Kinetics of the O+ ICN Reaction

Article pubs.acs.org/JPCA

Kinetics of the O + ICN Reaction Wenhui Feng and John F. Hershberger* Department of Chemistry and Biochemistry, Department 2735, P.O. Box 6050, North Dakota State University, Fargo, North Dakota 58108-6050, United States ABSTRACT: The kinetics of the O + ICN reaction was studied using a relative rate method, with O + C2H2 as the competing reaction. Carbon monoxide products formed in the competing reaction and subsequent secondary chemistry were detected as a function of reagent ICN pressure to obtain total rate constants for the O + ICN reaction. Analysis of the experimental data yields rate constants of k1 = (3.7 ± 1.0 to 26.2 ± 4.0) × 10−14 cm3 molecule−1 s−1 over the total pressure range 1.5−9.5 Torr. Product channel NCO + I, the only bimolecular exothermic channel of the reaction, was investigated by detection of N2O in the presence of NO and found to be insignificant. An ab initio calculation of the potential energy surface (PES) of the reaction at the CCSD(T)/CEP-31G//DFT-B3LYP/CEP-31G level of theory was also performed. The pathways leading to bimolecular product channels are kinetically unfavorable. Formation and subsequent stabilization of an ICNO adduct species appears to dominate the reaction, in agreement with the experimentally observed pressure dependent rate constants.

1. INTRODUCTION The CN radical is an important intermediate in combustion chemistry and astrochemical environments. The kinetics of this radical have therefore been extensively studied, with literature on reactions with O2,1−13 NO,14−17 NO2,17−20 and hydrocarbons.21−24 Cyanogen iodide, ICN, is used in many flash photolysis experiments as a photolytic precursor for CN radicals. One potential problem in these experiments is possible secondary chemistry involving the precursor. For example, in a flash photolysis study of the CN + O2 reaction, O atom products could react with ICN, potentially resulting in regeneration of CN radicals. This would lead to an observed decay rate for [CN] vs time that is slower than would be expected from the CN + O2 reaction alone, resulting in an erroneously small value of the CN + O2 rate constant. A similar issue was raised in our recent study of CN + SO2 kinetics, in which we found essentially no reaction with k ≤ 3.1 × 10−14 cm3 molecule−1 s−1,25 in contrast to an earlier literature value of 4.4 × 10−12 cm3 molecule−1 s−1.26 Is it possible that O atom formation (possibly via SO2 multiphoton dissociation or reaction of CN with trace O2) followed by the O + ICN reaction resulted in regeneration of CN radicals, and therefore led to an erroneously small value for the CN + SO2 rate constant? Although this possibility seems remote, we believe that an investigation of the kinetics of the O + ICN reaction is merited. The title reaction has several possible product channels: O + ICN → IO + CN ΔH °298 = 20 kcal/mol

endoergic, we do not expect formation of CN radicals in this reaction; however, there are substantial uncertainties in the thermochemistry of the iodine-containing species. No direct measurement of k1 has been reported in the literature, but a rate constant of k1 > 1.6 × 10−14 cm3 molecule−1 s−1 was suggested in an early discharge study.29 In this paper, we present an experimental measurement of the kinetics of this reaction, using a relative rate technique, as follows: O(3P) atoms were produced by 193 nm laser photolysis of SO2 SO2 + hν (193 nm) → SO + O(3P)

In the presence of ICN, C2H2, NO, and CF4 buffer gas, the following reactions consume O(3P) O + ICN → products O + C2H 2 → CO + CH 2

(1) (3a)

→ HCCO + H (3b)

Reaction 3 is fairly slow, with k3 = 1.40 × 10−13 cm3 molecule−1 s−1 at 298 K,30 and a product branching fraction of ϕ3b = 0.85.31 CO products are then detected by infrared absorption spectroscopy. If reaction 1 does not produce CO, then we expect the CO yield to decrease as the ratio of [ICN]/[C2H2] reagents is increased, due to the competition between reactions 1 and 3. Secondary reactions of the CH2 and HCCO products may complicate this picture slightly, but if excess NO reagent is included, the dominant secondary chemistry will be

