Kinetics of the NCCO+ NO2 Reaction

ARTICLE pubs.acs.org/JPCA

Kinetics of the NCCO + NO2 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 NCCO + NO2 reaction was studied by transient infrared laser absorption spectroscopy. The total rate constant of the reaction was measured to be k = (2.1 ( 0.1) 1011 cm3 molecule1 s1 at 298 K. Detection of products and consideration of possible secondary chemistry shows that CO2 + NO + CN is the primary product channel. The rate constants of the NCCO + CH4 and NCCO + C2H4 reactions were also measured, obtaining upper limits of k (NCCO + CH4) e 7.0 1014 cm3 molecule1 s1 and k (NCCO + C2H4) e 5.0 1015 cm3 molecule1 s1. Ab initio calculations on the singlet and triplet potential energy surfaces at B3LYP/6-311++G**// CCSD(T)/6-311++G** levels of theory show that the most favorable reaction pathway occurs on the singlet surface, leading to CO2 + NO + CN products, in agreement with experiment.

1. INTRODUCTION The NCCO radical has been the subject of recent spectroscopic and kinetics studies because of its role as a possible intermediate in the combustion chemistry of nitrogen.117 We have recently detected the NCCO radical at 18871889 cm1 in the gas phase using high resolution diode IR spectroscopy.15 Allen et al. used FTIR spectroscopy on matrix-isolated NCCO, finding a strong v2 transition at 1890 cm1.16 Recently, we have used this spectroscopic assignment to probe NCCO in several kinetics experiments. The kinetics of the reaction of NCCO with O2 has been studied by several groups.7,1315 It was shown that NCCO + O2 is slow with a pressure-dependent rate constant of (5.48.8) 1013 cm3 molecule1 s1 over the pressure range 2.015.5 Torr at 298 K.7 Our recent study shows that the primary product of the NCCO + O2 reaction is presumed to be a collisionally stabilized NCC(O)O2 adduct, but at low pressures, a significant yield of the CO2 + NCO product channel is also produced.15 Most recently, we have studied the kinetics of the NCCO + NO reaction by diode laser infrared spectroscopy and characterized the potential energy surface using ab initio methods.17 The results show that the reaction is highly pressure dependent, with k (NCCO + NO) = (3.227.1) 1013 cm3 molecule1 s1 at 325 Torr total pressure and 298 K. Both experiment and theory suggest that formation of an NCC(NO)O adduct, followed by collisional stabilization, dominates the NCCO + NO reaction.17 In this paper, we report the first kinetic study on the NCCO + NO2 reaction. The possible product channels of the reaction include:

Thermochemical information has been obtained from NISTJANAF standard tables18 as well as recent reports of the heats of formation of NCCO,11 NCO,19 and NCNO.20

2. EXPERIMENTAL SECTION Methyl cyanoformate (MCF) was photolyzed at 193 nm to produce NCCO radicals:

In a mass spectrometric study, this precursor was reported to be an efficient source of NCCO radicals with a quantum yield of Φ2a = 95%.6 In our previous study of the NCCO + O2 reaction, we estimated a somewhat lower value of Φ2a = 62%,15 with a significant yield of CO and CN product (eq 2c) as well. NCCO radicals were detected by time-resolved infrared laser absorption spectroscopy using lead salt diode lasers (Laser Components), as described in previous publications.21,22 Several NCCO (v = 1 r v = 0) transitions were located at 1887 1890 cm1, as described in our previous study.15 IR and UV light passed through a single-pass 143 cm absorption cell, and the infrared light was detected by a 1 mm diameter InSb detector (Cincinnati Electronics, ∼1 μs response time) and signal averaged on a digital oscilloscope. To account for probe laser thermal deflection affects, signals were collected, with the diode laser slightly detuned off the spectroscopic absorption lines, and such transients were subtracted from the on-resonant transients. These off-resonant signals were quite large in these experiments but could be minimized by careful alignment of the infrared probe laser. Received: July 26, 2011 Published: September 15, 2011

r 2011 American Chemical Society

12173

dx.doi.org/10.1021/jp207121p | J. Phys. Chem. A 2011, 115, 12173–12183

The Journal of Physical Chemistry A

Figure 1. Infrared absorption signal of NCCO at 1887.42 cm1 as a function of time. Reaction conditions: P(NO2) = 0.0 Torr (upper trace); P(NO2) = 0.080 Torr (lower trace); P(methyl cyanoformate) = 0.20 Torr; P(SF6) = 1.0 Torr; photolysis laser pulse energy = 2.6 mJ. Also shown on lower trace is a fit to a single exponential decay.

