Reaction dynamics of electronically state-specific CH2 with NO

Article abstract of DOI:10.1063/1.481992

Density functional theory (DFT) B3LYP calculations were carried out to explore the mechanisms related to the previously observed ³CH₂+NO and ¹CH₂+NO reaction channels. The potential energy surfaces of the observed reaction channels were described. As a result, the main reaction mechanisms of both ³CH₂+NO and ¹CH₂+NO were clarified.

Full text of DOI:10.1063/1.481992

Reaction dynamics of electronically state-specific CH 2 with NO
Hongmei Su, Fanao Kong, Bozhen Chen, Ming-Bao Huang, and Yajun Liu
Citation: The Journal of Chemical Physics 113, 1885 (2000); doi: 10.1063/1.481992
View online: http://dx.doi.org/10.1063/1.481992
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 113, NUMBER 5
1 AUGUST 2000
Reaction dynamics of electronically state-specific CH2 with NO
Hongmei Su and Fanao Konga)
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Bozhen Chen, Ming-Bao Huang, and Yajun Liu
Graduate School at Beijing, University of Science and Technology of China, Chinese Academy of Sciences,
Beijing 100039, People’s Republic of China
͑Received 25 October 1999; accepted 31 January 2000͒
With time-resolved Fourier transform infrared emission spectroscopy and DFT B3LYP quantum
3
a 1A1) with NO have been investigated.
˜
calculation, the reaction dynamics of CH2(X B1) and CH2(
˜
3
1
It is found that both CH2ϩNO and CH2ϩNO reactions follow the same reaction pathways and
produce same products arising from the same elementary channels. The primary products of
vibrationally excited CO( ), HCO( ), HOCN( ), OH( ), and NH ( 3) were detected for the
v1 v2
v
v
v
2
first time and four reaction channels have thus been identified. Theoretically, a doublet potential
3
1
energy surface is characterized. On the potential energy surfaces, both the CH2ϩNO and CH2
ϩNO systems reach a crucial intermediate OCHNH via a CNO ring-closure and ring-opening
process. From this intermediate, the four reaction pathways proceed: C–N bond rupture in OCHNH
simply leads to NHϩHCO; OCHNH rearranges either to H2NCO producing COϩNH2, or to
HOCHN generating HOCNϩH and HCNϩOH. © 2000 American Institute of Physics.
͓S0021-9606͑00͒00416-5͔
I. INTRODUCTION
with high pressures. Thus, the products observed were not
proved to be from elementary reactions. Obviously, a wide
survey of the nascent products is invoked.
The small hydrocarbon radicals play an important role in
the reburning of NOx . The reactions of CH, CH2, C2H or
even CH3 with NOx will produce NHi radicals (iϭ1,2)
which then effectively convert NO to N2.1 Among these pro-
cesses, the reaction of CH2 with NO is of particular interest
and has been investigated by several groups over the last two
decades.
Three groups12–15 have investigated the reaction mecha-
nism of 3CH2ϩNO by theoretical calculations. Melius calcu-
lated the enthalpies of formation of the possible intermedi-
ates and some of their decomposition barriers.1,10,11
Roggenbuck and Temps12 carried out the DFT ͑density
functional theory͒ B3LYP calculation on the reaction paths
of the 3CH2ϩNO reactions. They mainly studied the decom-
position channels via the intermediate CH2NO to HCNϩOH,
HϩHCNO, and H2ϩCNO. They concluded that the channel
͑3͒ leading to HϩHCNO is dominant. They did not explore
other possible channels.
Two low-lying electronic states of methylene are the
3
˜
ground triplet state CH2(X B1) and the lowest singlet state
CH2(˜
a 1A1). The energy gap of these two states is 9.00
2
3
kcal/mol. Hereafter, they are abbreviated to be CH2 and
1CH2. The overall rate constants for the reactions of either
1
3CH2 or CH2 with NO at 295 K have been measured by
Recently Shapley and Bacskay13–15 reported their ab
initio calculations at the G2, CASSCF, CASPT2, and
QCISD͑T͒ levels for the energies of all possible species oc-
curred in the formation, isomerization, and decomposition
reactions of CH2NO, and they also presented a comprehen-
sive study of the CH2NO potential energy surface.
using a variety of techniques, as shown in Table I.
