Quantification of the 248 nm photolysis products of HCNO (fulminic Acid)

Article abstract of DOI:10.1021/jp411209n

IR diode laser spectroscopy was used to detect the products of HCNO (fulminic acid) photolysis at 248 nm. Five product channels are energetically possible at this photolysis wavelength: O + HCN, H + (NCO), CN + OH, CO + NH, and HNCO. In some experiments, isotopically labeled ¹⁸O₂, ¹⁵N¹⁸O and C₂D₆ reagents were included into the photolysis mixture in order to suppress and/or isotopically label possible secondary reactions. HCN, OC¹⁸O, C¹⁸O, NCO, DCN, and NH molecules were detected upon laser photolysis of HCNO/reagents/buffer gas mixtures. Analysis of the yields of product molecules leads to the following photolysis quantum yields: _1a (O + HCN) = 0.39 ± 0.07, _1b (H + (NCO)) = 0.21 ± 0.04, _1c (CN + OH) = 0.16 ± 0.04, _1d (CN + NH(a¹Δ)) = 0.19 0.03, and _1e (HNCO) = 0.05 ± 0.02, respectively. The uncertainties include both random errors (1σ) and consideration of major sources of systematic error. In conjunction with the photolysis experiment, the H + HCNO reaction was investigated. Experimental data demonstrate that this reaction is very slow and does not contribute significantly to the secondary chemistry.

Full text of DOI:10.1021/jp411209n

Article
pubs.acs.org/JPCA
Quantification of the 248 nm Photolysis Products of HCNO (Fulminic
Acid)
Wenhui Feng and John F. Hershberger*
Department of Chemistry and Biochemistry, North Dakota State University, Dept. 2735, P.O. Box 6050, Fargo, North Dakota
58108-6050, United States
ABSTRACT: IR diode laser spectroscopy was used to detect the products of HCNO
(
fulminic acid) photolysis at 248 nm. Five product channels are energetically possible at
this photolysis wavelength: O + HCN, H + (NCO), CN + OH, CO + NH, and HNCO.
In some experiments, isotopically labeled O , N O and C D reagents were included
1
8
15 18
2
2
6
into the photolysis mixture in order to suppress and/or isotopically label possible
1
8
18
secondary reactions. HCN, OC O, C O, NCO, DCN, and NH molecules were detected
upon laser photolysis of HCNO/reagents/buffer gas mixtures. Analysis of the yields of
product molecules leads to the following photolysis quantum yields: ϕ (O + HCN) =
1a
0
.39 ± 0.07, ϕ (H + (NCO)) = 0.21 ± 0.04, ϕ (CN + OH) = 0.16 ± 0.04, ϕ (CN +
1b 1c 1d
1
NH(a Δ)) = 0.19 ± 0.03, and ϕ (HNCO) = 0.05 ± 0.02, respectively. The uncertainties
1
e
include both random errors (1σ) and consideration of major sources of systematic error.
In conjunction with the photolysis experiment, the H + HCNO reaction was investigated.
Experimental data demonstrate that this reaction is very slow and does not contribute
significantly to the secondary chemistry.
1
. INTRODUCTION
HCNO + hν(248 nm) → O + HCN
(1a)
Fulminic acid, HCNO, is an important intermediate in NO-
reburning processes for reduction of NO pollutants from
fossil-fuel combustion emission. HCNO is formed in
combustion primarily by the CH + NO and HCCO +
→
→
→
H + (NCO)
OH + CN
(1b)
(1c)
x
1
2
−5
2
6
−10
1
NO
reactions. The chemistry of HCNO is therefore of
CO + NH(a Δ)
(1d)
(1e)
great interest in the overall NO-reburning mechanism. In our
laboratory, we have previously studied the kinetics of the OH +
HCNO, CN + HCNO, NCO + HCNO and O + HCNO
→ HNCO
NCO is written as (NCO) to indicate that we cannot
distinguish between NCO and CNO isomers in this study.
Figure 1 shows the energetics of these species. The
thermochemical information has been obtained from standard
1
1−15
reactions,
molecules at 248 nm or 266 nm to form the radical species
OH, CN, NCO, or O). In the course of these experiments, we
using UV laser photolysis of precursor
(
observed significant yields of several product molecules in the
absence of the precursor molecule, which was attributed to
direct photolysis of HCNO. In those experiments, HCNO
photolysis yields were treated as background signals that were
generally subtracted from the product yields obtained in the
presence of the radical precursor. In this study, we
quantitatively examine the products produced by HCNO
photolysis.
18
tables as well as other literature for the heats of formation of
19
1
20
HCNO NCO and NH (a Δ). Formation of ground state
3
−
NH (X Σ ) is also possible, but is less likely as a spin-
forbidden process.