(1a)

HCCO + NO → HCN + CO2

→ NCO + I ΔH °298 = − 58 kcal/mol (1b) → ICNO

(2)

(4a)

→ HCNO + CO (4b)

(1c)

Thermochemical information has been obtained from NISTJANAF standard tables27 as well as recent report of the heat of formation of NCO.28 Obviously, if channel 1a is highly © 2012 American Chemical Society

Received: March 16, 2012 Revised: April 24, 2012 Published: May 9, 2012 4817

dx.doi.org/10.1021/jp302555p | J. Phys. Chem. A 2012, 116, 4817−4822

The Journal of Physical Chemistry A

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reactions 3 and 4. In the following, we give an analysis of the CO yield due to reactions 3 and 4. The reactions of O atoms with NO and SO2 are very slow and are therefore ignored. CO is produced from reactions 3 and 4 with a rate of

CH 2 + NO → HCNO + H (5a) → HCN + OH (5b)

The product branching ratio of ϕ4a = 0.22.32 Reactions 4 and 5 are very fast with the rate constants of k4 = 5.03 × 10−11 cm3 molecule−1 s−1 and k5 = 3.65 × 10−11 cm3 molecule−1 s−1, respectively, at 298 K.33,34 The presence of nitric oxide therefore will suppress other possible secondary chemistry of CH2 and HCCO, such as O + HCCO, O + CH2, ICN + HCCO, and ICN + CH2. Because reaction 4b creates additional CO, the measured CO yield in this experiment cannot yield a reliable value of ϕ3a. Nevertheless, if all of the CH2 and HCCO radicals are assumed to react with NO, then the dependence of the CO yield on [ICN] will depend only on the competition between reactions 1 and 3, permitting a relative rate determination of k1.

d[CO] = ϕ3ak 3[C2H 2][O] + ϕ4bk4[HCCO][NO] dt

The rate of HCCO production and removal by reactions 3 and 4 can be represented by r3 =

13

CO(v=1 ← v=0)

r4 = −

N2O(0001) ← (0000)

(7)

d[HCCO]4 = k4[HCCO][NO] dt

(8)

Because k4 ≫ k3, we can make a steady state approximation for HCCO radicals and obtain k4[HCCO][NO] = ϕ3bk 3[O][C2H 2]

(9)

Substituting eq 9 into eq 6, we obtain d[CO] = (ϕ3a + ϕ4bϕ3b)k 3[O][C2H 2] dt

(10)

where [O] is given by [O] = [O]0 exp{( −k1[ICN] − k 3[C2H 2])t }

(11)

which results from the integration of d[O] = −k1[O][ICN] − k 3[O][C2H 2] dt

(12)

Then we get d[CO] = (ϕ3a + ϕ4bϕ3b)k 3[C2H 2] dt [O]0 exp[( −k1[ICN] − k 3[C2H 2])t ]

R(2) at 2154.595 cm−1

R(14) at 2147.205 cm

d[HCCO]3 = ϕ3bk 3[O][C2H 2] dt

and

2. EXPERIMENTAL SECTION Transient infrared absorption spectroscopy was used to measure the yields of carbon monoxide product molecules upon photolysis of SO2/ICN/C2H2/NO/CF4 mixtures. N2O products (from secondary chemistry, described below) were also probed in some experiments. The infrared light source was a lead salt diode laser (Laser Components), as described in previous publications.35,36 Photolysis light was provided by a Compex 201 excimer laser (Coherent) operating at 193 nm. IR and UV light were made collinear and passed through a singlepass 143-cm absorption cell, and the infrared light was detected by a 1 mm diameter InSb detector (Kolmar Technology, 1 μs response time). Transient signals were recorded and averaged on a digital oscilloscope. The HITRAN molecular database was used to locate and identify the spectral lines of CO product molecules.37 The absorption lines chosen are generally near the peak of the rotational Boltzmann distribution, minimizing sensitivity to small heating effects. The absorption lines used are CO(v=1 ← v=0)

