Computational Study on Reaction Mechanisms and Kinetics of

Apr 15, 2010 - Taipei 116, Taiwan, Department of Chemistry and Institute of Applied Chemistry, Chinese Culture UniVersity,. Taipei 111, Taiwan, Nation...
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J. Phys. Chem. A 2010, 114, 5894–5901

Computational Study on Reaction Mechanisms and Kinetics of Diazocarbene Radical Reaction with NO Han-Jung Li,† Hui-Lung Chen,*,‡ Jee-Gong Chang,§ Hsin-Tsung Chen,§ Shiuan-Yau Wu,† and Shin-Pon Ju*,| Department of Chemistry, National Taiwan Normal UniVersity, 88, Section 4, Tingchow Road, Taipei 116, Taiwan, Department of Chemistry and Institute of Applied Chemistry, Chinese Culture UniVersity, Taipei 111, Taiwan, National Center for High-Performance Computing, No. 28, Nan-Ke Third Road, Hsin-Shi, Tainan 74147, Taiwan, and Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun-Yat-Sen UniVersity, Kaohsiung 80424, Taiwan ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 29, 2010

The mechanisms of the reaction of the diazocarbene radical (CNN) with the NO have been investigated by ab initio molecular orbital in conjunction with variational TST and RRKM calculations. The potential energy surface (PES) was calculated by the high-level CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. From the calculated potential energy surface, we have predicted that the formation of N2O + CN (P5) is most favorable, and the calculated energies of reaction intermediates and transition structures along this path are all below the starting reference point. The predicted total rate constants, ktotal, at a 760 Torr Ar pressure can be represented by the equations: ktotal ) 2.47 × 10-17T1.20 exp(1.60 kcal mol-1/RT) at T ) 300-650 K and 2.49 × 10-19T1.82 exp(2.29 kcal mol-1/RT) at T ) 660-3000 K cm3 molecule-1 s-1. The calculated results also indicate that the branching ratio for RP5 in the temperature range 300-3000 K has the largest value. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for the possible explanation. 1. Introduction Atmospheric nitrogen oxides (NOx) play an important role in the formation of photochemical smog and acid-rain precursors, the destruction of ozone in the stratosphere, and possibly in greenhouse effect.1 Therefore, the mechanisms and rate parameters for reactions involving nitrogen compounds have been extensively investigated in relation to such air pollutants.2 In 1975 Lyon reported ammonia to rapidly and nearly quantitatively reduce NO to N2 and H2O if used in the presence of O2 at temperatures 900-1100 °C.3,4 This chemical process was quickly developed into what is called the thermal DeNOx process.5-7 There have been numerous reports on theoretical and experimental approaches to eliminate NO,8-16 many of which employ cyanogen species as an effective reagent to remove NO. Our interest has been focused on the nonhydrogenated carbene molecule, diazocarbene (CNN). Since Milligan and Jacox17 first produced in 1966 this diazocarbene by irradiation of cyanogen azide (N3CN) in solid argon and nitrogen, experimentalists and theoreticians have found of interest both its structure and spectra.18,19 The diazocarbene radical CNN has been proposed as an intermediate in the CH + N2 reaction, which is important in hydrocarbon combustion because of its possible role in the formation of “prompt” NO.20-24 The organic radical CH is known to cleave N2 to generate HCN and N atoms, and it is conjectured that the * Corresponding authors. E-mail: H.-L.C., [email protected]; S.-P.J., [email protected]. Tel: +886-2-28610511 ext 25313. Fax: +886-2-28614212. † National Taiwan Normal University. ‡ Chinese Culture University. § National Center for High-Performance Computing. | National Sun-Yat-Sen University.

