Ab Initio Chemical Kinetics for the OH + HNCN Reaction
† Shucheng Xu*,‡and M. C. Lin‡,§Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322, and Institute of Molecular Science, Department of Applied Chemistry, National Chiao Tung UniVersity, Hsichu, Taiwan 300
ReceiVed: December 29, 2006; In Final Form: May 17, 2007
The kinetics and mechanism of the reaction of the cyanomidyl radical (HNCN) with the hydroxyl radical (OH) have been investigated by ab initio calculations with rate constants prediction. The single and triplet potential energy surfaces of this reaction have been calculated by single-point calculations at the CCSD(T)/ 6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++ G(d,p) levels. The rate constants for various product channels in the temperature range of 300-3000 K are predicted by variational transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate constants can be represented by the expressions ktotal) 2.66 × 10+2× T-4.50exp(-239/T) in which T ) 300-1000 K and 1.38× 10-20× T2.78exp(1578/T) cm3molecule-1s-1where T ) 1000-3000 K. The branching ratios of primary channels are predicted: k1for forming singlet HON(H)CN accounts for 0.32-0.28, and k4for forming singlet HONCNH accounts for 0.68-0.17 in the temperature range of 300-800 K. k2+ k7for producing H2O + NCN accounts for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k3for producing HCN + HNO, k6for producing H2N + NCO, k8for forming 3HN(OH)CN, k
9for producing CNOH +3NH, and k5+ k10for producing NH2+ NCO are negligible. The rate constants for key individual product channels are provided in a table for different temperature and pressure conditions.
Introduction
The cyanomidyl radical (HNCN) is a reactive transient species that plays an important role in a variety of chemical environ-ments, including prompt NO formation in hydrocarbon combus-tion, interstellar chemistry, and primordial reactions leading to the synthesis of amino acids from simple inorganic compounds. Experimentally, the HNCN radical was first identified spectro-scopically by Herzberg and Warsop in 1963.1More recently, Wu et al.2probed the B2A′
r X2A′′transition with laser-induced fluorescence, Yamamoto and Saito3 reported the microwave spectrum of HNCN, and Clifford et al.4studied the photoelectron spectrum of the HNCN-ion. In 2001, the photodissociation spectroscopy and dynamics of the HNCN radical were inves-tigated by Bise et al.5
Theoretically, ab initio calculations of the molecular geometry and vibrational frequencies of the HNCN ground state were first made by Tao et al. in 1994,6and then more recently by Puzzarini et al. in 2005.7In this laboratory, we first proposed HNCN to be the key stable intermediate of the new prompt NO formation reaction by CH+N2along a spin-allowed doublet electronic state path:8
where cycl-HC-NdN* includes two cyclic isomers and “/” denotes internal excitation. Under high-pressure combustion conditions, the collisional stabilization of the excited HNCN
by the second step given above may provide a high concentra-tion of HNCN radicals.
The reaction of HNCN with OH should therefore play an important role in the oxidation of HNCN, producing prompt NO precursors such as NCN, HNO, and HCN. In the literature, there have been no reports on the kinetics and mechanism for the reaction of OH with HNCN experimentally or theoretically. In this work, the singlet and triplet potential energy surfaces (PESs) of the OH + HNCN reaction have been calculated at the CCSD(T)/6-311+G(3df,2p) level of theory. 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.
Computational Methods
The optimized geometries of the reactants, transition states, intermediate complexes, and products for the reaction of OH + HNCN have been calculated at the B3LYP/6-311+G(3df,2p) level. In addition, the optimized geometries of primary channels for singlet PESs and triplet PESs for this reaction have been calculated at the higher CCSD/6-311++G(d,p) level in addition to B3LYP/6-311+G(3df,2p). The energies for the singlet and triplet PESs are improved by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level of theory based on the opti-mized geometries at the B3LYP/6-311+G(3df,2p) and CCSD/ 6-311++G(d,p) levels, which have been performed successfully for the reaction of OH + CH2O.9
The rate constants for the key product channels were computed with variational transition-state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory using the VariFlex code.10 All quantum chemistry calculations were carried out by the Gaussian 0311package using a PC cluster †Part of the special issue “M. C. Lin Festschrift”.
* Corresponding author. E-mail: sxu@emory.edu.
‡Emory University.
§National Chiao Tung University.
CH + N2f cycl-HC-NdN* f HNCN* f H + NCN f HNCN (+M)
6730 J. Phys. Chem. A 2007, 111, 6730-6740
10.1021/jp069038+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007
and the computers at the Cherry L. Emerson Center for Scientific Computation at Emory University.
