Product branching ratios of the NH2(X2B1) + NO2 reaction

Article abstract of DOI:10.1021/jp970994o

The reaction of NH₂(X²B₁) with NO₂ was studied at 298 K using time-resolved infrared diode laser spectroscopy to detect N₂O and NO products. The N₂O + H₂O channel was confirmed to be a rather minor contribution to the overall reaction, with a branching ratio of 0.24 ± 0.04. The branching ratio of the NO + H₂NO channel was measured to be 0.76 ± 0.1.

Full text of DOI:10.1021/jp970994o

J. Phys. Chem. A 1997, 101, 4991-4995
4991
Product Branching Ratios of the NH2(X2B1) + NO2 Reaction
Ned Lindholm and John F. Hershberger*
Center for Main Group Chemistry, Department of Chemistry, North Dakota State UniVersity,
Fargo, North Dakota 58105
ReceiVed: March 19, 1997; In Final Form: April 30, 1997X
The reaction of NH2(X2B1) with NO2 was studied at 298 K using time-resolved infrared diode laser spectroscopy
to detect N2O and NO products. The N2O + H2O channel was confirmed to be a rather minor contribution
to the overall reaction, with a branching ratio of 0.24 ( 0.04. The branching ratio of the NO + H2NO
channel was measured to be 0.76 ( 0.1.
Introduction
studies have indicated that channel 1a is only a minor contributor
to the total reaction rate. A recent study in the author’s
The NH2 radical is an important intermediate in a variety of
combustion environments including the thermal de-NOx pro-
cess1,2 and the chemistry of nitramine propellants.3 Reactions
of NH2 with NOx species are therefore of great interest. The
NH2 + NO reaction has been extensively studied4-14 and is
generally accepted to produce primarily H2O + N2 with a small
contribution by the OH + HN2 product channel. The product
channels of the NH2(X2B1) + NO2 reaction have until recently
received much less study, however. Several possible product
channels are thermodynamically accessible:15-16
laboratory used 193 nm photolysis of NH3 to produce NH2
radicals followed by infrared laser detection of N2O.22
A
branching ratio of 0.14 ( 0.02 into channel 1a at 298 K was
determined. Although ammonia is a highly efficient NH2 radical
source, secondary sources of NO in that study (primarily due
to reaction of photolytically produced hydrogen atoms with NO2)
prevented a quantitative determination of the yield of channel
1b. Park and Lin used mass spectrometry detection following
NH3 photolysis to obtain a branching ratio of φ1a ) 0.19 (
0.02, which was found to be independent of temperature over
the range 300-900 K.23,24 Electron impact cracking of NO2
produces a large amount of NO in their experiment, again
preventing determination of φ1b. Meunier et al., however, used
a pulse radiolysis source and infrared detection, estimating φ1a
) 0.59 ( 0.03 and φ1b )0.40 ( 0.05 at 298 K.25 Clearly,
further work is needed.
NH2(X2B1) + NO2 f N2O + H2O ∆H ) -385 kJ/mol
(1a)
f H2NO + NO ∆H ) -57.7 kJ/mol
(1b)
In this work, we use an alternative method of generating NH2.
Instead of photolyzing NH3, we first generate the CN radical
by the photolysis of cyanogen iodide at a wavelength where
NH3 has negligible absorption:
f N2 + H2O2 ∆H ) -364 kJ/mol
(1c)
f N2 + 2OH ∆H ) -146 kJ/mol
(1d)
ICN + hν (248 nm) f I + CN
NH2 radicals are formed by the reaction of CN with ammonia:
CN + NH3 f HCN + NH2 (3)
(2)
f 2HNO ∆H ) -25.1 kJ/mol
(1e)
The total rate constant of reaction 1 has been measured by
several groups.15,17-20 Most measurements at 298 K have been
This reaction is quite fast, with recent measurements indicating
in the range k1) (2.1-2.3) × 10-11 cm3 molecule-1 s-1
,
k ) (2.5-2.9) × 10-11 cm3 molecule-1 s-1 26-28
.