2
. EXPERIMENTAL SECTION
The photolysis laser was an excimer laser (Coherent, Compex-
pro) operating at 248 nm. Reaction products were detected by
infrared diode laser absorption spectroscopy. Several lead salt
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 light by means of a dichroic mirror, and both 0.6-
cm diameter beams were copropagated though a 1.43 m
absorption cell. After the UV light was removed by a second
The ultraviolet absorption spectrum of HCNO was
16
previously studied. The observed spectrum was a single
progression of vibronic bands over the range 244−285 nm. In
addition, the UV photolysis of HCNO has previously been
17
studied by Bondybey et al. They used FTIR spectroscopy to
observe HNCO as the primary final product. This molecule was
assumed to be a result of secondary chemistry subsequent to
the photolysis event. In the present study, time-resolved
infrared laser spectroscopy is employed to detect the transient
photolysis product signals to obtain a direct measurement of
the major photolysis product channels. There are five
energetically accessible channels at 248 nm:
Received: November 14, 2013
Revised: January 13, 2014
Published: January 24, 2014
©
2014 American Chemical Society
829
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The Journal of Physical Chemistry A
Article
II column to remove traces of CO . 15N O (Isotec) was
18
2
purified by repeated freeze−pump−thaw cycles at 153 K. The
following molecules were probed using infrared diode laser
absorption spectroscopy:
−
HCN (v = 1 ← v = 0)
DCN (v = 1 ← v = 0)
P(17) at 3258.441 cm 1
R(7) at 2648.9 cm 1
P(15) at 2320.46 cm
−
1
8
−1
OC O (ν1, v = 1 ← v = 0)
CO (v = 1 ← v = 0)
−
1
R(13) at 2193.359 cm
1
8
−1
C O (v = 1 ← v = 0)
NCO (v = 1 ← v = 0)
NH (v = 1 ← v = 0)
HNCO (v = 1 ← v = 0)
R(15) at 2146.198 cm
at 1904.14 cm 1
−
Figure 1. Relative Energy levels of possible photolysis products of
R(3) 3242.89 cm 1
−
HCNO photolysis. Thresholds (kJ/mol) compared to HCNO: −291.2
HNCO), 95.1 (CO+NH (X 3Σ )), 174.5 (H+NCO), 213.3 (O
−
(
+
+
1
−1
HCN), 261.9 (CO+ NH (a Δ)), 303.1 (OH+CN), 436.6 (H
CNO). 248 nm photon provides 482.7 kJ/mol.
(R10) 2276.71 cm
The HITRAN molecular database was used to locate and
1
8
18
identify the spectral lines of CO, C O, OC O, and HCN
dichroic mirror, the infrared beam was then passed into a 1/4
m monochromator and focused onto a 1 mm InSb detector
27
product molecules. Other published spectral data were used
28
29
30
25
to locate and identify DCN, NCO, NH and HNCO
lines.
(
Cincinnati Electronics, ∼1 μs response time). Transient
infrared absorption signals were recorded on a digital
oscilloscope (Lecroy, Wavesurfer 422) and transferred to a
computer for analysis. All experiments were performed at 298
K.
Typical experimental conditions were P(HCNO) = 0.1 Torr,
15
18
18
P( N O) = 1.0 Torr, P( O ) = 1.0−4.0 Torr, P(SF or CF )
2
6
4
=
2
1.50 Torr, P(C D ) = 0.5−7.0 Torr, P(Xe) = 2.0 Torr, and
2
6
2
48 nm laser pulse energies of 25 mJ (fluence of ∼88 mJ/cm ).
HCNO samples were synthesized as previously de-
2
0−23
scribed
by flash vacuum pyrolysis of 3-phenyl-4-oximino-
3
. RESULTS
isoxazol-5(4H)-one. The precursor (2.0 g) was sublimed from a
5
0 mL bulb in a 90 °C oil bath and passed through a horizontal
Infrared diode laser absorption was used to detect product
molecules (or products of secondary chemistry) from channels
1a−1e upon 248 nm laser photolysis of HCNO/buffer gas. In
order to modify and/or suppress potential secondary chemistry,
additional reagents were sometimes included in the reaction
mixture, as described below. Figure 2 shows some typical
signals for detection of HCN, CO, and CO2 molecules.
Typically, transient signals show a fast (∼10−100 μs) rise,
followed by a slow (∼1 ms) decay. The rise is attributed to
formation of the detected product by either direct photolysis
and/or subsequent secondary chemistry, as described below.