Results and Discussion
1. PESs and the Reaction Mechanism. The optimized geometries for the species involved in the reaction of OH with HNCN at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++ G(d,p) (data in parenthesis) levels are shown in Figure 1. The parameters of optimized geometries using both the B3LYP and CCSD methods are close to each other except for the distance of HO-HNCN in t-TS1. The energies for all the species obtained by the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+ G(3df,2p), and CCSD(T)/6-311+G(3df,2p)//CCSD/6-311++ G(d,p) methods are listed in Table 1. On average, the relative energies for the species using the CCSD geometries were found to be about 0.4 kcal/mol higher than those using the B3LYP geometries. The reaction of OH with HNCN can occur on both singlet and triplet PESs. For the singlet PES, it was found to be very complicated because of the existence of the resonance structures: HN-CtN S HNdCdN, which give rise to 3 association complexes with 14 isomerization channels and 15 dissociation processes, producing 19 products. For the triplet PES, it was found that there was one hydrogen abstraction channel and two addition channels. We discuss the PESs and reaction mechanism in the following four sections (1a, 1b, 1c, and 1d). In order to simplify the discussion of the singlet and triplet PESs, we only mention the energies using the B3LYP geometries in these sections because the energies using the CCSD geometries are close to those using the B3LYP geom-etries, as listed in Table 1 and shown in parentheses in Figure 2a-c.
1a. Formation and Isomerization of Singlet Intermediates. As shown in Scheme 1 and Figure 2a, the reaction of OH with HNCN first forms primary intermediates trans-HON(H)CN (denoted as t-HON(H)CN; dihedral angle HONC ) 118.8°) with a binding energy of 46.9 kcal/mol, cis-HON(H)CN (denoted as c-HON(H)CN; dihedral angle HONC ) -56.3°) with a binding energy of 44.2 kcal/mol when the OH attacks the N atom next to H, and also forms HONCNH with a binding energy of 41.4 kcal/mol when the OH associates with the terminal N atom.
As shown in Scheme 1, t-HON(H)CN and c-HON(H)CN are two conformers of HON(H)CN and can transform to each other by an internal rotation about the O-N bond via TS19(dihedral angle HONC ) -132.3°) with a barrier of 6.9 kcal/mol. HON-(H)CN can also isomerize to HONCNH via TS20with a barrier of 68.2 kcal/mol. Furthermore, HON(H)CN and HONCNH can isomerize to nine other intermediates. For example, t-HON(H)-CN can transform to ON(H2)CN via TS3with a barrier of 56.7 kcal/mol or to t-ONC(H)NH via TS4with a barrier of 71.1 kcal/ mol. HONCNH can isomerize to t-NC(NH)OH via TS9with a barrier of 39.9 kcal/mol or to ON(H)CNH via TS13with a barrier of 59.8 kcal/mol. Similarly, t-ONC(H)NH can transform to c-ONC(H)NH via TS8 with a barrier of 25.1 kcal/mol or to ONCNH2via TS6with a barrier of 73.6 kcal/mol, and t-NC-(NH)OH can transform to c-NCt-NC-(NH)OH via TS11with a barrier of 5.2 kcal/mol or t-HOCNNH via TS10with a barrier of 50.0 kcal/mol. In addition, NC(NH)OH can transform to c-HOCNNH through TS12with a barrier of 53.4 kcal/mol, and ON(H)CNH can transform to t-ONC(H)NH TS17with a barrier of 36.1 kcal/mol or to c-ONC(H)NH through TS14with a barrier of 33.0 kcal/mol. These isomerization reactions can also occur reversely, as one would expect.
1b. Primary Singlet Product Channels. As shown in Figure 2a, the OH + HNCN reaction may generate the following
primary products with predicted enthalpy changes: H2O + 1NCN (a˜1∆
g), -4.8 kcal/mol; HCN + HNO, -28.3 kcal/mol; HNC + HNO, -14.6 kcal/mol; and H2N + NCO, -11.6 kcal/ mol. The 1NCN product is formed by H
2O elimination from the primary intermediates, t-HON(H)CN and c-HON(H)CN, by overcoming the barriers of 49.4 kcal/mol at TS1 and 49.3 kcal/mol at TS2. The1NCN (a˜1∆g) product is the first excited state of NCN, which is predicted to be higher than the ground state X˜3Σ
g-by 30.1 kcal/mol at both the CCSD(T)//B3LYP and CCSD levels; the value is close to the previously predicted 30.7 kcal/mol calculated using the ROHF-CCSD(T)/pVTZ method by Martin et al.,12and 28.8 kcal/mol calculated using the CBS-QCI/APNO method by Clifford et al.4These values are clearly higher than the reported experimental result of 23.2 ( 0.2 kcal/ mol.13,14As the CCSD(T) is a single-reference method, we also performed a large-scale multireference calculation for the (a˜1∆
g -X˜3Σ
g-) energy difference using the CASPT2(8,8)/6-311+ G(3df,2p) method based on the geometries optimized at the CASSCF(8,8)/6-311+G(3df,2p) level. Here we selected eight active electrons of 3σµ21πµ41πg2and eight active orbitals for both states (1σg21σµ22σg23σg22σµ24σg23σµ21πµ41πg2). The predicted value is 29.4 kcal/mol, which is very close to the ones obtained by the single-reference methods cited above, but is noticeably higher than the 23.2 kcal/mol experimental result.