1
The CH2ϩNO reaction occurs with high rate, close to
the gas kinetic collision frequency. The rate constant of
3CH2ϩNO is about 20% of that of CH2ϩNO.
1
As a highly exothermic radical–radical reaction, CH2
with NO is expected to have multiple channels. The thermo-
dynamically allowed reaction channels of CH2ϩNO are
shown in Table II.
In the present work we performed DFT B3LYP calcula-
tions, exploring the mechanisms related to our observed
1
3CH2ϩNO and CH2ϩNO reaction channels. We also per-
The listed ⌬H0° values refer to the 3CH2ϩNO reactions.
Only a few products were identified in the previous experi-
mental investigations. For the reaction of 3CH2ϩNO, a small
amount of H, O atoms and OH radicals have been
observed.5,9,10 Recently, Wagner’s group11 analyzed the
stable products by Fourier transform infrared ͑FTIR͒ spec-
troscopy in a static reaction cell. They observed the products
of HCNO and HCN. However, the detection was performed
3
formed a comparative study on the reaction of CH2ϩNO
1
and CH2ϩNO experimentally and theoretically. The poten-
tial energy surfaces of these observed reaction channels were
characterized. The main reaction mechanisms of both
1
3CH2ϩNO and CH2ϩNO were clarified.
II. EXPERIMENT
The reaction chamber and the time-resolved ͑TR͒ FTIR
spectrometer have been described in detail elsewhere.16 Pure
CH2(X B1) radicals were produced via laser photolysis
a͒
Author to whom correspondence should be addressed. Electronic mail:
3
˜
kong@mrdlab.icas.ac.cn
0021-9606/2000/113(5)/1885/6/$17.00
1885
© 2000 American Institute of Physics
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1886
J. Chem. Phys., Vol. 113, No. 5, 1 August 2000
Su et al.
TABLE I. Overall reaction rate constants at 295 K.
Rate constants
Reaction
Group
k/cm3 moleculeϪ1 sϪ1
Techniques
Detection
Laufer ͑Ref. 3͒
Pilling ͑Ref. 4͒
Debruyn ͑Ref. 5͒
Wagner ͑Ref. 1͒
Moore ͑Ref. 6͒
(1–2)ϫ10Ϫ11
(1–2)ϫ10Ϫ11
(1–2)ϫ10Ϫ11
3.7ϫ10Ϫ11
Gas chromatography
Gas chromatography
Mass spectrometry
JMR
Indirect
Indirect
Indirect
Direct
3CH2ϩNO
2.9ϫ10Ϫ11
Diode laser absorption
Direct
Moore ͑Ref. 7͒
1.6ϫ10Ϫ10
Laser absorption
Direct
Direct
1CH2ϩNO
Hancock ͑Ref. 8͒
1.62ϫ10Ϫ10
CW resonance absorption
IV. RESULTS AND DISCUSSION
A. Products and channels
of ketene at 351 nm ͑XeF laser, Lambda Physik LPX305i,
170 mJ/pulse͒ and CH2(
a 1A1) via laser photolysis at 308
˜
nm ͑XeCl laser, Lambda Physik LPX200, 80 mJ/pulse͒.
Ketene was produced by the pyrolysis of acetone and puri-
fied by the trap-to-trap method.17 In the experiment, a gas
mixture of 30 Pa ketene and 70 Pa NO flowed into the reac-
tion chamber and was irradiated by the excimer laser beam,
slightly focused by a cylindrical lens (fϭ380 mm). The
transient IR emission from the reaction products was re-
corded by the TR–FTIR spectrometer ͑Nicolet 800͒. An
InSb detector was used. The spectral resolution of the instru-
ment was set at 16 cmϪ1. The data acquisition system
provided 10 time-sequenced interferograms with 18 ␮s spac-
ing after a laser pulse. The time resolution of the detection
was 3 ␮s.