The decay is attributed to diffusion of molecules out of the
probed volume. The rate of the rise is primarily determined by
the rate of collisional relaxation of a nascent vibrational
distribution to a Boltzmann distribution. The choice of buffer
gas (SF for detection of most molecules, but CF for detection
quartz tube heated to 450 °C in a tube furnace. The products,
which include HCNO, HNCO, CO , H O, and phenyl cyanide,
were collected over a 48 h period at 77 K. H O and phenyl
cyanide were removed by twice passing the products through a
2
2
2
240 K trap. CO and HCNO were removed by vacuum
2
distillation at 192 K. The purity of the HCNO samples was
characterized by FT-IR spectroscopy via a strong absorption
band at 2195 cm . The sample purity was estimated at 95%
or better, with only small CO and HNCO impurities. The
HNCO impurity was monitored by FT-IR at a strong band at
−1
24
2
−1
25
2
168 cm . 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 at below ∼0.5 Torr pressure in our
Pyrex absorption cell with minimal decomposition.
6
4
3
1,32
HNCO was synthesized by the reaction of NaOCN (Aldrich
of CO molecules)
was motivated by the desire to maximize
26
9
6%) with hydrogen chloride gas. NaOCN was contained in a
this relaxation rate, while keeping the total pressure sufficiently
low to minimize pressure broadening of spectral lines.
In order to estimate the yields, the slow decay portion of
each transient signal was fit to a single exponential decay
(diffusion out of a cylindrical volume is not strictly exponential,
but this is sufficient for our purpose). Transient signal
amplitudes were obtained by extrapolation to t = 0 (this
extrapolated amplitude was typically a small (∼5−15%)
increase compared to the peak−peak amplitudes). The
resulting amplitudes were converted into absolute concen-
trations using HITRAN line strengths for all of the detected
molecules except for NH radicals and DCN molecules, which
were calibrated as described below. Table 1 shows the resulting
yields of the detected molecules.
flask and evaluated. A measured pressure of HCl was filled into
the NaOCN-containing flask and then frozen onto the NaOCN
solid by immersing the flask into a liquid nitrogen bath. After
several cycles of warming and freezing of the sample, the
reaction products were purified by distillation at −193 K to
remove trace HCl and CO .
2
Hydrazoic acid (HN ) was synthesized by melting stearic
3
acid (Aldrich) over sodium azide (Aldrich) at 343 K and
purified by freeze−pump−thaw cycles at 193 K to remove
N O. Purity was checked by measurement of the UV
absorption coefficient and comparison with literature values.
SF and CF (Matheson) was purified by repeated freeze−
pump−thaw cycles at 77 K and by passing through an Ascarite
2
6
4
8
30
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Article
Table 1. Yields of Products of HCNO Photolysis and
a
Secondary Reactions
product
HCN
yield (1012 molecules cm−3
)
20.0 ± 1
6.0 ± 0.3
15 ± 2
8.3 ± 0.9
19.5 ± 2
9.7 ± 1
18
OC O
18
C O
DCN
CO
NH
HNCO
2.5 ± 0.2
a
15 18
Experimental conditions: P(HCNO) = 0.1 Torr, P( N O) = 1.5
18
Torr, P( O ) = 1.0−4.0 Torr, P(CF ) = 1.5 Torr (CO detection
only), P(SF ) = 1.5 Torr (HCN, NCO, DCN and CO detection), 248
2
4
6
nm photolysis laser pulse energy ∼25 mJ. Uncertainties represent one
standard deviation.
OH + HCNO → CO + H NO
(5a)
(5b)
2
→
→
HCO + HNO
NO + H CO
(5c)
(6)
2
NH + HCNO → products
H + HCNO → products
(7)
1
1−15
Reactions 2−6 have been studied previously in our lab,
resulting in the following rate constants: k = (5.32 ± 0.4) ×
2
−
12
3
−1 −1
−10
3
1
0
cm molecule s , k = (1.04 ± 0.1) × 10
cm
3
−1
−1
−11
3
−1 −1
molecule s , k = (1.58 ± 0.2) × 10 cm molecule s , k
4
5
(3.4 ± 0.3) × 10 cm molecule s−1 and k ≤ 2.1 × 10
−
11
3
−1
−13
=
6
3
−1 −1
cm molecule
s
at 298 K. As can be seen, reactions 2−5 are
fast and produce some of the same molecules as direct
photolysis, complicating the interpretation of product yields.
The approach used here is to include additional reagents in the
reaction mixture in order to suppress or redirect these
secondary reactions, as described in detail below.
3
.1. Product Channel 1a, O + HCN. In order to quantify
the yield of channel 1a, (O + HCN), we detected the HCN
product. If photolysis channel 1b is significant, however,
additional HCN may be produced by reaction 4. Isotopically
labeled 15N O reagent was therefore included in the reaction
mixture in order to suppress reaction 4:
18
1
8
1
5
15 18
NCO + N O → N N O + CO
(8a)
(8b)
1
5
18
→
N N + OC O
(
Product branching ratios at 296 K: φ = 0.44 ± 0.07; φ
=
8a
8b
32
0
.56 ± 0.07). By comparing the rate constants k = (1.58 ±
4
11 3
−
−1 −1
−11
Figure 2. Transient signals of HCN (top panel) and 16OC O (middle
18
0.2) × 10 cm molecule
s
and k = (4.3 ± 0.4) x10
8
15 18
15
18
3
−1 −1 33
panel) detected upon laser photolysis of HCNO(0.1 Torr)/ N O (1
cm molecule s , we conclude that a 10:1 ratio of N O
to HCNO pressure is sufficient to ensure that nearly all NCO
radicals react by reaction 8 rather than 4, and therefore do not
result in HCN production.