In addition, the products HCN + HNO may be produced by the dissociation of intermediates t-HON(H)CN, ON(H2)CN, and t-ONC(H)NH via TS21with a barrier of 87.2 kcal/mol, TS18with a barrier of 23.0 kcal/mol, and TS7with a barrier of 75.4 kcal/ mol, respectively. Furthermore, the products HNC + HNO may be produced by the dissociation of ON(H)CNH via TS15with a barrier of 19.2 kcal/mol. Similarly, the H2N + NCO products may be formed by the dissociation of t-NC(NH)OH via TS16 with a barrier of 47.6 kcal/mol.
The predicted heats of reaction for the formation of H2O + 3NCN (-34.9 kcal/mol at both the CCSD(T)//B3LYP and CCSD(T)//CCSD levels), HCN + HNO (-28.3 kcal/mol at both the CCSD(T)//B3LYP and CCSD(T)//CCSD levels), and HNC + HNO (-14.6 kcal/mol at the CCSD(T)//B3LYP and -13.8 kcal/mol at the CCSD(T)//CCSD level) from OH + HNCN listed in Table 1 are in reasonable agreement with the available experimental values at 0 K (-30.7 ( 4.0 kcal/mol, -25.8 ( 2.4 kcal/mol, and -12.4 ( 1.9 kcal/mol, respectively), based on∆fH0(OH) ) 8.87 ( 0.07 kcal/mol,15∆fH0(HNCN) ) 72.3 ( 0.7 kcal/mol,5 ∆
fH0(H2O) ) -57.10 ( 0.01 kcal/mol,15 and ∆
fH0(3NCN) ) 107.6 ( 3.2 kcal/mol derived from ∆fH298(3NCN) ) 107.7 ( 3.2 kcal/mol,4 ∆fH0(HCN) ) 30.9 ( 0.7 kcal/mol,5∆fH0(HCN) ) 44.3 ( 0.9 kcal/mol,5and∆
fH0(HNO) ) 24.5 kcal/mol.16For3NCN, Bise et al. obtained an experimental value of∆fH0(3NCN) ) 111.4 ( 0.7 kcal/mol.13This value would give rise to the exper-imental heat of reaction for H2O +3NCN formation, -26.8 ( 1.5 kcal/mol, which is 8.1 kcal/mol higher than the predicted result.
1c. Secondary Singlet Product Channels. As shown in Figure 2b, the OH + HNCN reaction may produce the following minor products with predicted enthalpy changes: H2+ ONCN, -9.4 kcal/mol; NO + t-HNCH, 1.3 kcal/mol; HNN + HOC, 25.7 kcal/mol; HNC + HON, 27.6 kcal/mol; CN + c-HNOH, 43.7 kcal/mol; CN + t-HNOH, 48.9 kcal/mol; and H2N + CNO, 51.0 kcal/mol. The H2 + ONCN products may be formed by the dissociation of the intermediate ON(H2)CN by over-coming the barrier of 46.3 kcal/mol at TS5. The products NO + t-HNCH may be produced by the direct barrierless
Figure 1. Optimized geometries of the reaction OH + HNCN computed at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) (data in
parenthesis) levels.
dissociation of t-ONC(H)NH and c-ONC(H)NH with dissoc-iation energies of 44.4 and 41.4 kcal/mol, respectively. Similarly, the production of other radical product pairs takes place
by barrierless dissociation processes with the predicted endothermicities: HNN + HOC from t-HOCNNH and c-HOCNNH, 53.3 and 50.3 kcal/mol, respectively; HNC + HON
TABLE 1: Total and Relative Energiesaof Reactants, Transition States, and Products of the Reaction OH + HNCN
B3LYP/6-311+G(3df,2p)
species or reactions ZPE energies
CCSD(T)b/6-311+G(3df,2p) B3LYP/6-311+G(3df,2p) CCSD(T)c/6-311+G(3df,2p) CCSD/6-311++G(d,p) ∆H0_exptd OH + HNCN 0.028085 -223.923551 -223.497423 -223.497792 (0.0) t-HON(H)CN 6.7 -41.3 -46.9 -46.5 c-HON(H)CN 6.5 -39.0 -44.2 -43.8 HONCNH 6.0 -40.6 -41.4 -41.3 t-NC(NH)OH 6.3 -28.6 -36.1 c-NC(NH)OH 6.4 -29.1 -36.7 -36.2 ON(H)CNH 5.7 -13.4 -12.4 -12.4 t-ONC(H)NH 5.7 -25.4 -33.1 c-ONC(H)NH 5.6 -22.9 -30.1 ONCNH2 5.9 -32.1 -31.7 t-HOCNNH 6.3 -26.4 -27.6 c-HOCNNH 5.9 -23.0 -24.6 ON(H2)CN 6.6 -2.4 -8.6 -8.1 TS1 1.7 4.8 2.5 3.2 TS2 