Ab initio chemical kinetics for the reactions
of HNCN with O(
3
P) and O
2
Shucheng Xu
a, M.C. Lin
a,b,*a
Department of Chemistry, Emory University, Atlanta, GA 30322, USA
b
Institute of Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsichu 300, Taiwan
Abstract
The kinetics and mechanisms of the reactions of cyanomidyl radical (HNCN) with oxygen atoms and molecules have been investigated by ab initio calculations with rate constant prediction. The doublet and quartet state potential energy surfaces (PESs) of the two reactions have been calculated by single-point cal-culations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/ 6-311++G(d, p) level. The rate constants for various product channels of the two reactions in the temper-ature range of 300–3000 K are predicted by variational transition state and RRKM theories. The predicted total rate constants of the O(3P) + HNCN reaction at 760 Torr Ar pressure can be represented by the expressions ktotal (O + HNCN) = 3.12 1010 T0.05 exp (37/T) cm3molecule1s1 at T = 300–
3000 K. The branching ratios of primary channels of the O(3P) + HNCN are predicted: k1for producing
the NO + CNH accounts for 0.72–0.64, k2+ k9for producing the3NH + NCO accounts for 0.27–0.32,
and k6for producing the CN + HNO accounts for 0.01–0.07 in the temperature range studied. Meanwhile,
the predicted total rate constants of the O2+ HNCN reaction at 760 Torr Ar pressure can be represented
by the expression, ktotal(O2+ HNCN) = 2.10 1016 T1.28exp (12200/T) cm3molecule1s1 at
T = 300–3000 K. The predicted branching ratio for k11+ k13 producing HO2+3NCN as the primary
products accounts for 0.98–1.00 in the temperature range studied.
Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Chemical kinetics; HNCN reactions with O(3P) and O2; Computational study
1. Introduction
The cyanomidyl radical (HNCN) is a reactive transient species which plays an important role in a variety of chemical environments such as in flame, interstellar space, and planetary atmo-sphere. Experimentally, the HNCN radical was
first identified spectroscopically by Herzberg and Warsop in 1963[1]. More recently, Wu et al.[2]
probed the B2A0 X2
A00 transition with
laser-induced fluorescence, Yamamoto and Saito [3]
reported the microwave spectrum of HNCN and Clifford et al. [4]studied the photoelectron spec-trum of the HNCNion. In 2001, the photodisso-ciation spectroscopy and dynamics of the HNCN radical have been investigated by Neumark and coworkers[5]. Theoretically, ab initio calculations of the molecular geometry and vibrational fre-quencies of the HNCN ground state were first made by Tao et al. in 1994[6]and more recently by Puzzarini et al. in 2005[7]. In our laboratory,
1540-7489/$ - see front matterÓ 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.07.011
*Corresponding author. Address: Department of
Chemistry, Emory University, Atlanta, GA 30322, USA. Fax: +1 404 727 6586.
E-mail addresses: sxu@emory.edu (S. Xu), chemm-cl@emory.edu(M.C. Lin).
Proceedings of the Combustion Institute 32 (2009) 99–106
www.elsevier.com/locate/proci
Proceedings
of the
Combustion
Institute
we first proposed HNCN to be the key stable intermediate of the prompt NO formation from CH + N2along its spin-allowed doublet electronic
state path[8].
Under high-pressure conditions, the oxidation of HNCN by OH radical, O(3P) atom, and O2
mol-ecule may play an important role in the prompt NO formation because they may produce the primary precursors for NO formation, CN, NCN, or NO products, where NCN as one of primary precursors was confirmed in the prompt NO formation exper-iments recently reported by Smith[9], Hanson and co-workers[10], and Williams and Fleming[11]. Recently, we have studied the kinetics and mecha-nism of the reaction of HNCN with OH by ab initio calculations[12]. In the OH + HNCN study, both singlet and triplet state PESs have been calculated at the CCSD(T)/6-311+G(3df, 2p)//B3LYP/6-311+G(3df, 2p) and CCSD/6-311++G(d, p) levels of theory and the rate constants for the primary channels of the reaction in the temperature range of 300–3000 K have been predicted. The results show that the primary products are H2O + NCN
at temperatures above 800 K. In present work, we continue to study the chemical kinetics of the reac-tions of HNCN with O(3P) and O2using the similar
methods as those employed for the OH + HNCN reaction; there have been no reports on the kinetics and mechanisms for the two reactions experimen-tally or theoretically.