3
1. Reaction of CH2¿NO
3
The CH2 radical was generated from the photolysis of
ketene at 351 nm:
351 nm
3
1
˜
CH CO —— CH X B ͒ϩCO X ⌸͒.
→
͑
2
͑
2
1
The product CH2 has been ascertained to be in the ground
2
3
˜
electronic state X B1 . The above reaction gives only a
small amount of available energy ͑5 Kcal/mol͒, therefore the
products are not expected to be vibrationally excited. In fact,
no IR emission was observed after the 351 nm photolysis of
pure ketene. Therefore, the photolysis of ketene at this wave-
III. COMPUTATIONAL METHODS
3
˜
length provides a neat ground state CH2(X B1) source as
The DFT18,19 B3LYP20,21 calculations were carried out
with the Gaussian 94W suite of programs22 using a 6-311
G(d,p) basis set23 ͓‘‘at the B3LYP/6-311 G(d,p) level’’͔.
The stationary points along the reaction paths were scanned
in the geometry optimization calculations. The frequency
analysis calculations were performed in order to characterize
stationary points as intermediate complexes ͑IMs͒ or transi-
tion states ͑TSs͒ and to evaluate zero point energies ͑ZPEs͒
for correcting the relative energies. For each of reaction
steps, the pointwise calculations and the IRC ͑intrinsic reac-
tion coordinate͒ calculations along the reaction path were
performed. Spin contamination in the ͑U͒B3LYP calcula-
tions for most IMs and TSs is not large.
well as a clean emission background for the further IR spec-
troscopic investigation.
When the gas mixture of ketene and NO was irradiated
by the 351 nm laser, a strong IR emission from the products
3
of CH2ϩNO reaction was recorded. Figure 1 shows the
transient IR emission spectra. The reaction reaches its maxi-
mum within 3 ␮s. This is consistent with the overall rate
constant of 3.3ϫ10Ϫ11 cm3 moleculeϪ1 sϪ1 reported by
Moore and Wagner.1,6
a. Product identification. Several emission bands ap-
pear in the IR spectra ͑Fig. 1͒. The band between 1820–2160
cmϪ1 is attributed to the emission of vibrationally excited
CO. Due to vibrational quenching, the peak position at 3 ␮s
shifts toward higher frequency at 5 ␮s and the emission al-
most disappears at 21 ␮s. Such rapid vibrational relaxation
3
TABLE II. Channels of the reaction CH2ϩNO.
can be interpreted as near-resonant
– energy transfer from
v v
CO( ) to the precursor molecule CH CO ͑␯ ϭ2152 cmϪ1
,
v
2
2
Products
⌬H0° ͑kcal/mol͒ ͑Ref. 11͒
Channel
C–O stretching mode͒.
The band of 2380–2560 cmϪ1 with peak position at
H2ϩCNO
NHϩHCO
HϩHCNO
H2COϩN
CNϩH2O
HϩHOCN
OHϩHCN
H2ϩNCO
HϩHNCO
NH2ϩCO
Ϫ16.7
Ϫ17.8
Ϫ19.5
Ϫ27.1
Ϫ63.8
Ϫ65.5
Ϫ72.8
Ϫ81.9
Ϫ90.5
Ϫ96.7
͑1͒
͑2͒
͑3͒
͑4͒
͑5͒
͑6͒
͑7͒
͑8͒
͑9͒
͑10͒
2472 cmϪ1 is assigned to the ␯ mode of HCO ͑fundamental
1
frequency being 2485 cmϪ1͒. The wide band of 2700–3510
cmϪ1 refers to the emission from OH. The sharp band cen-
tered at 3302 cmϪ1 may be either from the asymmetric
stretching mode (␯3) of NH2 ͑fundamental frequency being
3301.1 cmϪ1͒ or from NH ͑fundamental frequency being
3300 cmϪ1͒. It is likely from the contribution of NH2 which
is produced via channel ͑10͒ with a large exothermal energy
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1887
J. Chem. Phys., Vol. 113, No. 5, 1 August 2000
Reaction dynamics of CH2 with NO
TABLE III. The relative vibrational population of the product CO( ) of the
v
3
reaction of CH2ϩNO at 3 ␮s.