adducts of HCNN and CNN are both important intermediates in this process.25,26 The diazocarbene radical is an important combustion species because it could provide a low energy pathway for cleaving N2 to produce N atoms that are then rapidly oxidized to nitric oxide.27 Our purpose in this study is to find the possible reactants that may react with NO possibly with lower energy barriers to form other species that are not harmful to our environment. To the best of our knowledge, however, no thorough computational studies regarding the CNN + NO reaction mechanisms and kinetic analyses are available. Here, we report ab initio calculations of the potential energy surface of this reaction to investigate its possible role as an NO removal route. In addition, the rate constants and branching ratios for the primary reaction channels in the temperature range of 300-3000 K have been predicted for combustion modeling applications. 2. Computational Method With the Gaussian 03 suite of programs28 we perform molecular orbital calculations involving density-functional theory (DFT) with Becke’s three-parameter (B3) exchange functional and the Lee-Yang-Parr (LYP) correlation functional (B3LYP)29,30 with a 6-311++G(3df,2p) basis set. The calculated equilibrium structures (local minima and saddle points) are characterized by calculations of harmonic vibrational frequencies at the same level of theory, with calculations of intrinsic reaction coordinates (IRC)31 to establish the link between transition states and intermediates. To obtain reliable energies, we perform single-point calculations employing a coupled-cluster technique with single and double excitations and evaluation of triple contributions by perturbation theory, CCSD(T),32,33 based on the geometries optimized at the B3LYP/6-311++G(3df,2p)

10.1021/jp1008016  2010 American Chemical Society Published on Web 04/15/2010

Kinetics of Diazocarbene Radical Reaction with NO TABLE 1: Electron Affinities (EA) and Single-Triplet Splitting Energies of CNN Calculated at Various Levels of Theory and Some Experimental Data from the Literature level of theory MP2/6-31++G(d,p) MP2/6-311++G(3df,2p) B3LYP/6-31++G(d,p) B3LYP/6-311++G(3df,2p) CCSD(T)/aug-cc-PVDZ //B3LYP/6-311++G(3df,2p) CCSD(T)/aug-cc-PVTZ //B3LYP/6-311++G(3df,2p) CCSD(T)/aug-cc-PVQZ //B3LYP/6-311++G(3df,2p) experiment

EAa (eV)

S-T splittingb (eV)

1.639 2.006 1.795 1.795 1.518

1.261 1.247 1.139

1.680

1.061

1.730

1.043

1.771 ( 0.010c

0.846 ( 0.014c

a Energy difference between anion CNN- and neutral CNN. Energy difference between the triplet and singlet electronic states of CNN. c Reference 25. b

level. The highest level of theory attained in this work is thus denoted CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p). Unless otherwise specified, the CCSD(T) single-point energies are used in the following discussion. The rate constants for the key product channels are computed with the variational transition-state theory (VTST) and microcanonical Rice-RamspergerKassel-Marcus (RRKM) theory34-37 using the VariFlex38 program. 3. Results and Discussion 3.1. Computational Condition Tests. In Table 1, we present data for the electron affinities (EA) and singlet-triplet splitting energies of CNN radical calculated at various levels of theory with pertinent experimental data from the literature. The electron affinity (EA) predicted by the hybrid density functional B3LYP method with a 6-311++G(3df,2p) basis set is 1.795 eV, which

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5895 is in satisfactory agreement with experimental value (1.771 ( 0.01 eV),25 whereas the calculated B3LYP energy for singlettriplet splitting of CNN radical, 1.247 eV, substantially overestimates the experimental value (0.846 ( 0.014 eV, Table 1).25 For this reason, we perform a single-point energy calculation at the CCSD(T)/aug-cc-PVQZ level, based on the geometries obtained from B3LYP/6-311++G(3df,2p) calculations, and obtain a more satisfactory result, 1.043 eV, which is much closer to the experimental value. In addition, we also employ the same method to calculate the ionization energy of NO and the electron affinity of NCO and CN; calculated values, IE(NO) ) 9.20 eV, EA(NCO) ) 3.62 eV, and EA(CN) ) 3.90 eV, are in good agreement with the experimental data (9.263 ( 0.01,39 3.609 ( 0.005,40 and 3.862 ( 0.004 eV,40 respectively). For this reason, we therefore choose the CCSD(T)/aug-cc-PVQZ// B3LYP/6-311++G(3df,2p) approach as the method for the characterization of the potential energy surface of the title reaction. 3.2. Potential Energy Surface for the CNN + NO Reaction. As shown in Figure 1, the examined reaction can proceed in five pathways, A-E, corresponding to five possible product formation channels. The intermediates are correspondingly numbered IM1-IM11 and the products in these five channels, NCN + NO, CNO + N2, CON + N2, NCO + N2, and N2O + CN, are located in the same order, P1-P5, respectively. TS1-TS20 denotes a transition-state species connecting two intermediates located at local minima. The geometries of the intermediates and transition states optimized at the B3LYP/6311++G(3df,2p) level are shown in Figures 2 and 3, respectively. The potential energy surfaces (PESs) calculated at the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) level are shown in Figure 4. All the calculated energies for the reactants, intermediates, and products are listed in Table 2, for the transition states in Table 3. Among them, the zero-point vibration energy (ZPE) correction is given, and the energies

Figure 1. Schematic diagram of proposed paths for the reaction CNN + NO.