2. Computational methods
The geometries of the reactants, transition states, intermediate complexes, and products for the O(3P) + HNCN and O
2+ HNCN reactions
have been optimized at the CCSD/6-311++G(d, p) level. The energies for the doublet and quartet state PESs are improved by single point calcula-tions at the CCSD(T)/6-311+G(3df, 2p) level of theory based on the optimized geometries at CCSD/6-311++G(d, p) level, which have been performed successfully for the reactions of OH + CH2O[13]and OH + HNCN[12].
The rate constants for the key product channels have been computed with the variational TST and RRKM theory using the Variflex code[14]. All quantum chemistry calculations have been carried out by the Gaussian 03[15]using a PC cluster and the computers at Cherry L. Emerson Center for Sci-entific Computation at Emory University.
3. Results and discussion
3.1. Potential energy surfaces and reaction mech-anism of the O(3P) + HNCN
The reaction of O(3P) with HNCN can occur on both doublet and quartet reaction channels.
The optimized geometries and PESs for the dou-blet and quartet state reactions are shown inFigs. 1 and 2(a) and (b), respectively.
For the doublet state PES shown inFig. 2(a), the reaction for O(3P) with HNCN forms firstly primary intermediates D-O-IM1 (HNCNO) with
the binding energy of 53.4 kcal/mol when the O(3P) atom associates with the terminal N atom or D-O-IM2(HN(O)CN) with the binding energy
of 67.3 kcal/mol when the O(3P) associates with the N atom next to the H atom. D-O-IM1 and
D-O-IM2 can isomerize to other four
intermedi-ates. For example, D-O-IM1 can transform to
D-O-IM3 (trans-HNC(O)N) via D-O-TS3 with a
barrier of 12.5 kcal/mol or to D-O-IM4
(cis-HNC(O)N) via D-O-TS7 with a barrier of
15.5 kcal/mol. Similarly, D-O-IM2 can undergo
H-atom migration to D-O-IM6 (HONCN) via
D-O-TS6 with a barrier of 49.1 kcal/mol or
O-atom migration to D-O-IM4via D-O-TS8with
a barrier of 44.5 kcal/mol. Furthermore, D-O-IM3
can isomerize to D-O-IM5(HOC(N)N) via
D-O-TS4with a barrier of 55.3 kcal/mol. There is no
direct isomerization pathway found between D-O-IM1 and D-O-IM2. However, D-O-IM1 can
transform to D-O-IM2through D-O-IM4. These
isomerization reactions can also occur reversely as one would expect. As shown in Fig. 2(a), the doublet state PES of the O(3P) + HNCN reaction may generate the following primary products with the predicted enthalpy changes: NO + CNH, 66.9 kcal/mol; 3
NH + NCO, 20.9 kcal/mol; OH +3NCN,18.8 kcal/mol; CN + HNO, 2.5 kcal/mol; and 3NH + CNO, 41.7 kcal/mol. The predicted heats of reaction for the formation of
Fig. 1. Optimized geometries of the O(3P) + HNCN reaction computed at the CCSD/6-311++G(d, p) level.