ϭ1
ϭ2
ϭ3
ϭ4
ϭ5
v
v
v
v
v
1Ϯ0.08
0.61Ϯ0.06
0.33Ϯ0.05
0.34Ϯ0.04
0.25Ϯ0.04
ϭ6
ϭ7
ϭ8
ϭ9
ϭ10
v
v
v
v
v
0.35Ϯ0.03
0.36Ϯ0.03
0.36Ϯ0.02
0.27Ϯ0.02
0.19Ϯ0.014
HOCN ͑fundamental frequency being 2294 cmϪ1͒. There is
little possibility of the emission to be from the NCO
a-stretching mode of HNCO ͑fundamental frequency being
2267 cmϪ1͒. And it cannot be the CNO a-stretching mode of
HCNO ͑fundamental frequency being 2196 cmϪ1͒.
b. Vibrational population of the nascent CO. Simula-
tion of the IR emission spectra makes it possible to evalu-
ate the vibrational energy distribution of the products. The
emission spectrum of the CO ( → Ϫ1) was simulated. Fig-
v
v
ure 2 is a comparison of the experimental and simulated
spectra of CO( ). The spectral contour is composed of 10
v
rovibrational bands. Several simulated spectral bands are
also shown in Fig. 2. From the best-fitted simulation, the
relative vibrational population of CO( ) at 3 ␮s is obtained
v
and listed in Table III. The result shows that the nascent
products are highly excited, and deviate from the Boltzmann
distribution.
FIG. 1. The IR emission spectra of 3CH2ϩNO at different delay times after
laser shot.
c. Channel identification. The nascent product CO is
highly excited up to ϭ10. The vibrational energy of
v
v
ϭ10 corresponds to 60.9 kcal/mol. Such a high excitation
of 96.7 kcal/mol. In contrast, the contribution of NH arising
from channel ͑2͒ is small. Considering a small exothermal
energy of 17.8 kcal/mol released from channel ͑2͒, NH is not
expected to be vibrationally excited, as the energy of one
indicates that the CO( ) arises from the channel ͑10͒ which
v
releases a large amount of energy of 96.7 kcal/mol:5
vibrational quantum is 9.4 kcal/mol ( ϭ0 1).
v
3CH2ϩNO→NH2ϩCO.
͑10͒
It is interesting that a band centered at 2302 cmϪ1 ap-
pears at 5 ␮s and increases gradually with time. Its formation
is obviously slower than those of the other emissions. This
emission may arise from several isomers whose fundamental
vibrational frequency falls into this region. However, the
emission is most likely from the CN stretching mode (␯2) of
Other observed products are from the reaction channels ͑2͒,
͑6͒, and ͑7͒:
3CH2ϩNO→NHϩHCO
͑2͒
FIG. 2. The simulation of the CO emission spectrum.
The solid line is the spectrum taken from the 3CH2ϩNO
reaction at 3 ␮s. The dotted and dashed lines are the
simulation results.
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1888
J. Chem. Phys., Vol. 113, No. 5, 1 August 2000
Su et al.
3
sure. This fact rules out the possibility of a CH2ϩNO reac-
tion, which follows a fast quenching from 1CH2 state to 3CH2
state.
Comparing Fig. 3 with Fig. 1, we find that the reaction
products are almost the same in the spectra. Again, the band
of 1950 cmϪ1–2250 cmϪ1 is assigned to CO( ) and the band
v
centered at 3302 cmϪ1 is from NH2(␯3). Another band cen-
tered at 2302 cmϪ1, appearing at 3 ␮s and growing up gradu-
ally, is assigned to the CN stretching mode ( 2) of HOCN.
v
The OH and HCO cannot be recognized in Fig. 3 perhaps
due to poor S/N ratio.