(6)

(13)

Integration of eq 13 gives −1

[CO] =

P(28) at 2197.698 cm−1

(ϕ3a + ϕ4bϕ3b)k 3[C2H 2][O]0 k1[ICN] + k 3[C2H 2] {1 − exp[( −k1[ICN] − k 3[C2H 2])t ]}

SO2 (99.98%, Matheson), C2H2, CF4, and SF6 (Matheson) were purified by repeated freeze−pump−thaw cycles at 77 K. NO (Matheson) was purified by repeated freeze−pump−thaw cycles at 200 K. ICN (Aldrich) was purified by vacuum sublimation to remove dissolved air. 13C2H2 (13C 99%, Cambridge Isotope Laboratory) was used without purification. Typical experimental conditions were P(SO2) = 0.10 Torr, P(C2H2) = 0.05−0.2 Torr, P(ICN) = 0 − 0.3 Torr, P(NO) = 0.20 Torr, P(CF4 or SF6) = 1−9 Torr, and Excimer laser pulse energies of ∼4 mJ (fluence of 14 mJ/cm2). CF4 buffer gas was used for CO detection, whereas SF6 buffer gas was used for detection of N2O product molecules. The choice of buffer gas was motivated by the desire to relax any nascent vibrationally excited product molecules to a Boltzmann distribution.35,38

(14)

At t = ∞, [CO]∞ =

(ϕ3a + ϕ4bϕ3b)k 3[C2H 2][O]0 k1[ICN] + k 3[C2H 2]

(15)

Equation 15 can be transformed to k1[ICN] + k 3[C2H 2] 1 = [CO]∞ (ϕ3a + ϕ4bϕ3b)k 3[C2H 2][O]0

Equation 16 shows that [CO]∞ with a slope of slope =

3. RESULTS 3.1. Rate Constant. The rate constant k1 was measured using a relative rate technique. Experiments were conducted by laser photolysis of SO2/C2H2/ NO/ICN/CF4 mixtures, followed by detection of CO molecules produced from

−1

(16)

is a linear function of [ICN]

k1 (ϕ3a + ϕ4bϕ3b)k 3[C2H 2][O]0

(17)

and an intercept of intercept = 4818

1 (ϕ3a + ϕ4bϕ3b)[O]0

(18)



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of SO2 (0.1 Torr)/13C2H2(0.1 Torr)/ NO(0.2Torr)/ICN(variable pressures)/CF4(2.0 Torr) mixtures. As shown, the peak−peak amplitude of signal decreases with increased [ICN] due to the competition between reactions 1 and 3 for oxygen atoms. In addition, the rise time of the signals decreases with increased [ICN], consistent with faster depletion of oxygen atoms. All of the transients also have a slow decay, which is attributed to diffusion of product molecules out of the probed beam volume. To estimate the CO yields, the slow decay portion of each transient signal was fit to a single exponential decay (diffusion out of a cylindrical volume is not strictly exponential, but this approximation is sufficient for our purposes), and the decay was extrapolated to t = 0. This amplitude was then converted to an absolute [13CO] concentration using tabulated HITRAN line strengths as described previously.38 Figure 2 shows the plot of [13CO]∞−1

Equation 17 is divided by eq 18, resulting in ⎛ slope ⎞ k1 = ⎜ ⎟ × k 3[C2H 2] ⎝ intercept ⎠

(19)

Therefore, we can calculate k1 from the slope and intercept, which can be obtained from the plot of [CO]∞−1 as the function of ICN, and from a literature value of k3.30 The accuracy of above method depends on how exclusively CO is produced from reactions 3 and 4. If secondary chemistry involving ICN produce additional CO, errors would result, because the CO yield would no longer be completely suppressed in the limit of high ICN pressure. For example, 193 nm absorption and photolysis of ICN does occur to some extent: ICN + hν (193 nm) → CN + I

(20)

SO2 + hν (193 nm) → SO + O

(2)

O + CN → NCO

(21)