NO + CNH, OH +3NCN, and CN+HNO from the O(3P) + HNCN are in reasonable agreement with the available experimental values at 0 K, 66.1 ± 1.6 kcal/mol, 15.4 ± 4.6 kcal/mol, and 2.4 ± 1.4 kcal/mol, respectively, based on the heats of formation DfH0at 0 K as the following:
DfH0(O) = 59.56 ± 0.02 kcal/mol[16]; DfH0(HNCN)
= 72.3 ± 0.7 kcal/mol [5]; DfH0(NO) = 21.456
kcal/mol [17]; DfH0(CNH) = 44.3 ± 0.9 kcal/mol [5]; DfH0(OH) = 8.87 ± 0.07 kcal/mol [16]; DfH0
(3NCN) = 107.6 ± 3.2 kcal/mol derived from
DfH298(3NCN) = 107.7 ± 3.2 kcal/mol [4]; DfH0
(CN) = 104.1 ± 0.5 kcal/mol [5]; DfH0(HNO)
= 24.5 kcal/mol [17]. Comparing with the avail-able experimental heat of reaction producing
3
NH + NCO,15.2 ± 1.6 kcal/mol based on the experimental heats of formation, DfH0(3NH)
= 85.29 ± 0.14 kcal/mol [18] and DfH0(NCO) =
31.36 ± 0.69 kcal/mol [19], our predicted value (20.9 kcal/mol) is about 5.7 ± 1.6 kcal/mol lower than the experimental data. However, the predicted result is close to the value, 21.8 ± 0.7 kcal/mol, based on the computed heats of for-mation, DfH0(3NH) = 79.6 kcal/mol [20] and
DfH0(NCO) = 30.5 kcal/mol [21]. The
NO + CNH products are formed by the primary dissociation of the D-O-IM1 by overcoming a
small barrier of 15.2 kcal/mol at D-O-TS2. The 3NH + NCO products are from the next key
channel by the dissociation of D-O-IM3 and
D-O-IM4 with the dissociation energies of 30.4
and 30.2 kcal/mol, respectively. The OH +3NCN
products may be derived from the metathetical process by overcoming a barrier of 17.2 kcal/mol at D-O-TS1, or by the dissociation of the
D-O-IM5 through D-O-TS5 with a barrier of
64.4 kcal/mol or D-O-IM6with a direct
dissocia-tion energy of 48.9 kcal/mol. Furthermore, the CN + HNO products may be formed by the direct dissociation of D-O-IM2 with a dissociation
energy of 64.8 kcal/mol. Finally, 3NH + CNO may be produced by the direct dissociation of D-O-IM1with a much higher dissociation energy
and thus can be neglected.
For the quartet state PES shown inFig. 2(b), there is also a direct abstraction channel to pro-duce the OH +3NCN by overcoming a relatively lower barrier of 5.9 kcal/mol at Q-O-TS1.
How-ever, there are no barrierless association processes to form quartet intermediates. The quartet inter-mediates Q-O-IM1and Q-O-IM3are formed from
the reactants by overcoming barriers of 20.9 kcal/ mol at Q-O-TS2 and 8.7 kcal/mol at Q-O-TS3,
respectively. Similar to the doublet state PES, Q-O-IM1, and Q-O-IM3 can isomerize to other
intermediates. For example, Q-O-IM1can
isomer-ize to intermediate Q-O-IM2 via Q-O-TS4with a
barrier of 14.4 kcal/mol, Q-O-IM2 to Q-O-IM5
via Q-O-TS5 with a barrier of 34.0 kcal/mo and
Q-O-IM5to Q-O-IM6via Q-O-TS6with a barrier
of 5.9 kcal/mol. Meanwhile, Q-O-IM3 can also
transform to Q-O-IM4via Q-O-TS7with a
rota-tion barrier of 4.7 kcal/mol. These isomerizarota-tion reactions can also occur reversely as one would expect. As shown in Fig. 2(b), the quartet state PES of the O(3P) + HNCN reaction may generate
the following products: OH +3NCN, 3NH +
NCO, 3NH + CNO, and NO +3CNH. The
abstraction channel giving OH +3NCN is a
pri-mary channel. The 3NH + NCO products may
be produced by the dissociation of Q-O-IM3by
overcoming a barrier of 24.0 kcal/mol at Q-O-TS8 and Q-O-IM4 by overcoming a barrier of
25.8 kcal/mol at Q-O-TS9, respectively.