The following molecules were probed using infrared diode laser absorption spectroscopy: CN ðv ¼ 1 r v ¼ 0Þ CO ðv ¼ 1 r v ¼ 0Þ N2 O ð000 1Þ r ð000 0Þ NO ðv ¼ 1 r v ¼ 0Þ CO2 ð000 1Þ r ð000 0Þ NCCOðv ¼ 1 r v ¼ 0Þ

Rð7Þ at 2064:37 cm1 Rð13Þ at 2193:359 cm1 Pð23Þ at 2202:744 cm1 Rð7:5Þ at 1903:123 cm1 Rð12Þ at 2322:568 cm1 at 1887:42 cm1

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Figure 2. Pseudo-first-order decay rate constants of the NCCO radical as a function of NO2 pressures. P(MCF) = 0.20 Torr; P(NO2) = variable; 193 nm laser pulse energy = ∼2.6 mJ.

NCCO + CN, NCCO + CH3O, and NCCO + NCCO reactions, as well as diffusion out of the probed region of the reaction cell (reaction with trace O2 present in the absorption cell may contribute as well). Upon the addition of 0.08 Torr of NO2, a significant increase in NCCO decay rate is observed. Typical conditions were 0.2 Torr MCF, 1.0 Torr SF6, and variable pressure of NO2, so typical NCCO radical densities were ∼3.0 1013 molecules cm3. Under these conditions, if the NO2 concentration is greater than 1.2 1015 molecules cm3 = 40 mTorr, pseudo-first-order kinetics is expected, and thus, the time-dependent NCCO concentration has the form ½NCCOt ¼ ½NCCO0 expð k0 tÞ

ð3Þ

k0 ¼ k1 ½NO2 þ k0

ð4Þ

The HITRAN molecular database was used to locate and identify the spectral lines of CO, N2O, CO2, and NO product molecules.23 Other published spectral data were used to locate and identify CN.24,25 The spectral lines used are near the peak of the rotational Boltzmann distribution, minimizing sensitivity to small heating effects. Typical experimental conditions were P(MCF) = 0.10.2 Torr, P(NO2) = 010 Torr, P(SF6, CF4) = 1.0 Torr, and excimer laser pulse energies of 25 mJ. SF6 or CF4 buffer gas was used to relax any nascent vibrationally excited molecules to a Boltzmann distribution.21,22 Methyl cyanoformate (Aldrich, 99%) was purified by repeated freezepumpthaw cycles at 77 K. SF6, CF4, and C2H4 (Matheson) were purified by passing through an Ascarite II column to remove trace amounts of CO2. CH4 (Matheson, 99.99%) was used without further purification. NO2 (Matheson, 99.5%) were purified by repeated freeze pumpthaw cycles at 153 K to remove NO2 and N2O.

k0 is the observed pseudo-first-order NCCO signal decay rate, k1 is the desired bimolecular rate constant for the NCCO + NO2 reaction, and k0 represents other loss mechanisms as described above. The transient signals were fit to the above function to determine the decay rate k0 . Good fits were obtained for all signals except those with zero NO2 pressure, where radical radical reactions dominate and pseudo-first-order conditions are therefore not expected. Figure 2 shows the resulting decay rates as a function of NO2 pressures. Linear dependences are observed, as is expected under the pseudo-first-order kinetics conditions, in which [NO2] . [NCCO]. As per standard kinetic treatment, the slopes of these plots are the desired bimolecular rate constant k1:

3. RESULTS

where the error bar represents two standard deviations. Similarly we have measured the rate constant of the reactions of NCCO + CH4 and NCCO + C2H4:

3.1. Total Rate Constants. The total rate constant was measured by diode IR spectroscopy detection of NCCO. Figure 1 shows typical NCCO transient infrared absorption signals detected with and without the NO2 reagent. In the absence of NO2 reagent, an ∼8000 s1 decay rate over the time range 160 μs is observed. This decay is attributed to the removal of NCCO radicals by pathways other than the title reaction, including

k1 ¼ ð2:1 ( 0:1Þ 1011 cm3 molecule1 s1

NCCO þ CH4 f products

ð5Þ

NCCO þ C2 H4 f products

ð6Þ

We cannot observe any significant change in the NCCO decay rates upon addition of CH4 or C2H4 reagents up to pressures of 12174

dx.doi.org/10.1021/jp207121p |J. Phys. Chem. A 2011, 115, 12173–12183


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Figure 3. Infrared absorption signals of CO. Reaction condition: P(MCF) = 0.1 Torr; P(CF4) = 1.0 Torr; P(NO2) = variable; photolysis laser pulse enengy = 4.3 mJ.