NCO + NO → N2O + CO

(22)

Also, because there is no reagent included that reacts quickly with CN, reaction of CN with trace O2 impurity may also produce NCO. To suppress the effect of such secondary chemistry, we used isotopically labeled 13C2H2 instead of C2H2 in some experiments, and detected 13CO molecules formed in reactions 3 and 4, as follows: O + 13C2H 2 → 13CO + 13CH 2

(3a)

→ H13C13CO + H (3b) H13C13CO + NO → HCN + 13CO2

(4a)

Figure 2. [13CO]−1 as a function of [ICN]. Reaction conditions: P(SO2) = P(13C2H2) = 0.10 Torr, P(NO) = 0.2 Torr, P(CF4) = 2.0 Torr, P(ICN) = variable, T = 298 K, 193 nm laser pulse energy ∼4 mJ.

→ H13CNO + 13CO (4b)

In this case, carbon monoxide from reaction 22 and any other secondary chemistry due to ICN photolysis will be unlabeled (12CO) and therefore will not contribute to the resulting 13CO signals. Figure 1 shows the transient infrared absorption signals of 13 CO product molecules detected upon 193 nm laser photolysis

as a function of [ICN], which is predicted by eq 16 to be linear. By substituting the slope and intercept of these plots into eq 19, we determine k1(298K) = (7.72 ± 0.61) × 10 −14 cm3 molecule−1 s−1. Following the same procedure, we measured k1 value at different 13C2H2 pressures of 0.05 and 0.2 Torr, and obtained k1(298K) = (8.3 ± 0.3) × 10−14 cm3 and k1(298K) = (9.3 ± 0.9) × 10−14 cm3, respectively. These k1 values at 13C2H2 = 0.05−0.2 Torr, are in reasonable agreement, suggesting that our analysis of the data is valid over this 13C2H2 pressure range. As a comparison, we also conducted the experiment using unlabeled C2H2 and obtain k1(298K) = (6.05 ± 0.71) × 10−14 cm3 molecule−1 s−1 at 2.5 Torr and 298 K. The k1 values are close to that measured with 13C2H2 reagent, but as expected, the value obtained without isotopic substitution is somewhat smaller, presumably because of a modest contribution from secondary chemistry such as reactions 20−22. By the method shown above and using 13C2H2, we measured k1 at several different total pressures (the total pressure was varied by changing the CF4 buffer gas pressure). The results are shown in Figure 3. Figure 3 shows k1 varies over the range of (3.7 ± 1.0 to 26.2 ± 4.0) × 10−14 cm3 molecule−1 s−1 over the pressure range 1.5−9.5 Torr. This result shows the O + ICN reaction is significantly pressure dependent, suggesting that an adduct product channel may dominate this reaction. Although the above analysis is consistent with our experimental data, it does neglect some additional secondary

Figure 1. Transient infrared absorption signal of 13CO. Reaction conditions: P(SO2) = P(13C2H2) = 0.10 Torr, P(NO) = 0.2 Torr, P(CF4) = 2.0 Torr, P(ICN) = variable, T = 298 K, 193 nm laser pulse energy ∼4 mJ. 4819



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Figure 3. Rate constants of O + ICN reaction as a function of total pressure. The error bars represents one standard deviation. Reaction conditions as in Figure 2, except CF4 pressure adjusted to reach total pressure.

Figure 4. Kinetic modeling predictions for the time dependence of [CO]. Initial conditions: [O]0 = 7.8 × 1013 molecules cm−3, [C2H]0 = 1.0 × 1013 molecules cm−3, P(SO2) = P(13C2H2) = 0.10 Torr, P(NO) = 0.2 Torr. Solid curve: full mechanism from Table 1. Dashed curve: same mechanism except C2H + NO reaction removed.

chemistry, primarily due to photolysis of acetylene. If C2H is formed, several reactions can occur: C2H + C2H 2 → products

(23)

C2H + SO2 → products

(24)