Torr)/SF (1.5 Torr) mixture, and CO (bottom panel) detected upon
6
18
photolysis of HCNO(0.1 Torr)/ O (1.0 Torr) /CF (1.5 Torr)
2
4
mixture. 248 nm photolysis laser pulse energy = 25 mJ. Green curve in
top panel shows a typical single exponential fit to the decay portion of
the signal.
We note that if channel 1c is significant, the resulting CN
radicals will still react primarily with HCNO (reaction 3), since
the CN + NO reaction is slow at the low pressures used here.
1
2,15
Reaction 3 produces HCCN radicals,
which further react
Our previous studies show that HCNO is a highly reactive
molecule. Many secondary reactions following the photolysis
must therefore be considered:
with 15N O to produce isotopically labeled HCN:
18
CN + HCNO → NO + HCCN
(3)
(9)
O + HCNO → H + CO + NO
CN + HCNO → NO + HCCN
NCO + HCNO → HCN + CO + NO
(2)
(3)
(4)
18
1
5
15
18
HCCN + N O → HC N + NC O
1
4
Our experiment, however, detects the nonlabeled HC N
isotope, and is therefore not affected by this secondary
8
31
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The Journal of Physical Chemistry A
Article
chemistry. The nitrogen-15 labeling is therefore crucial for this
experiment (the oxygen-18 labeling is not).
Here, the notation 8b* is used to identify an isotopic variant of
reaction 8b. As can be seen, the carbon dioxide molecules in
18
18
Other potential secondary reactions include O + HCNO,
OH + HCNO, NH + NO, NH + HCNO, etc. To the best of
our knowledge, however, these reactions do not produce HCN.
Therefore, the HCN signal obtained upon photolysis of a
reaction 8b* are double labeled ( OC O) and do not
18
contribute to the yield of the singly labeled OC O, and
therefore do not interfere with this measurement.
We should note here that we have no method to detect
CNO, a high energy isomer of NCO. It is likely that any CNO
formed quickly isomerizes to NCO. Our experiment is unable
to distinguish whether this would happen during the photolysis
process or subsequent to fragmentation. In any case, our
measurement of the yield of channel 1b should be considered
the sum of H + CNO and H + NCO photolysis events.
3.3. Product Channel 1c, CN + OH. In order to detect the
presence of photolysis channel 1c, CN + OH, we attempted to
directly detect CN radicals. This was not successful, possibly
because CN is very rapidly depleted by the extremely fast
reaction 3. In principle, one could attempt to quantify this
channel by detection of NO molecules produced in reaction 3,
but this approach is complicated by other secondary chemistry
such as O + HCNO, reaction 2, which also produces NO. We
therefore used two alternative approaches.
1
5
18
HCNO(0.1 Torr)/ N O (1 Torr) /SF (1.5 Torr) mixture, as
6
shown in Figure 2, can be attributed to photolysis channel 1a.
3.2. Product Channel 1b, H + (NCO). In order to quantify
the yield of channel 1b, H + (NCO), several approaches are
possible. One is direct detection of NCO molecules upon
photolysis of a HCNO(0.1 Torr)/SF (1.5 Torr) mixture, as
6
shown in Figure 3. The NCO transient signal shows a ∼ 50 μs
18
The first approach was to add O reagent to the photolysis
2
mixture in order to initiate the following reaction sequence:
CN + O → NC O + 18O
1
8
18
(10a)
(10b)
(4*)
2
1
8
18
→
N O + C O
1
8
18
NC O + HCNO → HCN + NO + C O
1
8
18
O + HCNO → H + NO + C O
(2*)
Figure 3. Transient signal of NCO detected upon photolysis of
As described above, the asterisk notations in the reaction
numbering denote an isotopic variant. If channel 1c is
HCNO(0.2Torr)/SF (1.5 Torr) mixture and 248 nm photolysis; laser
6
18
pulse energy ∼25 mJ.