1.6 7.5 5.1 5.7 TS3 3.8 14.7 9.8 10.2 TS4 2.8 32.9 24.2 TS5 0.1 44.8 37.7 TS6 1.9 39.3 40.4 TS7 -0.1 49.0 42.3 TS8 3.8 -2.9 -8.0 TS9 4.2 0.6 -1.5 -0.1 TS10 4.3 20.6 13.9 TS11 5.5 -22.7 -30.8 TS12 4.3 22.1 16.6 TS13 2.0 16.4 18.4 18.8 TS14 1.8 19.3 20.6 TS15 3.3 7.5 5.8 6.2 TS16 2.1 17.0 11.4 12.2 TS17 1.5 22.1 23.7 TS18 3.1 17.3 14.4 14.9 TS19 6.0 -34.6 -40.0 -39.7 TS20 2.2 31.6 24.0 TS21 1.5 51.2 40.3 3OH‚‚‚N(H)CN 1.6 -1.3 -1.9 -1.9 t-TS1 1.8 -1.6 -1.1 -0.6 tc-3HN(OH)CN 5.1 -28.2 -30.9 -30.5 cc-3HN(OH)CN 5.3 -30.0 -32.2 -32.0 ct-3HN(OH)CN 5.5 -31.2 -33.9 -33.5 3H 2NC(O)N 5.5 -39.6 -42.2 -42.0 t-TS2 1.8 0.1 2.7 2.7 t-TS3 1.7 4.7 7.2 7.3 t-TS4 2.5 9.1 4.2 5.3 t-TS5 4.6 -25.2 -27.8 -27.3 t-TS6 4.2 -12.3 -14.7 -14.1 t-TS7 2.8 5.2 1.1 2.1 t-TS8 2.1 5.4 3.5 4.0 t-TS9 2.2 -5.7 -3.9 -2.9 H2O +1NCN 0.7 -0.9 -4.8 -4.8 H2O +3NCN 1.1 -33.2 -34.9 -34.9 -30.7 ( 4.0 HCN + HNO 1.3 -19.2 -28.3 -28.3 -25.8 ( 2.4 HNC + HNO 0.2 -6.4 -14.6 -13.7 -12.4 ( 1.9 H2N + NCO 0.6 -11.7 -11.6 -11.5 3NH + NCOH 0.6 2.3 -4.9 -4.9 H2+ ONCN -3.0 -1.4 -9.4 NO + t-HNCH 1.4 4.3 1.3 HNN + HOC -1.0 27.1 25.7 HNC + HON -0.1 33.8 27.6 CN + t-HNOH 2.4 49.7 43.7 CN + c-HNOH 2.0 54.7 48.9 H2N + CNO -0.1 50.7 51.0
aTotal energies for OH + HNCN are in a.u., and relative energies for others are in kcal mol-1.bSingle-point energies based on optimized
geometries calculated at the B3LYP/6-311+G(3df,2p) level.cSingle-point energies based on optimized geometries calculated at the
CCSD/6-311++G(d,p) level.dAt 0 K,∆
fH0values are as follows: ∆fH0(OH) ) 8.87 ( 0.07 kcal/mol,15∆fH0(HNCN) ) 72.3 ( 0.7 kcal/mol,5∆fH0(H2O)
) -57.10 ( 0.01 kcal/mol,15and∆
fH0(3NCN) ) 107.6 ( 3.2 kcal/mol, derived from∆fH298(3NCN) ) 107.7 ( 3.2 kcal/mol,4∆fH0(HCN) ) 30.9
( 0.7 kcal/mol,5∆
from HONCN, 69.0 kcal/mol; CN + t-HNOH from t-HON-(H)CN, 90.6 kcal/mol; CN + c-HNOH from c-HONt-HON-(H)CN, 93.1 kcal/mol, and, finally, H2N + CNO from ONCNH2, 82.7 kcal/mol.
1d. Triplet Product Channels. As shown in Figure 2c, for the OH + HNCN reaction, we have found one triplet abstraction channel and two triplet addition channels. For the triplet abstraction channel, the reactants of OH radical with HNCN radical first form a triplet precomplex3OH‚‚‚N(H)CN with a
1.9 kcal/mol binding energy, and the H-abstraction takes places via triplet t-TS1 with a 0.7 kcal/mol barrier to produce the products H2O +3NCN (X˜3Σg-). For the triplet addition channels, the reaction of OH with HNCN forms triplet intermediate tc-3HN(OH)CN with -30.9 kcal/mol exothermicity via triplet t-TS
2 with a 2.7 kcal/mol barrier or cc-3HN(OH)CN with -32.2 kcal/ mol exothermicity via triplet t-TS3with a 7.2 kcal/mol barrier. cc-3HN(OH)CN can transform to tc-3HN(OH)CN by an internal rotation about the C-O bond via t-TS6with a barrier of 17.5
Figure 2. (a) Singlet PES of the primary product channels of OH + HNCN calculated at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+
G(3df,2p) and CCSD(T)/6-311+G(3df,2p)//CCSD/6-311++G(d,p) (data in parenthesis) levels. (b) Singlet PES of the secondary product channels of OH + HNCN calculated at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) level. (c) Triplet PES of the reaction of OH + HNCN calculated at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) and CCSD(T)/6-311+G(3df,2p)//CCSD/6-311++G(d,p) (data in parenthesis) levels.