Figure 4. Product yield of CO molecules as a function of NO2 pressure. Reaction conditions: see Figure 3.

channels to be 2.0 Torr, indicating these two reactions are very slow. Following the standard kinetics treatment, we obtained upper limits of k5 e 7.0 1014 cm3 molecule1 s1 and k6 e 5.0 1015 cm3 molecule1 s1. The reaction NCCO + C2H4 is sufficiently slow that we can use C2H4 reagents in the experiments described below to rapidly remove CN radicals without consuming NCCO radicals. 3.2. Product Channel Measurements. IR diode laser absorption was used to detect CO, CO2, NO, N2O, and NCNO products from eq 1. The detected transient IR signal amplitudes (peakpeak) were converted into absolute concentration using HITRAN line-strengths as described in a previous publication.26 We first detected CO molecules upon photolysis of a MCF (0.1 Torr)/CF4 (1.0 Torr) mixture, to estimate the amount of background signal from the photolysis of MCF, eq 2c. Then we detected CO molecules upon photolysis of MCF/NO2/CF4 mixtures, expecting increased CO production if eq 1bd are significant. However, as shown in Figure 3, we observed the opposite effect, a significant decrease in the CO signal when NO2 was included in the reaction mixture. Figure 4 shows the resulting dependence of CO yield as a function of the reagent NO2 pressure. Our interpretation of this surprising result is that, first, eq 1bd are not the dominant product channels of the title reaction; second, CO produced in the photolysis of MCF (i.e., eq 2c) is probably produced in a stepwise mechanism involving eq 2a followed by dissociation of internally hot NCCO:6 NCCO f CO þ CN

ð7Þ

Reaction 7 is apparently slow enough under our conditions that the title reaction (eq 1) can compete for NCCO radicals if NO2 is included in the reaction mixture. This results in the decrease CO yield at high NO2 pressures. Close examination of Figure 4 shows that, at high NO2 reagent pressures, the CO yield levels off to a very small but constant value, which we attribute primarily to small contributions from eq 1bd, although other minor secondary reactions may also contribute. From this asymptotic CO yield, we estimate an upper limit to the total branching ratios of these three minor

Φ1b, 1c, 1d ¼ ½CO1b, 1c, 1d =½NCCO0 e ½COresidual =½CO0 ¼ 0:05 where [CO]1b,1c,1d represents CO yield from eq 1bd; [CO]residual represents the average level of CO yield at 510 Torr of NO2, which includes [CO]1b,1c,1d and that from other secondary reactions. [CO]0 represents the yield of CO molecules from reaction 7, which is less than or equal to the initial NCCO number density, that is, [NCCO]0 g [CO]0. We note here the following secondary reactions that take place if NO2 is present and CN radicals are produced, either via the stepwise photolysis, reaction 7, or from eq 1a of the title reaction:

These reactions are fast, with total rate constants of k8 = 8.9 1011 cm3 molecule1 s1 and k9 = 1.83 1011 cm3 molecule1 s1.27,28 The product branching ratios were taken from literature27,28 and indicate that these reactions represent at best very minor additional sources of CO but potentially very significant sources of CO2 and N2O. Large amounts of N2O were detected upon photolysis of a MCF (0.1Torr)/NO2 (0.1 Torr)/SF6 (1.0 Torr) mixture. As described above, however, substantial secondary sources of N2O exist in this reaction; thus, we cannot immediately assign this production to eq 1c of the title reaction. In particular, N2O is a major product of the reaction of NCO with NO2, eq 9. NCO radicals could originate either from eq 1b of the title reaction, or from photolytic (or eq 1a) CN radical formation followed by eq 8. To examine this issue, we performed experiments in which C2H4 was included in the reaction mixture. The primary effect 12175



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Table 1. Reactions Used in Kinetic Simulation of N2O Yields k (298 K) reaction a CN þ NO2 f NCO þ NO CN þ NO2 f CO2 þ N2 a NCO þ NO2 f CO2 þ N2 O NCO þ NO2 f CO þ 2NO CN þ C2 H4 f H þ CH2 CHCN NCO þ C2 H4 f products NCO þ NO f N2 O þ CO NCO þ NO f CO2 þ N2 a

cm3 molecule1 s1

ref

8.05 1011

27

5.2 10

12 11

1.64 10 1.3 10

12

2.91 10

10

12

2.85 10

11

1.45 10

11

1.85 10

27 28 28 29 30 26 26

Model B only.