significant, we expect C O in reactions 10, 4* and 2* to be
1
8
detected upon photolysis of an HCNO(0.1 Torr)/ O (4.0
2
Torr)/CF (1.5 Torr) mixture. Figure 4 shows the resulting
4
fast decay lifetime due to its reaction with HCNO. Although
this provides evidence for the existence of channel 1b, we have
not measured the infrared absorption coefficients of NCO
needed to quantify yields. Instead, we added isotopically labeled
−11
3
signal. Reaction 10 has a rate constant of k = 2.3 × 10 cm
1
0
molecule−
1
s
−1
at 298 K and a branching ratio of φ10a = 0.8
and φ10b = 0.2. As other secondary reactions, for instance (H,
O , OH + HCNO, etc., do not
2
produce C O, we can estimate that each CN molecule
34
35
18
1
5
18
O, NCO, NH or OH) +
N O reagent in the photolysis mixture as described above in
1
8
18
order to initiate reaction 8. Under these conditions, OC O is
produced in reaction 8b, as shown in Figure 2. Unlike NCO, we
produces 0.8 NCO molecules and 0.8 O molecules by reaction
18
1
0a; the former produces 0.8 C O by reaction 4*, and the
1
8
do have absorption coefficients for OC O from the HITRAN
database, allowing quantification (although the HITRAN line
strengths must be divided by the O-18 natural isotopic
abundance to account for the fact that HITRAN assumes
natural abundance, unlike our enriched sample). Therefore,
18
latter produces another 0.8 C O by reaction 2*. Adding the 0.2
C O from reaction 10b, we estimate a total of 1.8 C O
molecules should be produced from one CN molecules.
18
18
Therefore, the yield of photolysis channel 1c is [CN] =
18
[
C O]/1.8. In order for this to work, an overwhelming amount
1
5
18
assuming we have an excess of N O which results in all NCO
radicals reacting via eq 8, and knowing the branching ratio into
18
of O must be used in order to suppress the competing
2
18
reaction CN + HCNO. At [ O ] = 4.0 Torr, about 90% of CN
2
8
b, we quantify the yield of NCO molecules originally
+
HCNO reaction will be suppressed. Table 1 shows the
produced in photolysis channel 1b:
18
13
measured concentration of C O is (1.5 ± 0.2) × 10
1
8
−3
[
NCO]1b = [OC O]/0.56
molecules cm , from which we obtain [CN] = (8.3 ± 1.0)
−3
×
10 12 molecules cm . There is one disadvantage of this
We assume here that the isotopic substitution does not affect
the branching ratio. Additional secondary chemistry can
method. Among reactions 2−5, the O + HCNO reaction is
relatively slow, and it is not clear that all of the O atoms react
with HCNO under our conditions.
possibly produce CO , as follows:
2
CN + HCNO → NO + HCCN
(3)
(9)
The second approach to determining the yield of photolysis
channel 1c is to add deuterated ethane, C D , resulting in the
secondary reaction to produce DCN:
2
6
1
8
1
5
15
18
HCCN + N O → HC N + NC O
1
8
1
8
15
15
18
18
(8b*)
CN + C D → C D + DCN
2
6
2 5
(11)
NC O + N O → N N + OC O
8
32
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The Journal of Physical Chemistry A
Article
Figure 5. The yields of DCN as the function C D pressures. DCN
2
6
was detected upon photolysis of HCNO(0.1Torr)/C D (variable: 0−
2
6
7
.0 Torr)/SF (1.5 Torr) mixture; 248 nm photolysis laser pulse
6
energy ∼25 mJ. The experimental data was simulated by the function:
y = −2.40 exp(−x/6.44) −2.40 exp(−x/6.45) −6.82 exp(−x/0.39) +
8
.7.
combination of gases, so this effect was estimated by measuring
the absorption at line center of static DCN samples as a
function of added C D , as shown in Figure 6. As can be seen,
2
6
18
Figure 4. Transient signal of C O and DCN detected upon photolysis
18
of HCNO(0.1 Torr)/ O (4.0 Torr)/CF (1.5 Torr) mixture and
2
4
HCNO(0.1Torr)/ C D6 (5.0 Torr)/SF6 (1.5 Torr) mixture,
2
respectively. 248 nm photolysis laser pulse energy ∼25 mJ.
Figure 4 shows the resulting signals. We measured the DCN
signal as a function of C D pressure upon photolysis of
2
6
HCNO(0.1Torr)/C D (variable: 0.5−7.0 Torr)/SF6 (1.5
2
6
Torr) mixtures. These data, after calibration, are shown in
Figure 5.
Literature line strengths for DCN are not available, so we
calibrated the DCN signals as follows: several DCN transient
signals were detected upon 248 nm laser photolysis of an ICN
(
0.05−0.15Torr)/C D (1.0 Torr)/SF (1.5 Torr) mixture.
2
6
6
Under these conditions, CN is produced by photolysis of ICN,
and most of the CN molecules react to form DCN, so the
concentration of DCN is expected to be equal to that of CN.
The latter was calculated using the 248 nm absorption
Figure 6. The relative IR absorption intensities at the center of the
DCN R(7) line as the function of the pressure of C D .