Because the main products are the same as that of
3CH2ϩNO reaction, the reaction of 1CH2ϩNO probably pro-
3
ceeds via the channels similar to CH2ϩNO reaction.
B. Reaction mechanism
The DFT B3LYP reaction path calculations were carried
out for the identified channels:
3CH2͑1CH2͒ϩNO→NH2ϩCO
→HOCNϩH
͑10͒
͑6͒
→HCNϩOH
͑7͒
→HCOϩNH.
͑2͒
The reaction paths were calculated in the doublet state po-
tential energy surface ͑PES͒ of the CH2NO system, which is
related to all the products and to both the reactant groups
1
3CH2ϩNO and CH2ϩNO. Figure 4 shows a schematic dia-
FIG. 3. The IR emission spectra of 1CH2ϩNO at different delay times after
laser shot.
gram of the calculated reaction paths for the four channels.
The B3LYP/6-311G (d,p) energies of IMs, TSs, and the fi-
nal products relative to 3CH2ϩNO ͑corrected with the
B3LYP/6-311G (d,p) ZPEs͒ are given in Fig. 4.
HϩHOCN
OHϩHCN.
͑6͒
͑7͒
1. Initial attack leading to H2CNO
H2CNO ͑IM1, see Fig. 4͒ was found to be the most
stable product of the CH2 initial attack on NO. The 2A
Ј
3
1
reaction paths from CH2ϩNO to IM1 and from CH2ϩNO
to IM1 were checked by the B3LYP/6-311G (d,p) pointwise
calculations, and no energy barrier was found in either of the
The species underlined are the products observed in this ex-
periment.
3
paths. Our experimental results indicate that the CH2ϩNO
1
and CH2ϩNO reactions have the same four product chan-
nels. The 3CH2ϩNO and 1CH2ϩNO reactions have the same
reaction paths from IM1 to the final products of the four
channels.
1
2. Reaction of CH2¿NO
1
Pure CH2 radical was generated from the photolysis of
ketene at 308 nm:2
308 nm
2. From H2CNO to OCHNH
CH CO —— CH ˜
a 1A ͒ϩCO X 1⌸͒.
͑ ͑
2 1
→
2
The calculated reaction path from IM1 to IM4 describes
the following sequent processes: ͑i͒ a ring-closure process of
IM1 leading to IM2, ͑ii͒ a ring-reopening process of IM2
leading to IM3 ͑a nitrene14͒, a structure with the O–C–N
heavy atom chain, and ͑iii͒ an H-migration process leading
to IM4 ͑OCHNH͒, a key intermediate. Our calculations in-
dicate that the H-migration process cannot occur either ahead
of or simultaneously with the ring-reopening process. The
process from CH2ϩNO to IM2 was already calculated at the
B3LYP level by Roggenbuck and Temps,12 while the con-
version of the C–N–O chain to the O–C–N chain ͑in IM3
and IM4͒ was not considered.
No IR emission was observed in this condition. But when
NO was added to the system, strong IR emission was ob-
served.
Figure 3 exhibits the transient IR emission spectra of the
1CH2ϩNO reaction. It is noticed that the risetime of the CO
appearance is very short. The emission spectrum at 3 ␮s is
3
similar to that of the CH2ϩNO at 21 ␮s, indicating that the
1
IR emission of CH2ϩNO reaction have reached its maxi-
mum before 3 ␮s. Therefore, the IR emission bands recorded
at 3 ␮s have been somewhat reduced by relaxation. It agrees
with the rate estimated by the rate constant and the NO pres-
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1889
J. Chem. Phys., Vol. 113, No. 5, 1 August 2000
Reaction dynamics of CH2 with NO
FIG. 4. A schematic diagram of the potential energy curves along the calculated reaction paths for the 3CH2͑1CH2͒ϩNO reactions. The values in parentheses
3
are the B3LYP/6-311G(d,p) energies in kcal/mol related to CH2ϩNO ͑corrected with the B3LYP/6-311g(d,p) ZPEs͒.