Figure 5. Dependance of N2O yield on added C2H4 reagent. Experimental data (open squares) obtained under the reaction conditions: P(MCF) = 0.1 Torr; P(NO2) = 0.1 Torr; P(SF6) = 1.0 Torr; photolysis laser pulse energy = 5.0 mJ. Simulaton of model A (Black diamond) assumes N2O was produced from eq 1b + eq 9a; simulaton of model B (dots) assumes N2O was produced from eq 8a + eq 9a. N2O yields were nomalized to 1.0 at 0 Torr C2H4; for the experimental data, this represents 1.5 1013 molecules cm3.

of this reagent in these experiments is to efficiently remove CN radicals: C2 H4 þ CN f H þ C2 H3 CN

ð10Þ

The rate constant of reaction 10 was reported to be k10 = 2.9 1010 cm3 molecule 1 s1.29 If CN radicals are responsible for N2O formation via eq 8 and eq 9, we expect the N2O yield to decrease upon the addition of C2H4. The experimental result, shown as open squares in Figure 5, verifies that this is indeed the case. At a large pressure of [C2H4] = 1.0 Torr, the N2O yield has decreased to a very small value of ∼3.1 1011 molecules cm3. This represents an upper limit of N2O formation from eq 1c. An alternative explanation for the N2O yield dependence shown in Figure 5 is that instead of quenching CN radicals, the ethylene decreases the N2O yield by removing NCO radicals produced in eq 1b: C2 H4 þ NCO f products

ð11Þ 12

cm3 Reaction 11 has a total rate constant of k11 = 2.85 10 1 1 30 molecule s at 296 K. To determine whether the N2O arises from eq 1b and eq 9 or from eq 8 and eq 9, we carried out kinetics modeling simulations. Figure 5 (solid diamonds, designated model A) shows the simulated N2O yield versus ethylene pressure under the assumption that a significant yield of NCO was produced from eq 1b. Also shown in Figure 5 (solid circles, designated model B) is the simulated N2O yield under the assumption that eq 1b is not active, and all the NCO needed for eq 9 originated from CN radicals via eq 8. The kinetic mechanisms used in the simulations are shown in Table 1. As shown in Figure 5, model A predicts that large amounts of C2H4 would be necessary to quench the N2O yield, primarily because reaction 11 is relatively slow. In contrast, reaction 10 is very fast, so model B predicts much more efficient quenching behavior. Although neither model perfectly fits the experimental data, it is clear that model B is a much more accurate description.

Figure 6. Infrared absorption signal of CO2. Reaction conditions: P(MCF) = 0.1 Torr; P(NO2) = 0.1 Torr; P(SF6) = 1.0 Torr; P(C2H4) = 1.0 Torr.

We conclude that most of any NCO produced in this reaction system originated from eq 8 rather than eq 1b, and that, in agreement with the CO detection experiments described above, eq 1b is very minor. We estimate an upper limit to the total branching into eq 1b and eq 1c of Φ1b, 1c e 2% by comparison of the limiting N2O yield at high C2H4 pressures with the much larger CO2 yield of ∼21 1012 molecules cm3 from eq 1a, described below. Significant amounts of CO2 products were detected upon photolysis of MCF (0.1 Torr)/NO2 (variable)/C2H4 (1.0 Torr)/SF6 (1.0 Torr). A typical infrared transient signal is shown in Figure 6. Figure 7 shows the CO2 yield as a function of reagent NO2 pressure. The small yield detected in the absence of NO2 is attributed primarily to reaction of NCCO with trace amounts of O2 present, although minor photolysis channels to produce CO2 may also be present. Previous experiments showed that CO2 is a minor but significant product of that reaction.15 When NO2 is included in the reaction mixture, the title reaction (which is much faster than NCCO + O2), as well as secondary chemistry involving NO2, are expected to predominate. As shown in Figure 7, the CO2 yield levels off at NO2 pressures of about ∼0.05 Torr. 12176



Figure 7. Product yield of CO2 molecules as a function of NO2 presure. Reaction condition: P(MCF) = 0.1 Torr; P(NO2) = variable; P(C2H4) = 1.0; Torr P(SF6) = 1.0 Torr; photolysis laser pulse enengy = 4.3 mJ.