2
6
pressure broadening results in a ∼25−30% decrease in
absorption at the higher pressures (7−10 Torr) used. This is
somewhat larger than typical pressure broadening affects. The
data in Figure 5 was therefore corrected for pressure
broadening, a modest but significant (∼25%) correction.
As shown in Figure 5, the DCN yield has not completely
peaked even at the highest C D pressures used. This is
−1
−1
coefficient of ICN (α = 0.009 Torr cm ) and a quantum
yield of unity and measurement of the photolysis laser energy.
From the plot of DCN IR absorptions as the functions of DCN
concentrations, we can obtain the infrared absorption
coefficient of the probed DCN line under low pressure (<2
Torr) conditions. One additional correction must be
mentioned, however. Because Figure 5 extends to substantially
greater pressures than the 1−2 Torr values typical in our
experiments, pressure broadening effects may substantially
reduce the DCN absorption coefficients below those
determined at low pressures. Literature values of pressure
broadening coefficients are not available for this specific
2
6
consistent with the kinetics involved: the rate constant of
−
11
reaction 11 was measured in our lab to be k ∼ 1.2 × 10
11
3
−1
−1
cm molecule cm . (This is about four times slower than
that of the CN + C H reaction, a reasonable kinetic isotope
2
6
effect). Using our value of k and k , we estimate that about 10
11
3
Torr C D (versus 0.1 Torr HCNO) is needed to suppress
2
6
8
33
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The Journal of Physical Chemistry A
Article
most of the competing CN + HCNO reaction. In order to
estimate the DCN yield expected at 20 Torr C D , where
likely that not all of the O atoms formed in 1a react with
HCNO, as k is quite small, and diffusional loss likely competes
2
6
2
reaction 3 should be completely suppressed, the data was
simulated by using a three parameter exponential equation, as
shown in Figure 5. Using this simulation equation, we
for O atoms. If one assumes that only half of the O atoms react
via reaction2, one obtains [CO]1d ∼ 0. Based on the large
uncertainties in the secondary chemistry, we conclude that
detection of CO is not a very good method to quantify the yield
of channel 1d.
1
2
−3
estimated the [DCN] = 8.26 × 10
molecules cm at
[
[
C D ] = 20 Torr, which, by comparison, is very close to the
2
6
18
CN] value obtained by the first approach (involving O ,
Another possible approach to quantifying channel 1d is direct
detection of the NH radical. We have successfully detected the
2
described above). We therefore estimate the yield of channel 1c
3
−
1
2
−3
NH(X Σ ) radical upon photolysis of HCNO(0.1Torr)/Xe
to be [CN] = (8.3 ± 0.9) × 10 molecules cm . This result is
(
2.0 Torr)/SF (2.0 Torr) mixture, as shown in Figure 7.
shown in Table 2.
6
a
Table 2. Yields of Products of HCNO Photolysis
product
yield (1012 molecules cm−3
)
HCN
NCO
CN
20.0 ± 1
10.7 ± 0.5
8.3 ± 0.9
9.7 ± 1
NH
HNCO
Total
2.5 ± 0.5
51.2
a
Experimental conditions: P(HCNO) = 0.1 Torr, P(CF or SF ) = 1.5
4
6
Torr, and 248 nm photolysis laser pulse energy ∼25 mJ. Uncertainties
represent one standard deviation.
3.4. Product Channel 1d, CO + NH. We have also
attempted to quantify the yield of channel 1d, by detecting CO
product molecules. Unfortunately, multiple secondary reactions
can contribute to the CO yield, including O + HCNO, NCO +
HCNO, and OH + HCNO reactions. Reaction 5, OH +
HCNO reaction has product branching ratios of φ = 0.61, φ
5b
Figure 7. Transient signal of NH radical detected upon photolysis of
5
a
HCNO(0.1Torr)/Xe(2.0 Torr)/SF (2.0 Torr) mixture and 248 nm
11
6
=
0.35 and φ5c = 0.04. The HCO product in 5b could
photolysis, laser pulse energy ∼25 mJ.
possibly dissociate to H + CO to add more CO molecules. In
1
8
order to simplify the analysis, we included isotope labeled O
2
1
1
6
Xenon gas was used to relax electronic excited state NH(a Δ)
in the photolysis mixture, but still detected the unlabeled C O
product, obtaining the signal shown in Figure 2 (bottom
panel). This removes some but not all of the secondary
3
−
36,37
3 −
to ground state NH(X Σ ).
No NH(X Σ ) signal was
detected in the absence of xenon, suggesting that direct
photolysis of HCNO produces NH exclusively in the excited
(a Δ) state. There is no IR absorption line strength of NH was
reported, so we measured it by the following experiment. NH
1
8
chemistry: any CN produced in 1c will react with O to form
2
1
1
8
18
O and/or NC O. These species, when reacted with HCNO,
1
8
16
will probably make C O rather than the detected C O.