kcal/mol, and can also transform to another conformer ct-3 HN-(OH)CN with -33.9 kcal/mol exothermicity by an internal rotation about the C-N bond via t-TS5with a barrier of 5.4 kcal/mol. The intermediates cc-3HN(OH)CN and ct-3 HN(OH)-CN can dissociate to the products of singlet HN(OH)-CNOH +3NH with -4.9 kcal/mol exothermicity via t-TS4 with a barrier of 36.4 kcal/mol and t-TS7with a barrier of 35.0 kcal/mol, respectively. In addition, ct-3HN(OH)CN can isomerize to another intermedi-ate3H
2NC(O)N with -42.2 kcal/mol exothermicity via t-TS8 with a barrier of 37.4 kcal/mol. Furthermore,3H
2NC(O)N can dissociate to produce H2N +NCO via t-TS9with a barrier of 46.1 kcal/mol. Interestingly, here we found that the geometries of all triplet intermediates are planar.
2. Rate Constant Calculations for the Primary Reaction Channels of OH + HNCN. 2a. Methods Employed for Rate Constant Calculations. The rate constants for the following primary singlet and triplet reaction channels of OH + HNCN have been predicted by statistical calculations:
The rate constants for the reactions of OH with HNCN with the primary six singlet and four triplet channels have been calculated using variational TST and RRKM theory by the VariFlex Code10in the temperature range 300-3000 K with Ar as the bath gas. Channel 1 is an association reaction forming the intermediate HON(H)CN, whose two conformers, t-HON-(H)CN and c-HONt-HON-(H)CN, are treated as one intermediate via hindered rotation about the O-N bond with a barrier of 6.9
kcal/mol. Channel 2 is a dissociation reaction via the intermedi-ate HON(H)CN and two dissociation paths by transition stintermedi-ates TS1 and TS2 to produce the same H2O + 1NCN products through the t- and c-conformers, respectively. Channel 3 is a dissociation reaction via the intermediate HON(H)CN and transition states TS3 and TS18to produce the HCN + HNO products, where the primary controlling transition state TS18 and secondary transition state TS3 with a relatively shallow intermediate ON(H2)CN can be treated as a combined transition state by the multiple reflection treatment.9 Channel 4 is an association reaction forming the intermediate HONCNH. Chan-nel 5 is treated as a dissociation reaction to H2N + NCO via the intermediate HONCNH and the primary transition state TS16 because the barrier of TS16is 13.0 kcal/mol higher than that of the secondary transition state TS9lying -1.5 kcal/mol below the reactants. Channel 6 is treated as a dissociation reaction to HNC + HNO via the intermediate HONCNH and the primary transition state TS13because the barrier of TS13is 14.6 kcal/ mol higher than that of the exit transition state TS15.
Channel 7 is a triplet H-abstraction reaction through the precomplex3OH‚‚‚N(H)CN and the triplet transition state t-TS
1 to produce the H2O + 3NCN products, where the energy of t-TS1is 0.6 kcal/mol lower than the reactants calculated at the CCSD(T)//CCSD level. The existence of the preassociation complex has been shown to have a significant effect on the predicted rate constants due to multiple reflections above the well of the complex in previous studies,9,17when the energy of an exit transition state is close to that of the reactants. Therefore, the effect of multiple reflections was examined for channel 7. Channel 8 is an association reaction forming the triplet intermediate tc-3HN(OH)CN via t-TS
2 or its conformer cc-3HN(OH)CN via t-TS
3. Channel 9 is a dissociation reaction via the intermediate 3HN(OH)CN and two dissociation paths by transition states t-TS4and t-TS7to produce the same CNOH + 3NH products. Channel 10 is treated as a dissociation reaction to H2N + NCO via transition states t-TS3, t-TS8, and t-TS9, where t-TS9is treated as a secondary transition state of t-TS8. Although these treatments for channels 3, 5, and 6 may have systematic errors, they have negligible effects on the total rate constants because the values of the rate constants of channels 3, 5, and 6 are negligibly small, even at high temperatures, as will be discussed later. The rate constants for these channels are based on the molecular parameters, including the geometries, vibrational frequencies, and rotational constants calculated at SCHEME 1: Formation and Isomerization of Intermediates of OH + HNCN, Where Data in Parentheses Are Relative Energies in kcal/mol OH + HNCN f1HON(H)CN* f HON(H)CN (+M) (1) f H2O + 1NCN (2) f HCN + HNO (3) OH + HNCN f1HONCNH* f HONCNH (+M) (4) f H2N + NCO (5) f HNC + HNO (6) OH + HNCN f3OH‚‚‚N(H)CN* f H2O + 3 NCN (7) OH + HNCN f3HN(OH)CN* f3HN(OH)CN (+M) (8) f CNOH +3NH (9) f H2N + NCO (10)
the higher CCSD/6-311++G(d,p) level and the PESs calculated at the CCSD(T)/6-311+G(3df,2p) level listed in Table 1.