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Figure 2. Optimized geometries of the relevant reactions, intermediates, and products on potential energy surfaces of CNN + NO reactions, calculated at the B3LYP/6-311++G(3df,2p) level. Bond lengths are given in Å and angles in degrees.

with respect to the reactant (CNN + NO) calculated at the CCSD(T) level are denoted as CRE. As shown in Figure 4, it is found that there exist two possible orientations for C-N bond addition, IM1 and IM4. Our calculated results for channels A and B are R f TS1 f IM1 f TS7 f IM2 f TS8 f IM3 f TS9 f P1 and R f TS1 f IM1 f TS11 f P2, respectively. In the former path, CNN and NO undergo the C-N bond addition producing the IM1, which then overcomes a barrier of 22.51 kcal/mol height forming a ring structure intermediate, IM2. The IM2 undergoes further reaction by passing a barrier (TS8, 8.79 kcal/mol) to open the ring by breaking the O-N bond and then forms the products NCN + NO (P1), with an overall exothermicity of 28.57 kcal/mol. However, for channel B, the IM1 adduct will proceed along another pathway, via TS11 (barrier height ) -23.37 kcal/mol, with respect to the reactants), to directly produce the final product CNO + N2 (P2) with an exothermicity of 64.86 kcal/mol. There exist two possible intermediate with different orientations for C-O bond addition, IM5 and IM6. Our calculated channel C is R f TS4 f IM6 f TS15 f P3. The highest barrier in this path is 29.17 kcal/mol at TS4 with an overall exothermicity of 7.10 kcal/ mol. In addition, with regard to N-N coordination, our calculated results for the channels D and E are R f IM7 f TS16 f IM10 f TS18 f P4 and R f IM7 f TS16 f IM10 f TS19 f P5, respectively. Similarly, these pathways involve the formation of another adduct CNNNO (IM7, -6.44 kcal/ mol) via a barrierless addition. This adduct, IM7, would overcome a relatively small energy barrier 5.20 kcal/mol (TS16), to form a more stable intermediate, IM10 (6.70 kcal/mol).

Finally, the intermediate IM10 might proceed through a C-O bond formation and N-N and N-O bonds dissociation on passing the transition structure (TS18) with a reaction barrier 15.54 kcal/mol, forming P4 (NCO + N2) with exothermicity of 127.25 kcal/mol. An alternative pathway is via a dissociation of N-N bond on crossing the transition structure (TS19) with a smaller barrier 11.63 kcal/mol and exothermic by 31.70 kcal/ mol. 3.3. Fukui Function Analysis. From the aforementioned results, it is found that the CNN + NO reaction may form the primary adduct, IM7, which is energetically more stable than the reactants by 6.44 kcal/mol. Besides, the formation of the six secondary adducts, IM1, IM4, IM5, IM6, IM8, and IM9, is also possible, although these pathways involve significant energy barriers. Obviously, the first adduct, IM7, possesses much higher stability than the others. To investigate this phenomenon, we calculate the Fukui functions41,42 and applied the theory of hard-and-soft acid-and-base (HSAB) to seek for the possible explanation. The extrapolation of the general behavior “soft likes soft” and “hard likes hard” locally, together with the idea that the larger the value of the Fukui function, the greater the reactivity, is also a very useful approach to explain the reactivity of many chemical systems.43-49 Clearly, the determination of the specific sites at which the interaction between two chemical species is going to occur is of fundamental importance for the determination of the path and the products of a given reaction. Ga´zquez et al.50 also stated that the largest value of the Fukui function is, in general, associated with the most reactive site. In our calculation for N electrons

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Figure 3. Optimized geometries of the relevant transition states on the potential energy surfaces of CNN + NO reactions, calculated at the B3LYP/6-311++G(3df,2p) level. Bond lengths are given in Å and angles in degrees.