Nevertheless, substantial secondary chemistry remains: any
HCO produced in 5b reacts as follows:
transient signal was detected upon photolysis of HN (variable:
3
0
.1−0.4 Torr)/Xe (2.0 Torr)/SF6 (2.0 Torr) mixtures.
1
Hydrazoic acid (HN ) photolysis produces NH(a Δ):
3
1
8
18
HCO + O → CO + H O
(12)
2
2
1
HN + hν(248nm) → NH(a Δ) + N
(13a)
3
2
Therefore one OH molecule is estimated to form 0.96 CO
→
H + N3
(13b)
38
molecules (the sum of φ and φ ). In addition, any O atoms
5
a
5b
formed in 1a or NCO radicals formed in 1b will react,
producing additional CO. We can very roughly estimate
(
The photolysis quantum yields are φ13a = 0.76; φ13b = 0.24).
The NH concentration in our experiment was estimated from
the 248 nm absorption coefficient of HN (α = 0.0022
3
[
CO]1d = [CO]total − [CO]O+HCNO − [CO]NCO+HCNO
−1
−1
Torr cm ) and a quantum yield of 0.76 and measurement of
the photolysis laser energy. From the slope of a plot of ln(I/I0)
as a function of NH concentration, we obtain the NH
−
[CO]
OH+HCNO
[CO]
[CO]
[CO]
= [O] = [HCN]
−18
2
−1
O+HCNO
1a
1a
absorption cross section of 6.57 × 10 cm molecule for the
R(10) line, and use it to convert the absorption amplitude of
the detected NH signal to number density to be (9.7 ± 1) ×
= [NCO]1b
NCO+HCNO
12
3
1
0
cm .
= [OH] = [CN] × 0.96
OH+HCNO
1c
1c
Other possible approaches to quantifying channel 1d,
The [CO]total denotes the total CO concentration detected
involving N O detection from the NH + NO reaction were
considered, but were rejected because they also had large
amounts of competing secondary chemistry.
2
1
8
upon laser photolysis of HCNO(0.1 Torr)/ O (1.0 Torr)/
2
CF (1.5 Torr) mixture, as shown in Table 1. Unfortunately, this
4
calculation results in a negative value for [CO] , indicating that
some of our assumptions are unfounded. For example, it is
3.5. Product Channel 1e, HNCO. The HNCO photolysis
product was detected upon laser photolysis of an HCNO(0.1
1
d
8
34
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The Journal of Physical Chemistry A
Article
Torr)/SF (1.0 Torr) mixture using an HNCO R(10)
out a small contribution. Therefore, we included the reagents
NO, NO , and O in the reaction mixture in order to quench
these radicals:
6
−1
absorption line at 2276.71 cm . The IR absorption cross
2
2
−17
2
section at line center was measured to be 4.75 × 10 cm
molecule− by measurement of the absorption of static samples
of HNCO. One difficulty is that our HCNO samples have a
small level of HNCO impurity, typically a few percent. This
amount is sufficient to substantially interfere with detection of
HNCO transient signals. Therefore, for the HNCO detection
experiments, additional purification of the HCNO sample was
conducted to reduce the HNCO impurity to ∼0.2%; this
resulted in about ∼3% absorption of the infrared light at the
HNCO R(10) line. Although still not ideal, slow dark reactions
that convert HCNO to HNCO prevented better purification.
Figure 8a shows the HNCO transient signal obtained from
1
CN + O → NCO + O
(10)
(8)
2
NCO + NO → products
O + NO → NO + O2
(14)
2
As shown in Figure 8b, a smaller but still significant HNCO
transient absorption signal was detected upon photolysis of a
HCNO(0.1 Torr)/SF (1.0 Torr)/O (4.0 Torr)/NO(1.0
6
2
Torr)/NO (1.0 Torr) mixture. The slower decay in Figure
2
(
8b) compared to Figure (8a) is due to the greater total
the photolysis of HCNO(0.1 Torr)/SF (1.0 Torr). Before
6
pressure, resulting in slower diffusional loss. Unfortunately, at
least one of these reagents, probably NO , apparently reacts
2
slowly with HCNO, resulting in an increase in the static
HNCO impurity by a factor of ∼2. We therefore believe that
this signal is less reliable, and have chosen to use the signal
from Figure 8a to quantify channel 1e. Although this results in
substantial uncertainty, we do conclude that there is a small but
nonzero yield of HNCO produced through the photo-
isomerization channel 1e.
3
.6. Summary of Quantum Yields. Table 2 shows the
resulting yields of the five HCNO photolysis channels after the
above analysis to account for secondary chemistry. The sum of
1
3
the channels gives a total product yield of (5.12 ± 0.39) × 10
−3
molecules cm . Assuming no other product channels, we
calculate the product branching ratios to be (39.1 ± 2)%, (20.9
±
1)%, (16.2 ± 2)%, (18.9 ± 2)%, and (4.8 ± 1)% for channels
1
a, 1b, 1c, 1d, and 1e, respectively, where the uncertainties are
random errors (one standard deviation).