3. From OCHNH to final products
sis of our B3LYP calculations are supported by the recent ab
initio calculations of Shaplay and Bacskay.13–15
The IM4→NH2ϩCO ͓the final products of channel ͑10͔͒
reaction step consists of two processes: H migration from the
C atom to N atom in IM4 leading to IM5, followed by a
dissociation process.
The IM4→HOCNϩH ͓the final products of channel ͑6͔͒
reaction step also consists of two processes: H migration
from the N atom to O atom in IM4 leading to IM6͑a͒
͑HOCHN͒, followed by a C–H bond cleavage process of
IM6 ͑a͒ leading to trans-HOCNϩH. A chemical process
from IM2 ͑or from IM3͒ directly to IM6 without passing
through IM4 was not found in our calculations. An alterna-
tive process from IM4 to HOCNϩH via a species HOCNH
was not found in our calculations.
V. CONCLUDING REMARKS
3
͑a͒ The elementary reaction dynamics of CH2ϩNO and
1CH2ϩNO has been investigated experimentally and
theoretically. The theoretical results agree well with the
experimental observations.
͑b͒ The vibrationally excited primary products of CO( ),
v
HCO(␯ ), HOCN(␯ ), OH( ), and NH (␯ ) were ob-
v
1
2
2
3
served for the first time.
͑c͒ The reaction channels were identified to be
Reactants
Products
Reactants
The IM4→HCNϩOH ͓the final products of channel ͑7͔͒
reaction step consists of the following sequential processes:
͑i͒ H migration from the N atom to O atom in IM4 leading to
IM6 ͑a͒, ͑ii͒ internal rotation about the O–C bond leading to
IM6͑b͒, and ͑iii͒ a cleavage process of the O–C bond in
IM6͑b͒. Channel ͑7͒ was previously studied at the B3LYP
level by Roggenbuck and Temps12 and they reported a dif-
ferent reaction path via HCNOH, different from that of the
present paper.
3CH2ϩNO→NH2ϩCO
3CH2ϩNO→OHϩHCN
3CH2ϩNO→HϩHOCN
3CH2ϩNO→NHϩHCO
3
1CH2ϩNO
1CH2ϩNO
1
͑d͒ Both the CH2ϩNO and CH2ϩNO reactions follow a
doublet potential energy surface via an addition–
elimination mechanism.
The IM4→HCOϩNH ͓the final products of channel ͑2͔͒
͑e͒ The initial step of CH2ϩNO reaction is a barrierless
addition process forming a planar intermediate IM1,
H2CNO. Following CNO ring forming and sequential
ring opening steps, the OCN-chain type of intermediate
IM3, OCH2N is formed. A H-atom transfer from C
atom to N atom in IM3 leads to IM4 OCHNH. All of
the four sequential reaction channels arise from the key
intermediate IM4.
reaction step is a C–N bond cleavage process.
The relative energy values of Ϫ92.8, Ϫ61.9, Ϫ70.4, and
Ϫ21.2 kcal/mol for products NH2ϩCO, HOCNϩH,
HCNϩOH, and HCOϩNH are comparable with the ⌬H°
values of Ϫ96.7, Ϫ65.5, Ϫ72.8, Ϫ17.8 kcal/mol for the
3
channels ͑10͒, ͑6͒, ͑7͒, and ͑2͒ of the CH2ϩNO reaction,
respectively. The reaction mechanisms suggested on the ba-
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Su et al.
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101, 134 ͑1997͒.
ACKNOWLEDGMENTS
This project was financially supported by China Natural
Science Foundation Contracts No. 29773052 and 29892162.
The authors would like to thank Jixin Yang, Wenhui Feng,
and Na Ji for their helps.
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