in a system, independent calculations are made for the corresponding (N - 1), N, and (N + 1) electron systems with the same geometry. A natural population analysis yields qk(N 1), qk(N), and qk(N +1) for the predicted possible sites of reaction of CNN and NO molecules, and the Fukui function is calculated as a difference of populations between N and N + 1 or N and N - 1 electron systems.16 We choose the f 0 value for comparison since the CNN + NO reaction is more characteristic of a radical system.42 According to our calculated data in Table 4, it is found that the largest Fukui function (f 0, ca. 0.53) is on the N-t atom in the CNN radical, and that of the other reactant NO is on the N atom (ca. 0.63), which accounts for the formation of the complex IM7 to be more favorable than other complexes (IM1, IM4, IM5, IM6, IM8, and IM9). In addition, applying the HSAB theory, we find also that the largest values of the local softness s0 for both reactants are on the N-t atom of CNN and the N atom of NO (ca. 1.09 and 1.95), which also accounts for the favorable formation of the adduct of IM7. On the contrary, the smaller calculated values of the Fukui function, f 0 ) 0.372, and the local softness, s0 ) 1.155, on the O atom in NO, indicates an unfavorable formation through its O terminus site (such as IM5, IM6, IM8, and IM9). 3.4. Rate Constant Calculation. Variational TST and RRKM calculations have been carried out for this reaction with

the VariFlex code38 including the following more favorable reaction channels:

In our kinetic calculations, the effect of multiple reflections51,52 is included since this type of mechanism involved a molecular complex with a small well (IM7, -6.44 kcal/mol). The energies used in the calculation are plotted in Figure 4, and the vibrational frequencies and moments of inertia are listed in Table S1 (Supporting Information). In the scheme above, an activated molecule is marked by * and M is the third particle (Ar in this work). The Lennard-Jones (LJ) parameters employed for the CNN + NO reaction are as follows: for Ar,53 σ ) 3.47 Å, ε/k ) 114.0 K and for CNN-NO, σ ) 3.90 Å, ε/k ) 205.0 K, which are approximated to be the same as that of the NCO-NO system.9 For the variational rate constant calculations by the VariFlex code, a statistical treatment of the transitional-mode

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Figure 4. Calculated profiles of the potential-energy surface for possible paths in the reaction CNN + NO at the level of CCSD(T)/aug-cc-PVQZ// B3LYP/6-311++G(3df,2p).

TABLE 2: ZPE (hartree), Total Energies (TE, hartree), and Relative Energies (RE, kcal/mol) of Reactants, Intermediates, and Products Calculated at the Level B3LYP/ 6-311++G(3df,2p) (BTE), and at the Levels CCSD(T)/ aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) (CTE and CRE) for the Systems of CNN plus NO

TABLE 3: ZPE (hartree), Total Energies (TE, hartree), and Relative Energies (RE, kcal/mol) of Transition States Calculated at the Level B3LYP/6-311++G(3df,2p) (BTE) and at the Levels CCSD(T)/aug-cc-PVQZ//B3LYP/ 6-311++G(3df,2p) (CTE and CRE) for the Systems of CNN plus NO

species

ZPEa

BTE + ZPEb

CTE + ZPEb

CREc

species

ZPEa

BTE+ZPEb

CTE + ZPEb

CREc

R(CNN+NO) IM1 IM2 IM3 IM4 IM5 IM6 IM7 IM8d IM9d IM10 IM11 P1(NCN + NO) P2(CNO + N2) P3(CON + N2) P4(NCO + N2) P5(N2O + CN)

0.011577 0.018489 0.020041 0.019512 0.018779 0.016049 0.016334 0.016382 0.016414 0.017744 0.018136 0.018266 0.013015 0.014450 0.012432 0.015603 0.016097

-277.424227 -277.471958 -277.446647 -277.503259 -277.485992 -277.399212 -277.403606 -277.430564 -276.756595 -276.783526 -277.445532 -277.452776 -277.469834 -277.518929 -277.430078 -277.61833 -277.461908

-276.9917792 -277.033044 -277.025519 -277.0690741 -277.045202 -276.959840 -276.966477 -277.002048 -276.928398 -276.923104 -277.012723 -277.017898 -277.037315 -277.095142 -277.003098 -277.194570 -277.042296

0 -25.89 -21.17 -48.50 -33.52 20.04 15.88 -6.44 39.77 43.09 -13.14 -16.39 -28.57 -64.86 -7.10 -127.25 -31.70