Further-more, the products 3NH + CNO and NO +
3
CNH formed by direct dissociations with much higher dissociation energies should be negligible. 3.2. Potential energy surfaces and reaction mech-anism of the O2+ HNCN
Analogous to the O(3P) + HNCN reaction, the reaction of O2with HNCN can also occur on both
doublet and quartet state PESs. The optimized geometries and PESs for the doublet and quartet reaction channels are shown in Figs. 3 and 4(a) and (b), respectively.
For the doublet state PES shown inFig. 4(a), the reactants O2+ HNCN have to overcome 34–
40 kcal/mol barriers to form doublet intermedi-ates except for forming D-O2-IM1by overcoming
12.0 kcal/mol at D-O2-TS1 because of its loose
Fig. 2. (a and b) The doublet and quartet state PESs of the O(3P) + HNCN reaction calculated at the CCSD(T)/
structure for O2-N(H)CN. In addition, the direct
O-abstraction channel for O2 attacking NCNH
producing O + HNCNO in the doublet state is neglected due to its 78 kcal/mol barrier. The D-O2-IM6may dissociate to products OH +1NCNO
by overcoming a barrier of 14.0 kcal/mol at D-O2
-TS7or HO2+3NCN with the dissociation energy
of 25.9 kcal/mol. In addition, the D-O2-IM4and
D-O2-IM5 may dissociate directly to products
O + D-O-IM3 with the dissociation energies of
7.4 and 5.2 kcal/mol, respectively. Finally, the D-O2-IM2and D-O2-IM3may dissociate directly
to products O + D-O-IM4 with the dissociation
energies of 18.4 and 19.0 kcal/mol, respectively. The quartet state PES shown in Fig. 4(b) is similar to the doublet state PES. However, the reactants O2+ HNCN have to overcome
rela-tively lower barriers of 22-34 kcal/mol to form quartet intermediates Q-O2-IM1, Q-O2-IM2,
Q-O2-IM3, and Q-O2-IM4. The Q-O2-IM1 is a
product complex, which can dissociate directly to products HO2+3NCN with a lower
dissocia-tion energy of 3.2 kcal/mol. Similarly as in the doublet state, the direct O-abstraction channel for O2attacking NCNH to produce O + HNCNO
in the quartet state is neglected due to its high (83 kcal/mol) barrier. Finally, the direct dissocia-tion processes can also occur from Q-O2-IM2,
Q-O2-IM3, and Q-O2-IM4 to produce 3NH +
NCOO, O + Q-O-IM3, and Q-O-IM4with
consid-erably higher dissociation energies, they should be negligible.
3.3. Rate constants calculations for the primary reaction channels of the O(3P) + HNCN and O2+ HNCN
3.3.1. Methods employed for rate constant calculations
The rate constants for the following primary doublet and quartet state reaction channels of the O(3P) + HNCN and O2+ HNCN have been
pre-dicted using variational TST and RRKM theory by the Variflex Code[14]in the temperature range 300–3000 K and pressure range of 1–7600 Torr based on the obtained PESs at the CCSD(T)/6-311+G(3df, 2p) level with the optimized structures at the CCSD/6-311++G(d, p) level: Oþ HNCN ! D-O-IM 1! NO þ CNH ð1Þ !3NHþ NCO ð2Þ ! OHþ3NCN ð3Þ Oþ HNCN ! D-O-IM 2! D-O-IM2ðþMÞ ð4Þ ! OH þ3NCN ð5Þ ! CN þ HNO ð6Þ Oþ HNCN ! D-O-TS1! OHþ3NCN ð7Þ Oþ HNCN ! Q-O-TS1! OHþ3NCN ð8Þ Oþ HNCN ! Q-O-TS3!3NHþ NCO ð9Þ O2þ HNCN ! D-O2-TS1! OH þ NCNO ð10Þ ! OOHþ3 NCN ð11Þ
Fig. 3. Optimized geometries of the O2+ HNCN
reac-tion computed at the CCSD/6-311++G(d, p) level.