C2H + NO → CO + HCN

(25)

not attempt to modify the model to obtain perfect agreement, as that would primarily involve rather arbitrary adjustments to the diffusion rate. The primary result of the model is that it shows that inclusion of the C2H+NO reaction only results in a modest increase in the CO yield, showing that the neglect of C2H chemistry in our analysis was a reasonable approximation within the overall uncertainties of the experiment. 3.2. Product Channels. The possible existence of the exothermic product channel 1b was investigated. If NCO is a significant product of the title reaction 1, N2O is expected to be produced by the reaction of NCO with NO, reaction 22. Under the conditions SO2 (0.0−0.4 Torr)/ NO (0.2 Torr)/ICN (0.1 Torr)/SF6 (2.0 Torr), we detected very small transient signals attributed to N2O. The signal amplitudes (peak−peak) were converted into absolute concentrations,38 which are shown in Figure 5. As shown, only about ∼3 × 1011 molecules cm−3 of N2O molecules were produced. The measured N2O yield is

Reaction 25 in particular represents a potential additional source of carbon monoxide. Using an absorption coefficient of 0.0079 cm−1 Torr−1 for C2H2 at 193 nm,39 and assuming a quantum yield for C2H production of unity, we estimate an initial concentration of [C2H]0 ∼ 1.0 × 1013 molecules cm−3. Similarly, using an absorption coefficient for SO2 of 0.122 cm−1 Torr−1,40 we estimate [O]0 ∼ 8.0 × 1013 molecules cm−3. We then used kinetic modeling to predict the time dependence of [CO] under typical experimental conditions. Table 1 shows the Table 1. Reactions Used in Kinetic Modeling of the Time Dependence of [CO] reaction SO2 → SO + O O + ICN → ICNO O + C2H2 → CO + CH2 O + C2H2 → HCCO + H HCCO + NO → HCN + CO HCCO + NO → HCNO + CO C2H + C2H2 → products C2H + SO2 → products C2H + NO → HCN + CO

k(298K), cm3 molecule−1 s−1 3.7 2.1 1.2 1.1 3.9 1.3 1.1 3.8

× × × × × × × ×

10−14 10−14 10 −13 10 −11 10−11 10−10 10−11 10−11

reference this work 30, 31 30, 31 32, 33 32, 33 41 42 43

mechanism used in the model; it differs from the mechanism described above in that it includes several reactions involving C2H chemistry.41−43 A first-order decay of CO with a rate of 300 s−1 was included in the model to approximate the diffusion of CO molecules out of the probed volume. Figure 4 shows the model predictions for the transient behavior of [CO]. Several observations are relevant. First, the predicted behavior is in reasonable agreement with the experimental transient signals, both in the time dependence and in the approximate magnitude of the CO signals. We did

Figure 5. Yield of N2O as a function of SO2 pressure. P(ICN) = 0.1 Torr, P(SO2) = variable, P(NO) = 0.2 Torr, P(SF6) = 2.0 Torr, 193 nm laser pulse energy ∼4 mJ. 4820



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therefore very small compared to the initial [O]0 of ∼8.0 × 1013 molecules cm−3 (see above). Furthermore, if N2O is formed by reaction 1b followed by reaction 22, we would expect to see the yields that scale roughly linearly with increasing SO2 pressures. However, such a trend is not reflected from Figure 5. We therefore conclude that the observed small yield of N2O is not due to reaction 1b but is probably due to minor secondary chemistry such as ICN photolysis, reaction 20, followed by reaction 21 and 22.