TS1 TS2 TS3 TS4d TS5d TS6d TS7 TS8 TS9 TS10 TS11 TS12 TS13 TS14 TS15 TS16 TS17 TS18 TS19 TS20

0.014116 0.013793 0.014262 0.014037 0.019438 0.014258 0.016672 0.018233 0.014903 0.015724 0.016620 0.016110 0.013709 0.014541 0.013338 0.014814 0.015389 0.016569 0.015643 0.015299

-277.413145 -277.410098 -277.382272 -276.752397 -276.750608 -276.75479 -277.432564 -277.437105 -277.431974 -277.435861 -277.472000 -277.480255 -277.389757 -277.375704 -277.385461 -277.427206 -277.418387 -277.415327 -277.428447 -277.419285

-276.964077 -276.954211 -276.944360 -276.945294 -276.917724 -276.920879 -276.997162 -277.0115111 -277.0148092 -277.011031 -277.029019 -277.042817 -276.9618603 -276.937139 -276.957076 -276.993757 -276.982661 -276.987952 -276.994178 -276.983857

17.38 23.57 29.76 29.17 46.47 44.49 -3.38 -12.38 -14.45 -12.08 -23.37 -32.03 18.77 34.29 21.78 -1.24 5.72 2.40 -1.51 4.97

a Zero-point energy (au) at the level B3LYP/6-311++G(3df,2p). The unit of energy is hartree. c Relative energy (kcal/mol) with respect to the reactants. d The value is calculated at CCSD(T)/ aug-cc-PVQZ//MP2/6-311++G(3df,2p) level. b

a

contributions to the transition-state partition functions is performed variationally. On account of the absence of well-defined transition states for the initial association process, the potential function was computed variationally as a function of the bond length along the reaction coordinate R, which was evaluated according to the variable reaction coordinate flexible transitionstate theory.34-37,54 The Morse potential function V(R) ) De{1 - exp[-β(R - R0)]}2, was employed to approximate the

Zero-point energy (au) at the level B3LYP/6-311++G(3df,2p). The unit of energy is hartree. c Relative energy (kcal/mol) with respect to the reactants. d The value is calculated at CCSD(T)/ aug-cc-PVQZ//MP2/6-311++G(3df,2p) level. b

minimum energy path in our rate constant calculations. In the above equation, R is the reaction coordinate (i.e., the distance between the two interacting atoms), De is the bond energy excluding zero-point energy, and R0 is the equilibrium value of

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TABLE 4: Condensed Fukui Functions for C, N-c,a and N-tb Atoms in CNN, and N and O atoms in NO, and Global and Local Softnesses of the Molecules Calculated at the Level B3LYP/6-311++G(3df,2p) f0c molecule

N

O

NO CNN

0.628

0.372

a

local softness (s0)e

C

N-ca

N-tb

global softness Sd

N

O 1.155

0.035

0.527

3.104 2.076

1.949

0.508 b

C

N-ca

N-tb

1.055

0.073

1.094

c

N atom in the central position of CNN. N atom in the terminal position of CNN. Atomic charges according to a natural population analysis. d S ) 1/(IE - EA), with ionization energy IE and electron affinity EA; the energy unit is hartree. e s0 ) f 0 · S.

Figure 6. Predicted branching ratios for the primary reaction channels of CNN + NO reactions at 760 Torr Ar pressure in the temperature range 300-3000 K.

Figure 5. Predicted rate constants of (a) kIM10, kP4, and kP5 and (b) the total rate constants (ktotal ) kIM10 + kP4 + kP5) at Ar pressures of 760 Torr in the temperature range 300-3000 K.

R. The three parameters of this Morse potential are R0 ) 1.804 Å, β ) 1.974 Å-1, and De ) 6.44 kcal/mol for the IM7 formation. In addition, an energy grain size of 1.00 cm-1 is used for the convolution of the conserved mode vibrations, and a grain size of 80.00 cm-1 is used for the generation of the transitional-mode numbers of states of the transitional-mode contribution to the transition-state number of states for a given energy is evaluated via Monte Carlo integration with 10 000 configuration numbers. The energy-transfer process is computed on the basis of the exponential down model with a 〈∆E〉down value (the mean energy transferred per collision) of 400 cm-1 for Ar. In principle, the different 〈∆E〉down values should be determined by comparing the experimental pressure-dependent rate constants with the calculated values using the exponentialdown model in the solution of the master equation. Since there is no experimental value available for this system, our selected value of 〈∆E〉down for Ar, 400 cm-1, is taken from the published