Fig. 4. (a and b) The doublet and quartet state PESs of the O2+ HNCN reaction calculated at the
O2þ HNCN ! D-O2-TS4! O þ D-O-IM3 ð12Þ
O2þ HNCN ! Q-O2-TS1! OOH þ3NCN ð13Þ
where Channels(1)–(7) are the doublet channels and Channels(8), (9)are the quartet channels of the O(3P) + HNCN reaction and Channels(10)– (12) are the doublet channels and Channel (13)
is the quartet channel of the O2+ HNCN
reac-tion. Here the formation of D-O-IM1is negligible
because the dissociation barriers for production of NO + CNH and3NH + NCO are small and thus no significant stabilization of intermediate is expected.
Channels(1)–(5), (6), (12), (13)involve barrier-less association processes or direct dissociation processes, which are treated using the variational TST. The minimum energy paths (MEPs) for O + HNCN ? D-O-IM1 (MEP1), O + HNCN
? D-O-IM2(MEP2), D-O-IM2?CN + HNO
(MEP3), D-O-IM3? 3
NH + NCO (MEP4),
D-O-IM4?3NH + NCO (MEP5), D-O-IM6? OH
+3NCN (MEP6), D-O2-IM6?HO2+3NCN
(MEP7), D-O2-IM4 ? O + D-O-IM3(MEP8),
and Q-O2-IM1? HO2+3NCN (MEP9), were
obtained by computing the potential energy curves along the reaction coordinate from their equilibrium bond length to 5.0 A˚ with a step size of 0.1 A˚ estimated at the UB3LYP/6-311+G(3df, 2p) level. The calculated MEPs for the above-mentioned processes could be fitted to the Morse potential function with the parameters, b= 2.600 A˚1 with R0= 1.220 A˚ , and De=
56.4kcal/mol for MEP1; b = 2.960 A˚1with R0=
1.2767 A˚ , and De= 71.5 kcal/mol for MEP2;
b= 2.220 A˚1 with R0= 1.346 A˚ , and De= 69.3
kcal/mol for MEP3; b = 2.485 A˚1 with R0=
1.238 A˚ , and De= 35.8 kcal/mol for MEP4;
b= 4.471 A˚1 with R0= 1.238 A˚ , and De= 35.7
kcal/mol for MEP5; b = 2.984 A˚1 with R0=
1.367 A˚ , and De= 54.9 kcal/mol for MEP6;
b= 4.718 A˚1 with R0= 1.356 A˚ , and De=
29.9 kcal/mol for MEP7; b = 4.012 A˚1with R0=
1.318 A˚ , and De= 8.6 kcal/mol for MEP8;
b= 1.615 A˚1 with R0= 1.990 A˚ , and De= 4.3
kcal/mol for MEP9; respectively, where the
ener-gies for De were scaled to the CCSD(T)//CCSD
level without ZPE corrections. For the variational rate constant calculations by the Variflex code, a statistical treatment of the transitional-mode con-tributions to the transition-state partition func-tions was performed variationally. The numbers of states are evaluated according to the variable reaction coordinate flexible transition state theory
[14,22]. The energy-transfer process was com-puted on the basis of the exponential down model with thehDEidownvalue of 400 cm1for Ar. The
Morse potentials with the above-mentioned parameters, the Lennard-Jones pair-wise potential and the anisotropic potential are added together to form the final potential, similar to that employed in the OH + CH2O[13], OH + CH3OH
[23], OH + C2H5OH[23], and OH + HNCN[12]
reactions. For the Lennard-Jones potential, the function of VLJ= 4e [(r/r)12 (r/r)6] was used,
where r is the distance between the non-bonding atoms, e is the well depth and r is the hard sphere radius. In this calculation, e and r are 42.80 cm1, 2.95 A˚ ; 35.6 cm1, 3.35 A˚ ; 25.90 cm1, 3.31 A˚ ; and 5.98 cm1, 2.81 A˚ for O, C, N, and H atoms, respectively, as recommended in the Variflex code
[14]. For the anisotropic potential, the stretching potential is used in conjunction with the potential form of VAni= V0[1 (cos2(h1 h1e) cos2(h2
h2e)]. Where, V0is the stretching potential which
is represented by a Morse potential in this work as mentioned above; h1and h2are the rotational
angles between the fragments 1 and 2 and the refer-ence axis; h1eand h2erepresent the equilibrium
bon-ging angles of fragments 1 and 2.