4. POTENTIAL ENERGY SURFACE The Gaussian-03 set of programs44 were used to characterize the singlet and triplet potential energy surfaces (PES) of the title reaction. Geometry optimization at the DFT-B3LYP/CEP31G level was used to obtain geometries of reactants, products, intermediates (local minima), and saddle points through the reaction. Vibrational analysis was used to identify intermediates and saddle points according to the number of vibrational imaginary frequencies. Intrinsic reaction coordinate (IRC) calculations were used to link each saddle point structure with corresponding intermediate geometries to develop an energy profile. Following geometry optimization, single point energies were computed at the CCSD(T)/CEP-31G level of theory. Values from these calculations were combined with zero point energy (ZPE) corrections, determined using force constants calculated at the DFT-B3LYP/CEP-31G level, to give a total energy. These values were compared to the total energy of the reactants, O + ICN, to give the relative energies shown in Table 2. Figure 6 shows the calculated structures. Figure 7 shows the resulting potential energy surface of the reaction in singlet and triplet state. Table 2. Total Energies (hartrees), Zero-Point Energies (ZPE, hartrees), and Relative Energies (ΔE, kcal/mol) for Species on the Potential Energy Surface Relevant to the O + ICN Reaction Obtained at DFT and CCST(T) Levels of Theory geometry optimization

a

species

B3LYP/CEP31G

O + ICN NCO + I CNO + I CN + IO 1 M1 3 M2 3 M3 3 Tr/2 3 T2/3

−42.735635 −42.794531 −42.713259 −42.674995 −42.777909 −42.792526 −42.806355 −42.724115 −42.788964

Figure 7. Potential energy surface of the O + ICN reaction in (a) singlet and (b) triplet. Relative energies are taken from the CCSD(T)/ CEP-31G values, on the basis of optimized geometries and zero-point energy corrections calculated at the B3LYP/CEP-31G level.

single point energy

ZPE

CCSD(T)/CEP31G

ΔE

0.006772 0.008781 0.008066 0.005819 0.010983 0.008833 0.009962 0.007247 0.008173

−42.1780919 −42.2163935 −42.1328863 −42.1148897 −42.1955013 −42.2057956 −42.2125114 −42.1536766 −42.1967066

0 −22.8 29.2 39.1 −8.3 −16.1 −19.6 15.6 −10.8

As shown of Figure 7a, the reactants can associate to form the adduct M1 in the singlet state, which is energetically lower than the reactants by 8.3 kcal/mol. Because the reactant O atom is a ground state triplet, reaching the adduct M1 requires an intersystem crossing. Such a triplet−singlet crossing is probably quite facile given the presence of the heavy iodine. It is possible that there is a barrier between the reactants and M1, but detailed investigation of the triplet−singlet crossing is beyond the scope of this study. We also note that the dissociation of M1 to I + CNO is difficult because the process is endothermic. We found no other existing pathways involving M1. Figure 7b shows a pathway in the triplet state involving O atom attack of the C atom of ICN to form an intermediate 3M2 via the saddle point 3Tr/2, which is 15.6 kcal/mol above the reactants. 3M1 can break the I−C bond to form a loose molecule with structure 3M3 via a transition state 3T2/3, which is 5.3 kcal/mol above 3M2. 3M3 dissociates to NCO + I products without an energy barrier. In the pathway, the transition state 3Tr/2 is higher than the reactants so the NCO + I product channel is kinetically unfavorable. In summary, there is no kinetically favorable bimolecular product channel on either the singlet or triplet PES. The only available pathway, at least at moderate temperatures, is high pressure stabilization of a singlet ICNO adduct (1 M1). This is

a

Relative to the reactants, after correction for ZPE.

Figure 6. DFT-B3LYP/CEP-31G optimized geometries of the intermediates and the saddle points in the O + ICN reaction. Bond lengths are given in Å and bond angles in degrees.