For the barrierless association processes OH + HNCN f HON(H)CN shown in Figure 3 a and OH + HNCN f HONCNH shown in Figure 3b, the minimum energy paths (MEPs) for forming the primary intermediates, HON(H)CN and HONCNH, were obtained by computing the potential energy curves along the reaction coordinate from 1.5 to 5.0 Å with a step size of 0.1 Å estimated at the UB3LYP/6-311+G(3df,2p) level. Similarly, for the barrierless association processes OH + HNCN f triplet precomplex3OH‚‚‚N(H)CN shown in Figure 3c, the MEP was obtained by computing the potential energy curves along the reaction coordinate from 2.1 to 5.0 Å with a step size of 0.1 Å at the UB3LYP/6-311+G(3df,2p) level. The calculated MEPs could be fitted to the Morse potential function with the parameters,β ) 1.502 Å-1with R0) 1.428 Å and De ) 53.6 kcal/mol; β ) 1.416 Å-1with R
0) 1.415 Å and De) 47.5 kcal/mol; andβ ) 1.573 Å-1with R0) 2.109 Å and De
) 3.4 kcal/mol for forming HON(H)CN, HONCNH, and triplet 3OH‚‚‚N(H)CN, respectively, where the energies for D
e were scaled at 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 contributions to the transition-state partition functions was performed varia-tionally. The numbers of state are evaluated according to the variable reaction coordinate flexible transition-state theory;10,18 an energy grain size of 1.00 cm-1was used for the convolution of the conserved mode vibrations, and a grain size of 50.00 cm-1 was used for the generation of the transitional-mode numbers of states. The estimate 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 was com-puted on the basis of the exponential down model with a 〈∆E〉downvalue (the mean energy transferred per collision) of 400 cm-1 for Ar. In order to achieve convergence in the
Figure 3. (a) MEPs (b) of OH + HNCN f HON(H)CN along the reaction coordinate of O-N calculated at the B3LYP/6-311+G(3df,2p) level
with the fitted Morse curves (solid curve). (b) MEPs (9) of OH + HNCN f HONCNH along the reaction coordinate of O-N calculated at the B3LYP/6-311+G(3df,2p) level with the fitted Morse curves (dotted curve). (c) MEPs (O) of OH + HNCN f3OH‚‚‚N(H)CN along the reaction coordinate of H-N calculated at the B3LYP/6-311+G(3df,2p) level with the fitted Morse curves (dotted curve).
integration over the energy range, an energy grain size of 100 cm-1 was used. The total angular momentum J covered the range from 1 to 250 in steps of 10 for the E,J-resolved calculation. The Morse potentials with the above-mentioned parameters, the Lennard-Jones pairwise potential, and the
anisotropic potential are added together to form the final potential, similar to that employed in the OH + CH2O,9OH + CH3OH, and OH + C2H5OH reactions.17
The tunneling effect on the transition statessTS1and TS2in channel 2, TS18and TS3in channel 3, TS16in channel 5, TS13
Figure 4. Predicted rate constants of k1(a), k2(b), k4(c), k9(e), and ktotal(f) at Ar pressures of 1 Torr, 10 Torr, 100 Torr, 300 Torr, 760 Torr, and 10 atm, and k7(d) at a high-pressure limit in the temperature range of 300-3000 K.
Figure 5. Predicted branching ratios for the 10 primary singlet and triplet reaction channels of the reaction OH + HNCN at 760 Torr Ar pressure
in channel 6, and t-TS2, t-TS3t-TS4, t-TS7, and t-TS8in channels 8, 9, and 10sare considered because their barriers are much higher than those of the reactants.9,17In this study, the tunneling effects are treated using Eckart’s tunneling corrections.
Finally, HON(H)CN, HONCNH, TS18, and TS3in channel 3, TS16in channel 5, and TS13in channel 6 have their own optical isomers, so a statistical factor of 2 is employed in these rate constant calculations.
2b. Predicted Rate Constants. The predicted values for k1 forming singlet HON(H)CN at six specific pressures between 1 and 7600 Torr in the temperature range of 300-3000 K are shown in Figure 4 a and are also listed in Table 2. The values of k1decrease with increasing temperature from 300 to 3000 K. In addition, k1 has a strong pressure dependence in the temperature range below 1000 K, as one expects. When the pressure increases from 1 Torr to 10 atm, k1increases, as clearly illustrated by Figure 4a and the results listed in Table 2.