By use of a
although one study20 reported a substantially lower value of k1
) 9.3 × 10-12 cm3 molecule-1 s-1. Measurements at elevated
temperatures15,17,20 have all shown that this reaction has a
negative temperature dependence, although there is substantial
disagreement on the rate constants at high temperatures. Bulatov
et al. observed no dependence of the total rate constant on
pressure over the range 10-650 Torr.15
bimolecular reaction to form NH2 rather than direct photolysis,
no hydrogen atoms are produced, and therefore, one can obtain
a reliable yield of NO from channel 1b. This approach is similar
to that of Meunier et al. in which F atoms created by pulse
radiolysis react with NH3 to form HF and NH2.25 The primary
difference between the present study and that of ref 25 is that
our study uses a much lower radical density.
Literature data on active product channels of this reaction
are more contradictory, however. Ab initio calculations have
suggested that channels 1a and 1b are the most likely.16 Hack
et al. used mass spectrometry to detect N2O and H2O products.20
Although no quantitative branching ratio was quoted, they
suggested that channel 1a dominates the reaction. A recent flow
reactor and kinetic modeling study of the NH3/NO2 system
suggested that channel 1a is dominant at low temperatures but
that 1b becomes important at high temperatures (∼800-1300
K).21 In contrast to those results, two out of three recent direct
Experimental Section
The experimental apparatus is similar to that used in previous
product branching studies.22,29-32 The photolysis source was
an excimer laser (Lambda Physik COMPex 200) operating at
248 nm. The infrared probe beam was a lead salt diode laser
(Laser Photonics) operating in the 80-110 K temperature range.
Photolysis and probe beams were made collinear using a
dichroic mirror and transmitted through a 146 cm single-pass
absorption cell with CaF2 windows. Iris diaphragms (6 mm
diameter) were placed at each end of the reaction cell in order
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
S1089-5639(97)00994-8 CCC: $14.00 © 1997 American Chemical Society

4992 J. Phys. Chem. A, Vol. 101, No. 27, 1997
Lindholm and Hershberger
to ensure reproducible beam overlap. After the cell, the UV
light was removed with a second dichroic mirror, and the IR
probe beam was focused through a 0.25 m grating monochro-
mator and onto a 1 mm InSb detector (∼1 µs rise time).
Transient absorption signals were averaged on a LeCroy 9310A
digital oscilloscope and stored on a personal computer for further
analysis.
Typical experimental conditions were 0.03-0.05 Torr ICN,
0-2 Torr NH3, 0.05 Torr NO2, and 1 Torr SF6 buffer gas. SF6
buffer gas was chosen because it is efficient at relaxing
vibrational excitation of the N2O and NO products. It is also
expected to rapidly relax any internal excitation of the nascent
CN and NH2 radicals. The reaction mixture was allowed to
stand for ∼5 min in the cell in order to ensure complete mixing.
Only 4-8 shots per transient signal were obtained in order to
prevent depletion of reactants or buildup of products. All
experiments were performed at room temperature (298 ( 3 K).
In some of the experiments it was necessary to estimate the
initial radical concentration. This requires knowledge of the
incident photolysis laser pulse energy as well as the absorption
coefficient of ICN at 248 nm. A joule meter (Molectron) was
therefore used to measure the photolysis energy immediately
after the second iris. These measurements were taken under
the following conditions: with the evacuated reaction cell in
place, with the evacuated reaction cell removed from the optical
path, with the second iris opening set at 6 mm, and with the
second iris set fully open at 25 mm. Owing to window losses
with the cell in place, and the slight beam divergence of the
photolysis laser, there was about a 30% spread in the energy
measurements. Therefore, the average of the four measurements
was taken as the best estimate. Typical values for photolysis
laser energy were ∼10-20 mJ. An absorption coefficient of
0.009 cm-1 Torr-1 for ICN at 248 nm29 was used to calculate
[CN]0.