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(18) Wooldridge, S. T.; Mertens, J. D.; Hanson, R. K.; Bowman, C. T. Symp. Int. Combust. Proc. 1994, 25, 983−991. (19) Park, J.; Hershberger, J. F. J. Chem. Phys. 1993, 99, 3488−3493. (20) Zhang, J.; Li, Z.; Liu, J.; Sun, C. J. Phys. Chem. A 2005, 109, 10307−10313. (21) Gannon, K. L.; Glowacki, D. R.; Blitz, M. A.; Hughes, K. J.; Pilling, M. J.; Seakins, P. W. J. Phys. Chem. A 2007, 111, 6679−6692. (22) Choi, N.; Blitz, M. A.; McKee, K.; Pilling, M. J.; Seakins, P. W. Chem. Phys. Lett. 2004, 384, 68−72. (23) Sims, I. R.; Queffelec, J.-L.; Travers, D.; Rowe, B. R.; Herbert, L. B.; Karthauser, J.; Smith, I. W. M. Chem. Phys. Lett. 1993, 211, 461− 468. (24) Sayah, N.; Li, X.; Caballero, J. F.; Jackson, W. M. J. Photochem. Photobiol. A 1988, 45, 177−194. (25) Feng, W.; Hershberger, J. F. J. Phys. Chem. 2011, 115, 286−290. (26) Titarchuk, T. A.; Halpern, J. B. Chem. Phys. Lett. 1995, 232, 192−196. (27) Chase, M. W. NIST-JANAF Thermochemical Tables. J. Phys. Chem. Ref. Data, 4th ed.; 1998. (28) Schuurman, M. S.; Muir, S. R.; Allen, W. D.; Schaefer, H. F., III. J. Chem. Phys. 2004, 120, 11586−11599. (29) Grady, Q. J. F.; Freeman, C. G.; Phillips, L. F. J. Phys. Chem. 1968, 72, 743−744. (30) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. JPL Publication 97-4 1997, 1−266. (31) Boullart, W.; Peeten, J. J. Phys. Chem. 1992, 96, 9810−9816. (32) Meyer, J.; Hershberger, J. F. J. Phys. Chem. B 2005, 109, 8363− 8366. (33) Carl, S. A.; Sun, Q.; Vereecken, L.; Peeters, J. J. Phys. Chem. A 2002, 106, 12242−12247. (34) Seidler, V.; Temps, F.; Wagner, H. G.; Wolf, M. J. Phys. Chem. A 1989, 93, 1070−1073. (35) Cooper, W. F.; Hershberger, J. F. J. Phys. Chem. 1992, 96, 771− 775. (36) Cooper, W. F.; Hershberger, J. F. J. Phys. Chem. 1992, 96, 5405−5410. (37) Rothman, L. S.; et al. J. Quant. Spectrosc. Radiat. Transfer 1992, 48, 469−507. (38) Cooper, W. F.; Park, J.; Hershberger, J. F. J. Phys. Chem. 1993, 97, 3283−3290. (39) Benilan, Y.; Smith, N.; Jolly, A.; Ravlin, F. Planet. Space Sci. 2000, 48, 463−471. (40) Ahmed, S. M.; Kumar, V. J. Quant. Spectrosc. Radiat. Transfer 1992, 47, 359−373. (41) Van Look, H.; Peeters, J. J. Phys. Chem. 1995, 99, 16284−16289. (42) Chikan, V.; Nizamov, B.; Leone, S. R. J. Phys. Chem. A 2004, 108, 10770−10782. (43) Peeters, J.; Van Look, H.; Ceursters, B. J. Phys. Chem. 1996, 100, 15124−15129. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al., Gaussian03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.

consistent with our experimental observation of a pressuredependent value of k1. Our original concern was that the O + ICN reaction could potentially represent a route for regeneration of CN radicals. The ab initio calculations clearly show that formation of CN is a highly endoergic process and is therefore not significant. Furthermore, both the experiment and the calculations indicate that this slow reaction proceeds primarily by adduct stabilization.

5. CONCLUSION The kinetics of the O + ICN reaction was studied by using IR diode absorption spectrum. We found that this is a slow reaction with a pressure dependent rate constant characteristic of adduct stabilization. No bimolecular product channels were observed. The potential energy surface of the reaction at singlet and triplet state, calculated by ab initio method at the DFTB3LYP/CEP-31G//CCSD(T)/CEP-31G level, is in agreement with the experimental conclusions. We therefore conclude that this reaction does not adversely affect previous or future measurements of CN kinetics using ICN precursor molecules.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by Division of Chemical Sciences, Office of Basic Energy Sciences of the Department of Energy, Grant DE-FG03-96ER14645.

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Kinetics of the O+ ICN Reaction

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