Next we converted the product yields into photolysis
quantum yields. We are not aware of any literature information
on UV absorption cross sections. We therefore measured the
UV absorption coefficient of HCNO at 248 nm by measuring
the transmitted excimer laser energy as a function of HCNO
pressure, and making a standard Beer−Lambert plot. We obtain
= (4.93 ± 0.50) × 10− Torr cm , equivalent to a cross
3
−1
−1
α
248
−
19 2
−1
section of σ248 = (1.52 ± 0.15) × 10
Although not used in the present study, we also measured
cm molecule .
absorption coefficients at other common photolysis wave-
−
2
−1
−1
−19
lengths: α1 = 1.75× 10 Torr cm (or σ = 5.39 × 10
93
193
2
−1
−3
−1
−1
cm molecule ), and α = 4.59× 10 Torr cm (or σ266
=
266
19
2
−1
1
.42 × 10− cm molecule ). Using the absorption coefficient
of HCNO at 248 nm and the photolysis laser energy (25 mJ/
pulse) we estimated the number density of absorbed photons
upon photolysis of 0.1 Torr HCNO at 248 nm to be (5.14 ±
1
3
−3
0
.40) × 10 cm . Comparing with the product yield of
channels 1a−1e, we can obtain photolysis quantum yields of
ϕ = 0.39 ± 0.07, ϕ = 0.21 ± 0.04, ϕ = 0.16 ± 0.04, ϕ
Figure 8. The HNCO transient IR absorption signal detected upon
=
1d
photolysis of mixture (a) HCNO (0.1 Torr)/SF (1.5 Torr) and (b)
1a
1b
1c
6
0
.19 ± 0.03, and ϕ = 0.05 ± 0.02. The sum of these quantum
HCNO(0.1Torr)/NO(1.0 Torr)/NO (1.0 Torr)/O (4.0 Torr)/
SF (1.5 Torr) using R10 line of HNCO.
6
1e
2
2
yields is unity, 1.00 ± 0.08, indicating that these experiments
account for all of the major photolysis channels (the exact
agreement with unity is probably fortuitous). The quoted
uncertainties are obtained by propagation of the random errors
inherent in the measurements as well as consideration of
systematic errors due to estimated uncertainties in the infrared
line strengths (typically ∼5−10%), the UV laser energy (∼5%),
and the HCNO UV absorption coefficient (∼10%).
attributing the signal to photolysis channel 1e, we must
consider the possibility that some of the secondary chemistry,
reactions 2−5, may result in collision-induced isomerization of
HCNO to HNCO. Ab initio studies of the potential energy
39−44
surface studies of these reactions
do not show any low
energy pathways leading to HNCO, and the previous
experimental observations of fast radical decay suggest that
isomerization does not play a major role, but we cannot rule
One last issue concerns the fate of H atoms produced in
channel 1b. If H atoms react quickly with HCNO (reaction 7),
additional routes to some of the product molecules detected
8
35
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The Journal of Physical Chemistry A
Article
may exist. No experimental measurement of k has been
that O + HCN (1a) is the major but not dominant product
7
reported, but there are two computational studies in the
literature. A potential energy surface study by Wang et al. shows
channel. Photolysis quantum yields are ϕ (O + HCN) = 0.39
1a
± 0.07, ϕ1b (H + (NCO)) = 0.21 ± 0.04, ϕ (CN + OH) =
1c
45
1
only one kinetically allowed product channel: HCN + OH,
0.16 ± 0.04, ϕ1d (CN + NH(a Δ)) = 0.19 ± 0.03, and ϕ
1e
and suggests that this is a fast reaction. If true, our
interpretation of the HCN yield experiments are clearly in
(HNCO) = 0.05 ± 0.02.
1
error. A theoretical calculation by Miller et al., however, shows
AUTHOR INFORMATION
■
*
this reaction is slow with a rate constant of k = 1.2 × 10 cm 3
−13
Corresponding Author
E-mail: john.hershberger@ndsu.edu. FAX: (701)-231-8831.
−1
−1
s
molec . If this is true, most H atoms would be lost to
diffusional decay under our experimental conditions rather than
react with HCNO. We do not have any way to directly detect
H atoms, but we have investigated possible products:
Notes
The authors declare no competing financial interest.
H + HCNO → CO + NH2
(7a)
ACKNOWLEDGMENTS
■
→
CN + H O
(7b)
(7c)
This work was supported by Division of Chemical Sciences,
Office of Basic Energy Sciences of the Department of Energy,
Grant DE-FG03-96ER14645.
2
→
→
HCN + OH
H CO + N
(7d)
2
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