Figure 1. Transient infrared absorption signals for N2O and NO
product molecules. Probed lines are NO V ) 0 R(4.5) and N2O (0000)
P(23). Reaction conditions are the following: PICN ) 0.05, PNO
0.05, PNH ) 1.0, and PSF ) 1.0 Torr.
)
2
3
6
ground vibrational state that is probed in these experiments. This
phenomenon has been routinely observed in our previous
product yield measurements.22,29-32
A possible secondary source of NO products in addition to
reaction 1b is direct photodissociation of the NO2 reagent.
When the ICN precursor was omitted from the reaction mixture,
transient signals for NO were found to be negligible at the low
NO2 pressures used, indicating that NO2 photodissociation is
not a significant factor in these experiments.
Transient absorption signals were converted into absolute
number densities using equations described in previous
publications.22,29-32 The only change is that the infrared
absorption coefficients of the NO lines probed were found, by
direct measurement of fractional absorption of standard NO
samples, to be ∼30% smaller than those tabulated in the
HITRAN database. This discrepancy is probably at least partly
due to instrumental broadening of the particular laser diode used
for NO detection. Since the same diode was used in both the
absorption coefficient and product yield experiments, it was
deemed appropriate to use our absorption coefficients in the
number density calculations.
NH3 and SF6 (Matheson) were purified by repeated freeze-
pump-thaw cycles at 77 K. NO2 (Matheson) was purified at
223 K to remove NO and N2O impurities. ICN (Aldrich) was
purified by vacuum sublimation to remove dissolved air.
Several transitions were used to probe reaction products:
NO (V ) 0 f V ) 1) R(4.5) line at 1893.863 cm-1
R(6.5) line at 1900.071 cm-1
The most important secondary reactions that must be con-
sidered involve reaction of photolytically produced CN radicals
with NO2 rather than the desired reaction with NH3:
N2O (0000 f 0001) P(19) line at 2206.659 cm-1
CN + NO2 f NCO + NO
f N2O + CO
(4a)
(4b)
(4c)
P(23) line at 2202.744 cm-1
R(16) line at 2236.943 cm-1
f CO2 + N2
CO2 (0000 f 0001) P(4) line at 2345.985 cm-1
Any NCO produced in reaction 4a would then quickly react
with NO2:
The experimental results were independent of which spectral
line was used for a given molecule. The HITRAN database
was used as an aid for the assignment of spectral lines.33
NCO + NO2 f N2O + CO2
f 2NO + CO
(5a)
(5b)
Results
These reactions are fast, with k4 ) (7.4-8.1) × 10-11 and k5 )
Typical transient signals for N2O and NO product molecules
are shown in Figure 1. Off-resonant signals obtained with the
infrared laser detuned ∼0.02 cm-1 off the probed absorption
lines were found to be negligible. The rise at early times is
due to formation of the product molecules. The rise time is
slower than would be predicted by the rate constant of the title
reaction because under our conditions product molecules formed
in vibrationally excited states take ∼100 µs to relax to the
(1.8-2.7) × 10-11 cm3 molecule-1 s-1 30-31,34-35
.
Channel 4a
is the major channel in reaction 4,30 while channel 5a dominates
reaction 5.31 Clearly, both the detected products are produced
in the above sequence, and the complexity of the system
combined with uncertainties in the data for reactions 4 and 5
imply that it would be difficult to obtain a reliable branching
ratio for reaction 1 if these secondary reactions are occurring
to a large extent. We therefore suppress reaction 4 by

NH2(X2B1) + NO2
J. Phys. Chem. A, Vol. 101, No. 27, 1997 4993
TABLE 1: Branching Ratios of the NH2 + NO2 Reaction
φ1a
φ1b
(N2O + H2O) (NO + H2NO)
ref
this work
Park and Lin
Quandt and Hershberger
Meunier, Pagsberg, and
Sillesen
0.24 ( 0.04
0.19 ( 0.02
0.14 ( 0.02
0.59 ( 0.03
0.76 ( 0.1
23, 24
22
25
0.40 ( 0.05
formation of NH2 radicals from the reaction
OH + NH3 f H2O + NH2
(6)
Although this reaction is rather slow, with a recommended rate
constant of k ) (1.5-1.6) × 10-13 cm3 molecule-1 s-1 at 298
K,36,37 it could represent a significant OH removal mechanism
at the rather high NH3 pressures used in our experiment. Since
NH2 is formed, this reaction, when combined with the title
reaction, represents a possible chain mechanism. We cannot
detect OH directly, but we can efficiently detect carbon dioxide
formed by the reaction
Figure 2. N2O and NO product yields as a function of NH3 pressure.