The tunneling effect on the transition states, D-O-TS1 in Channel (7), Q-O-TS1 in Channel (8), Q-O-TS3in Channel(9), D-O2-TS1in
Chan-nel (10), D-O2-TS4 in Channel (12), and Q-O2
-TS1in Channel(13)are considered because their
barriers are higher than the reactants [13,23]. In this study, the tunneling effects are treated using the Eckart’s tunneling corrections.
3.3.2. Predicted rate constants for O(3P) + HNCN
For the reaction channels of the O(3P) +
HNCN, only k4 forming D-O-IM2has a strong
pressure dependence in the temperature range of 300–3000 K. When the pressure increases from 1 Torr to 10 atmospheres, k4 producing HN
(O)CN (shown inFigure S1 of Supplemental data) increases too. In addition, the values of k4
decrease with increasing temperature from 300 to 3000 K. The rate constants k1, k2, k3, k5, k6,
k7, k8, and k9 have no pressure dependence in
the whole temperature range. The predicted indi-vidual and total rate constants for the primary products at 760 Torr Ar pressure in the tempera-ture range of 300–3000 K are shown inFig. 5(a). As shown in the figure, the primary products of the O(3P) + HNCN reaction are NO + CNH with the rate constants (1.26–1.53) 1010 cm3 mole-cule1s1in the whole temperature range investi-gated. The second, third, and fourth key products are the3NH + NCO with the rate constants in the
range (5.70–6.70) 1011cm3molecule1s1, the CN + HNO with the rate constants in the range (2.59–14.30) 1012cm3molecule1s1, and the OH +3NCN with the rate constants varying in the range (2.30–6.74) 1013cm3molecule1s1, respectively. The production of D-O-IM2through
collisional quenching is negligible.
The total rate constant ktotal (O + HNCN)¼
P
kifor all accessible channels have no pressure
dependence either in the temperature range 300– 3000 K because the primary channels have no pressure effects.
The predicted individual and total rate con-stants given in units of cm3molecule1s1at the
760 Torr Ar pressure in the temperature range 300–3000 K can be represented by:
k1¼ 2:46 1010 T0:08expð11=T Þ; k2¼ 1:90 1010 T0:13expð156=T Þ; k3¼ 6:64 1028 T4:02expð1288=T Þ; k4¼ 1:57 10þ16 T10:47expð2675=T Þ; k5¼ 3:46 1015 T0:47expð844=T Þ; k6¼ 1:05 1013 T0:62expð95=T Þ; k7¼ 5:89 10þ4 T5:79expð8674=T Þ; k8¼ 2:45 102 T3:37expð2732=T Þ; k9¼ 1:43 10þ2 T5:14expð5039=T Þ; ktotalðO þ HNCNÞ ¼ 3:12 1010 T0:05expð37=T Þ:
Comparing with the OH + HNCN reaction
[12], ktotal(O + HNCN) is 2–15 times larger than
that of the OH + HNCN at T > 500 K under the atmospheric pressure condition. In addition, the primary products of O(3P) + HNCN are
NO + CNH instead of H2O + NCN in the
OH + HNCN reaction. Therefore, the O + HNCN reaction may be as important as OH + HNCN for prompt NO production.
3.3.3. Predicted rate constants for O2+ HNCN
In the O2+ HNCN reaction, all product
chan-nels have no pressure dependence in the whole temperature range studied. The predicted individ-ual rate constants for the primary products at 760 Torr Ar pressure in the temperature range of 300–3000 K are shown inFig. 5(b). As shown in
Fig. 5(b), the primary products for the O2+ HNCN are the HO2+3NCN with the rate
constants ranging from 6.62 1031 to 1.09 1013cm3molecule1s1 in the whole
tempera-ture range. The other products OH + NCNO with the rate constants varying in the range 1.53 1032–4.87 1017cm3molecule1s1 and O +
D-O-IM3 in the range 2.62 1041–1.00 1015
cm3molecule1s1 are small and can be neglected.
