
observations.
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The NCN + NO reaction proceeds primarily through adduct
formation as evidenced by significant pressure dependence of
the rate constant. At 298 K, this reaction has a rate constant of
99.
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(
(
(
27) Chase, M. S. J. J. Phys. Chem. Ref. Data, 1985, 14, 1.
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-13
3
-1 -1
(
2.88 ( 0.2) × 10 cm molecule
of ∼3 Torr, but the rate constant increased with pressure,
3
s
at a low total pressure
Chem. 1997, 101, 134.
-
12
reaching a high-pressure limit of (5.0 ( 0.5) × 10
cm
(30) Su, H.; Kong, F. J. Chem. Phys. 2000, 113, 1885.
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-
1
-1
molecule s . The reaction displays a negative temperature
dependence typical of radical-radical reactions. Only very small
amounts of N2O products were detected. These results suggest
that NCNNO adduct formation is the dominant product channel.
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(
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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.
Smith, M. A. H.; Benner, D. C.; Devi, V. M.; Flaud, J. M.; Camy-Peyret,
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