Reaction conditions are the following: PICN ) 0.05, PNO ) 0.05, PNH
2
3
) variable, and PSF ) 1.0 Torr.
6
OH + CO f CO2 + H
(7)
Experiments were therefore conducted with the addition of 1.0
Torr of CO to the standard reaction mixture. Under these
conditions, reaction 7 should occur on a roughly ∼100 µs time
scale, although some OH would be removed by reaction 6 as
well. No increase in CO2 yield was observed upon addition of
CO, indicating that the OH concentration in this system is
negligible. We estimate an upper limit of φ1d < 0.03.
There are two approaches for the determination of branching
ratios from our product yield data. The first method is to simply
assume that all CN radicals are converted to NH2 by reaction 3
and that all NH2 radicals subsequently react with NO2 and
therefore report φ1a as the ratio of N2O formed to initial [NH2]0,
and similarly for φ1b. Alternatively, one may assume that only
Figure 3. CO2 product yields as a function of NH3 pressure. Reaction
conditions are the following: PICN ) 0.03, PNO ) 0.05, PNH3 ) variable,
2
and PSF ) 1.0 Torr.
6
the detected channels 1a and 1b are active and quote φ1a
)
[N2O]/([N2O] + [NO]) and φ1b ) [NO]/([N2O] + [NO]). The
first method has the advantage of determining whether channels
1c-1e contribute but is somewhat less precise than the second
method because of the uncertainties in the determination of
[NH2]0. Using the first method, we obtain φ1a ) 0.287 and φ1b
) 0.92. These numbers add up to somewhat greater than 1,
indicating that our calculated [NH2]0 is underestimated by about
∼20%. These data are consistent, however, with the assumption
that channels 1c-1e do not contribute significantly. The second
method yields φ1a ) 0.24 ( 0.04 and φ1b ) 0.76 ( 0.1 (average
of six experimental runs at 1.0 Torr NH3 pressure), where the
uncertainties represent two standard deviations. We believe the
second method provides the more reliable determination in this
instance because all active product channels are probed.
performing experiments with a large excess of NH3 over NO2.
Figure 2 shows the effect of varying the initial [NH3] on the
observed yields of N2O and NO products, while Figure 3 shows
the effect on the yield of CO2 products. Note that reaction 5a
is the only plausible source of CO2 in this system. The decay
in the CO2 yield with increasing [NH3] indicates that if the
[NH3]/[NO2] ratio is at least ∼20, reactions 4 and 5 are
effectively suppressed and nearly all the CN radicals are
removed by reaction 3. Additional evidence for this hypothesis
is apparent in the fact that the N2O and NO yield curves (Figure
2) have leveled off at these conditions. The lower N2O and
especially NO yields at low [NH3] were somewhat unexpected,
since reactions 4 and 5 should produce both NO and N2O in
high yield. Under these conditions, however, additional CN
loss mechanisms (possibly CN + ICN or radical-radical
reactions) may be present. Some evidence for this may be found
in Figure 2 of our previous study of reaction 4,30 which shows
that CN produced in the photolysis of an ICN/buffer gas mixture
decays faster than expected by diffusional processes alone. Most
of the product yield studies in the present work were performed
using 0.05 Torr NO2 and 1.0 Torr NH3. Under these conditions,
secondary chemistry is largely suppressed.