computational works.55,56 To achieve convergence in the integration over the energy range, an energy grain size of 120 cm-1 is used. The total angular momentum J covered the range from 1 to 250 in steps of 10 for the E, J-resolved calculation. In the RRKM calculations, we neglect the pathways of the P1-P3 formation since their energy barriers are much higher than TS16, which is the rate-controlling step of our proposed predominant channel. The predicted values for kIM10, kP4, and kP5 and the total rate constants (ktotal ) kIM10 + kP4 + kP5) at Ar pressures of 760 Torr in the temperature range 300-3000 K are shown in Figure 5a,b, respectively. It can be seen that the value of kIM10 substantially decreases when the temperature increases from 660 to 3000 K. The values of kP4 and kP5 appear to be nearly independent of temperature, while the value of kP5 is constantly larger than kP4 in the temperature range 300-3000 K. The branching ratios for the three primary reaction channels (RIM10, RP4, and RP5) at Ar pressure of 760 Torr in the temperature range 300-3000 K are shown in Figure 6. The results indicate that the IM10 completely dissociates when the temperature goes higher than 660 K. In addition, P5 (N2O + CN) is the most significant product while the branching ratio of P4 (NCO + N2) approximates zero in the temperature range 300-3000 K. Figure 7 displays different 〈∆E〉down tests for the total rate constants (ktotal) as a function of pressure (Torr) at four different temperatures (300, 400, 500, and 750 K). Both the 300 and 400 cm-1 values for total rate constants are shown on the plot. At lower temperatures (300, 400, and 500 K), the 400 cm-1 result for the total rate constants is larger than the 300 cm-1 result. However, the difference becomes continuously smaller as the temperature increases to 750 K. We summarize the components of rate expressions (kIM10, kP4, and kP5) at eight specific pressures between 1 Torr and 100 atm in the temperature range 300-3000 K and list them in Table 5. As we can see, in this temperature range, the values of kIM10

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Figure 7. Different 〈∆E〉down tests for the total rate constants as a function of pressure (Torr) at four different temperatures (300, 400, 500, and 750 K).

TABLE 5: Predicted Rate Expressionsa of kIM10, kP4, and kP5 at Ar Pressures of 1, 10, 50, 380, and 760 Torr and 10, 50, and 100 atm in the Temperature Range 300-3000 K reaction kIM10

kP4 kP5

P

A

1 Torr 10 Torr 50 Torr 380 Torr 760 Torr 10 atm 50 atm 100 atm all pressures all pressures

1.29 × 10 5.67 × 1024 2.73 × 1026 7.99 × 1028 5.69 × 1029 7.01 × 1032 1.36 × 1035 1.19 × 1036 3.78 × 10-21 1.84 × 10-18 23

n

B

-14.20 -14.37 -14.62 -15.02 -15.16 -15.70 -16.12 -16.28 1.86 1.58

-4.85 -4.95 -5.31 -6.82 -6.73 -8.70 -10.43 -11.23 -0.51 1.68

Rate constants are represented by by k ) ATn exp(B kcal mol-1/ RT) in units of cm3 molecule-1 s-1. a

have pronounced pressure dependence; however, the values of kP4 and kP5 show nearly pressure-independence. The predicted total rate constants, ktotal, at a 760 Torr Ar pressure can be represented by the equations: ktotal ) 2.47 × 10-17T1.20 exp(1.60 kcal mol-1/RT) at T ) 300-650 K and 2.49 × 10-19T1.82 exp(2.29 kcal mol-1/RT) at T ) 660-3000 K, in units of cm3 molecule-1 s-1. At present, no comparison can be made for the calculated and experimental data. For this newly identified, potentially important, prompt NO precursor reaction, our results are recommended for high-temperature combustion modeling applications. 4. Conclusion In this Article, a combined high-level CCSD(T) method and RRKM based rate constant calculation study is performed on the mechanism of the CNN + NO reaction. The total and individual rate constants for the primary channels of the aforementioned reaction in the temperature range 300-3000 K are predicted. The most favorable association adduct, IM7, is dominant in the low-temperature range (T ) 300-650 K); over 660 K, formation of N2O + CN (P5) is the primary channel through the PESs of the CNN + NO reaction, while the branching ratio of NCO + N2 (P4) is nearly zero over the whole temperature range 300-3000 K. Our predicted total and individual rate constants and product branching ratios for this crucial reaction may be employed for combustion kinetic modeling applications, and the future experimental reinvestiga-

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