The predicted individual and total rate con-stants given in units of cm3 molecule1 s1 at the 760 Torr Ar pressure in the temperature range 300–3000 K can be represented by:
k10¼ 1:38 1045 T8:55expð6090=T Þ; k11¼ 1:72 1034 T5:92expð10900=T Þ; k12¼ 3:29 1015 T0:64expð19200=T Þ; k13¼ 2:67 1016 T1:25expð12300=T Þ; ktotalðO2þ HNCNÞ ¼ 2:10 1016 T1:28expð12200=T Þ:
Comparing with the OH + HNCN and O(3P) + HNCN reactions, ktotal(O2+ HNCN)
with the rate constants of 6.77 1031– 1.10 1013cm3molecule1s1 in the whole temperature range are much smaller than those of the OH + HNCN [12] and O(3P) + HNCN reactions. The results are similar to those pre-dicted for the NCN reactions with O, OH, and O2, in which the molecular oxygen reaction was
found to be extremely slow due to its high reaction barrier[24,25].
3.3.4. Predicted branching ratios
The branching ratios of the products for the O(3P) + HNCN and O
2+ HNCN at the 760 Torr
Ar-pressure in the temperature range of 300– 3000 K are shown in Fig. 6(a) and (b), respec-tively. For the O(3P) + HNCN reaction, k1 for
producing the NO + CNH accounts for 0.72– 0.64, k2+ k9 for producing the3NH + NCO
accounts for 0.27–0.32, and k6for producing the
CN + HNO accounts for 0.01–0.07 in the temper-ature range of 300–800 K. The branching ratios of k3+ k5+ k7+ k8for producing the OH +3NCN
and k4 for forming D-O-IM2 are negligible in
the whole temperature range. For the
Fig. 5. (a) The predicted rate constants of individual products and ktotalof the O(
3
P) + HNCN reaction at the 760 Torr Ar pressure in the temperature range of 300– 3000 K. (b) The predicted rate constants of individual products of the O2+ HNCN reaction at the 760 Torr Ar
O2+ HNCN reaction, k11+ k13for producing the
HO2+3NCN accounts for 0.98–1.00. The
branching ratios of k10 for producing the OH + 1
NCNO and k12 for producing the O +
D-O-IM3are negligible in the whole temperature range
studied.
4. Conclusions
The kinetics and mechanisms for the O(3P) + HNCN and O2+ HNCN reactions
tak-ing place via the doublet and quartet state PESs have been studied at the CCSD(T)/6-311+G(3df, 2p)//CCSD/6-311++G(d, p) level of theory. The total and individual rate constants for the primary channels of the two reactions in the temperature range of 300–3000 K are predicted. The total rate constants of the O(3P) + HNCN reaction produc-ing the primary products of NO + CNH is 2–15 times larger than that of the OH + HNCN pro-ducing the primary products of H2O + NCN
when the temperature is over 500 K. However, the total rate constants of the O2+ HNCN
reac-tion producing the primary products, HO2+3NCN, are much slower than those for
the OH + HNCN or O + HNCN reactions due to the much higher reaction barriers. Our
pre-dicted total and individual rate constants, and product branching ratios for the O(3P) + HNCN
and O2+ HNCN reactions may be employed for
combustion kinetic modeling of prompt NO formation.
Acknowledgements
The authors are grateful for the support of this work from the Basic Energy Sciences, Department of Energy, under contract no. DE-FG02-97-ER14784. MCL acknowledges the support by Taiwan’s National Science Council for a distin-guished visiting professorship at National Chiao Tung University, Hsinchu, Taiwan.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
doi:10.1016/j.proci.2008.07.011.
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