The predicted results for k2producing H2O +1NCN products at six specific pressures between 1 and 7600 Torr in the temperature range of 300-3000 K are shown in Figure 4b and are also listed in Table 2. k2is the sum of contributions from the reactions via TS1and TS2, in which the rate constants from the contribution via TS1account for 0.96-0.53 in the temper-ature range of 300-3000 K. The properties of k2are completely different from those of k1. k2increases with increasing temper-ature from 300 to 3000 K; it has a weak pressure dependence
only at temperatures below 500 K due to competition with k1 resulting from collisional deactivation of the excited HON(H)-CN.
The predicted values for k4forming singlet HONCNH at six specific pressures between 1 and 7600 Torr in the temperature range of 300-3000 K are shown in Figure 4c and are also listed in Table 2. k4as an association process has pressure/temperature (P,T) dependences similar to those of k1. k4 decreases with increasing temperature from 300 to 3000 K. In addition, k4also has a strong pressure dependence in the temperature range below 1000 K, as does k1. When the pressure increases from 1 Torr to 10 atm, k4increases, as clearly illustrated by Figure 4c and the results listed in Table 2.
The predicted pressure-independent values for k7, producing H2O + 3NCN in the temperature range of 300-3000 K, are shown in Figure 4d and are also listed in Table 2. k7 is an abstraction process by the triplet precomplex 3OH‚‚‚N(H)CN and the transition state t-TS1. The properties of k7are completely different from those of k1, k2, and k4because of the absence of pressure effect on this abstraction process.
The predicted results for k9producing the products of CNOH +3NH at six specific pressures between 1 and 7600 Torr in the temperature range of 300-3000 K are shown in Figure 4e and are also listed in Table 2. k9is the sum of contributions from the reactions via t-TS4 and t-TS7, in which the rate constants from the process via t-TS4account for 0.13-0.49 in
TABLE 2: Predicted Rate Expressionsaof k
1, k2, k3, k4, k5, k6, k7, k8, k9, k10, and ktotalat Ar Pressures of 1, 10, 100, 300, 760,
and 7600 Torr in the Temperature Range of 300-3000 K
reaction P (Torr) A n B reaction P (Torr) A n B
k1 1 4.03× 1016 -10.14 -2144 k7 1 1.72× 10-19 2.48 949 10 5.24× 1017 -10.17 -2284 10 1.72× 10-19 2.48 949 100 8.59× 1018 -10.22 -2549 100 1.72× 10-19 2.48 949 300 3.25× 1019 -10.24 -2730 300 1.72× 10-19 2.48 949 760 9.08× 1019 -10.24 -2905 760 1.72× 10-19 2.48 949 7600 7.22× 1020 -10.19 -3431 7600 1.72× 10-19 2.48 949 k2 1 3.93× 10-23 2.99 174 k8 1 2.86× 1013 -9.55 -3532 10 4.85× 10-23 2.97 143 10 3.25× 1014 -9.54 -3943 100 4.96× 10-23 2.97 143 100 1.40× 1015 -9.39 -3363 300 5.44× 10-23 2.95 133 300 1.42× 1015 -9.23 -4712 760 6.91× 10-23 2.92 102 760 8.33× 1014 -9.03 -4897 7600 4.63× 10-22 2.69 173 7600 1.44× 1013 -8.19 -5165 k3 1 9.16× 10-16 0.74 -6749 k9 1 3.38× 10-18 1.72 -2965 10 9.16× 10-16 0.74 -6749 10 7.52× 10-18 1.62 -3104 100 9.16× 10-16 0.74 -6749 100 5.27× 10-17 1.39 -3455 300 9.16× 10-16 0.74 -6749 300 2.65× 10-16 1.19 -3760 760 9.16× 10-16 0.74 -6749 760 1.49× 10-15 1.00 -4105 7600 9.09× 10-16 0.74 -6749 7600 1.00× 10-13 0.51 -5172 k4 1 1.47× 1012 -9.02 -656 k10 1 8.47× 10-16 0.74 -3102 10 1.04× 1015 -9.54 -1298 10 1.84× 10-15 0.65 -3225 100 3.41× 1017 -9.95 -1896 100 4.52× 10-14 0.27 -3862 300 9.11× 1017 -9.93 -1949 300 3.95× 10-13 0.11 -4329 760 3.11× 1019 -10.25 -2345 760 2.21× 10-12 -0.18 -4768 7600 4.09× 1020 -10.26 -2755 7600 1.36× 10-11 -0.36 -5780 k5 1 2.56× 10-33 5.51 342 ktotalb 1 T1 2.16× 10 -26 4.59 2405 10 2.15× 10-31 4.98 -326 T2 1.48× 10-18 2.24 520 100 9.86× 10-31 4.79 -549 10 T1 6.86× 10-29 5.26 3393 300 1.34× 10-30 4.76 -592 T2 1.32× 10-18 2.25 537 760 1.73× 10-30 4.72 -629 100 T1 1.05× 10-16 1.30 2216 7600 6.57× 10-30 4.56 -822 T2 6.54× 10-19 2.33 697 k6 1 3.15× 10-29 4.60 -4778 300 T1 1.56× 10-7 -1.61 1104 10 8.59× 10-29 4.48 -4917 T2 1.76× 10-19 2.48 996 100 1.03× 10-28 4.45 -4942 760 T1 2.66× 10 2 -4.50 -239 300 1.08× 10-28 4.45 -4949 T2 1.38× 10-20 2.78 1578 760 1.13× 10-28 4.44 -4955 7600 T1 1.23× 1015 -8.31 -2189 7600 1.53× 10-28 4.41 -4998 T2 2.76× 10-27 4.56 5189
aRate constants are represented by k ) ATnexp(B/T) in units of cm3molecule-1s-1.bT
1, temperature range of 300-1000 K; T2, temperature
range of 1000-3000 K.