Discussion
Table 1 shows a comparison of our results with other recent
work. The branching ratio for channel 1a obtained in the present
work is in reasonable, although not in outstanding agreement
with the study of Park and Lin.23,24 The value reported here is
significantly higher than in our previous study using 193 nm
NH3 photolysis,22 although the qualitative conclusion that
channel 1a is a minor contribution at room temperature remains
unchanged. Our results, however, differ dramatically from those
obtained in the pulse radiolysis study by Meunier et al.,25 who
used the F + NH3 reaction to form NH2 radicals. They report
φ1a ) 0.59 ( 0.03 and φ1b ) 0.40 ( 0.05. This discrepancy is
especially surprising in view of the similarity of the experimental
Of the potential minor channels 1c-1e, channel 1d appears
the most likely, although ab initio calculations have indicated
that the potential energy barrier to form these products is
somewhat higher than for channels 1a and 1b.16 If significant
amounts of OH radicals are produced in this reaction, the data
analysis would be considerably complicated by the possible re-

4994 J. Phys. Chem. A, Vol. 101, No. 27, 1997
Lindholm and Hershberger
approaches in ref 25 and this work. There are two possible
explanations for this discrepancy. One is that the experiments
of ref 25 were performed at a total pressure of ∼30 Torr, while
our data are at ∼2 Torr pressure. It is possible that the
branching ratio depends on pressure, although this would be
quite surprising, since the total rate constant displays no pressure
dependence.15,24 Another difference between their study and
ours lies in the radical densities employed. Meunier et al. quote
[F]0 ) 5.7 × 1014 molecules/cm3, in contrast to our typical [CN]0
of ∼(2-4) × 1013. At such high densities, we believe that the
reaction
experiments. Obviously, additional experiments are necessary
to resolve this issue.
Conclusions
The branching ratios of the NH2 + NO2 reaction were
measured by infrared detection of products. The most important
result of this study is that we believe it represents the first
reliable estimate of the contribution of the major NO formation
channel. This study also confirms earlier reports that N2O +
H2O formation represents only a minor channel, although the
measured value of the contribution of this channel is somewhat
greater than that obtained in our previous determination.
F + NH2 f HF + NH
(8)
Acknowledgment. The authors thank J. Park and M. C. Lin
of Emory University for valuable discussions and for making a
manuscript available prior to publication. This work was
supported by the Division of Chemical Sciences, Office of Basic
Energy Sciences of the Department of Energy, Grants DE-FG06-
93ER14390 and DE-FG03-96ER14645). Acknowledgment is
also made to the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for partial
support of this work.
is significant in their experiments and cannot be ignored. No
direct measurements of the rate constant for this reaction have
been reported, although Fagerstroem et al. inferred k8 ) 1.16
× 10-10 cm3 molecule-1 s-1 by fitting a kinetic model to
experimental data38 and Donaldson et al. performed energy
disposal experiments on the HF products of this reaction.39 It
seems quite likely, in any case, that this reaction proceeds much
faster than F + NH3 (although we note that there is significant
disagreement in the literature regarding the F + NH3 rate
constant).40-42 Since the [NO2]/[F]0 ratio in the experiments
of ref 25 was only about 10, reaction 8 may be able to compete
with the title reaction for NH2 radicals. The NH + NO2 reaction
has been previously estimated in our laboratory to produce N2O
+ OH in 41% yield.32 The reaction F + NH f N + HF may
further complicate this system. It is therefore quite possible
that much of the N2O observed in the experiments of ref 25
was due to the NH + NO2 and possibly N + NO2 reactions.
Simple kinetic modeling calculations on their system suggest
that N2O formation from these secondary reactions can be
significant, although the detailed comparisons depend on the
values of the rate constants used as well as initial reagent
concentrations.
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NH2(X2B1) + NO2
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