the temperature range of 300-3000 K. k9as an dissociation process is similar to k2. k9increases with increasing temperature from 300 to 3000 K; it is pressure-dependent at temperatures below 1000 K.
The predicted total rate constants for ktotal) k1+ k2+ k3+ k4+ k5+ k6 + k7 + k8 + k9 + k10 at 10 specific pressures between 1 and 7600 Torr in the temperature range 300-3000 K are shown in Figure 4f and listed in Table 2. The P,T dependences of ktotalare closely parallel with those of k1and k4 in the temperature range below about 1000 K. ktotal decreases with increasing temperature from 300 to about 1000 K and also has a strong pressure dependence in the temperature range below 1000 K. However, with temperature over 1000 K, the property of ktotal is close to that of k7; it increases upon increasing temperature from 1000 to 3000 K. In comparison, the values of ktotalin the lower temperature range are still higher than those in the higher temperature range due to dominant k1 and k4 through collisional deactivation.
The predicted individual rate constants given in units of molecules per cubic centimeter and time in seconds (cm3 molecule-1s-1) at a 760 Torr Ar pressure in the temperature range 300-3000 K can be represented by
The total rate constants at 760 Torr of Ar pressure can be represented by two fitting equations: ktotal ) 2.66 × 102 × T-4.50exp(-239/T) at T ) 300-1000 K and 1.38× 10-20× T2.78exp(1578/T) cm3molecule-1 s-1 at T ) 1000-3000 K. At present, no comparison can be made for the calculated and experimental data. For this newly identified, potentially impor-tant, prompt NO precursor reaction, our results are recommended for high-temperature combustion modeling applications.
2c. Predicted Branching Ratios. The branching ratios of the rate constants k1-k610 at an Ar pressure of 760 Torr in the temperature range of 300-3000 K are shown in Figure 5. k1 accounts for 0.32-0.28 and k4accounts for 0.68-0.17 in the temperature range of 300-800 K. k7accounts for 0.55-0.98 in the high-temperature range of 800-3000 K. Because both k2 and k7 can produce H2O, and excited1NCN from k1 can transfer to its ground state 3NCN finally, the total branching ratios of H2O and NCN with the sum of k2and k7account for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k3 for producing HCN + HNO, k6 for producing H2N + NCO, k8for forming 3HN(OH)CN, k9 for
producing CNOH +3NH, and k
5+ k10for producing NH2and NCO are negligible, even in the higher temperature range. Therefore, the singlet primary intermediates, HON(H)CN and HONCNH, are expected to be stable in the low-temperature range and begin to dissociate when temperature is over 800 K, producing H2O and NCN as the primary products.
Conclusions
The kinetics and mechanism for the OH + HNCN reaction with singlet and triplet PESs have been studied 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. The total and individual rate constants for the primary channels of the reaction in the temperature range of 300-3000 K are predicted. The primary intermediates formedssinglet HON(H)CN and HONCNHsare stable in the low-temperature range and begin to dissociate when temperature is higher than 800 K, giving rise to H2O and NCN as the primary products through the singlet and triplet PESs of the OH + HNCN reaction. Our predicted total and individual rate constants and product branching ratios for this critical reaction may be employed for combustion kinetic modeling applications.
Acknowledgment. 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. M.C.L. acknowledges the support from the National Science Council of Taiwan for a distinguished visiting professorship and the Taiwan Semiconductor Manufacturing Company for the TSMC Distinguished Professorship.
Supporting Information Available: Table S1: Cartesian coordinates of the optimized geometries of intermediates and transition sates of the reaction OH + HNCN at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) levels. Table S2: The frequencies and moments of inertia of the optimized geometries of the reaction OH + HNCN at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) levels. Figure S1. The whole PES of the reaction OH + HNCN calculated at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) levels. This material is available free of charge via the Internet at http:// pubs.acs.org.
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