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Figure 9. CN decay rates as a function of NO2 pressure. CN was produced from photolysis of methyl cyanoformate at 193 nm. Reaction condition: P(MCF) = 0.1 Torr; P(NO2) = 00.5 Torr.

upper limit of the branching fraction toward CO-forming channels of 0.09. There are several possible approaches to distinguishing between eq 1a and eq 1e. The most obvious approach is to directly detect NCNO produced in eq 1e. We attempted to detect infrared transient signals of NCNO near its ν1 fundamental frequency (2176.9 cm1).33 No such signals were found; however, the lack of line strength data for NCNO transitions prevents a quantitative measurement. A second approach is to detect NO products from eq 1a. We have detected large amounts of NO, with yields of ∼1.6 1013 molecules cm3 upon photolysis of an NO2 (0.1 Torr)/SF6 (1.0 Torr) mixture, and ∼5.8 1013 molecules cm3 upon photolysis of an MCF (0.1Torr)/NO2 (0.1 Torr)/SF6 (1.0 Torr) mixture. The lower yield is presumably due to NO2 dissociation and reaction of the resulting O atoms with NO2:

Figure 8. Product yield of CO2 as a function of C2H4 pressure. Reaction condition: P(MCF) = 0.1 Torr; P(NO2) = 0.1 Torr; P(C2H4) = variable; P(SF6) = 1.0 Torr; photolysis laser pulse energy = 4.0 mJ.

As described above in regards to CO and N2O yields, secondary eq 8 and eq 9 must be considered. In particular, eq 9a can produce large amounts of CO2. This secondary chemistry can be suppressed by addition of C2H4 reagent, resulting in reactions 10 and 11. Figure 8 shows the CO2 yield as a function of C2H4 pressure. As with the N2O yield (Figure 5), addition of C2H4 reduces the CO2 product yield, but unlike N2O, the CO2 yield curve levels off at a constant value at C2H4 pressures above ∼1.0 Torr. Under these conditions, eq 8 is completely suppressed, and the remaining CO2 yield can therefore be attributed to the title reaction, eq 1a, eq 1d, or eq 1e. The CO detection experiments described above ruled out eq 1d as a major product channel, indicating that eq 1a and eq 1e dominate the reaction. We note that C2H4 is included in the data shown in Figure 7, so the CO2 yields shown there have little or no contribution from eq 8 and eq 9. By comparing the limiting CO2 yield in Figure 7 (∼21 1012 molecules cm3) to the limiting CO yield of Figure 4, obtained using identical photolysis laser energy (∼2 1012 molecules cm3), we estimate an

NO2 þ hν ð193 nmÞ f O þ NO

ð12Þ

O þ NO2 f NO þ O2

ð13Þ

The larger yield produced when the NCCO precursor is included in the reaction mixture includes NO production from the title reaction, but also includes large amounts from secondary reactions, especially eq 8a, where the CN radicals can originate from either eq 2c or eq 1a. Unfortunately, addition of ethylene cannot suppress this secondary chemistry, because H atoms produced in reaction 10 would be expected to produce additional NO: H þ NO2 f OH þ NO

ð14Þ

Other possible CN radical quenchers suffer similar problems; for example, if C2H6 were used rather than C2H4, we would expect the following sequence of reactions, also leading to NO production: CN þ C2 H6 f HCN þ C2 H5

ð15Þ

C2 H5 þ NO2 f C2 H5 O þ NO

ð16Þ

Therefore, the NO detection experiments, while suggestive of the existence of eq 1a, do not provide unequivocal evidence. 12177



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Table 2. Total Energies (hartree), Zero-Point Energies (ZPE, hartree), and Relative Energies (ΔE, kcal/mol) for Species on the Singlet Surface to NCCO + NO2 Reaction Obtained at DFT and CCST(T) Levels of Theory geometry optimization

single point energy

species

B3LYP/6-311++G**

ZPE

CCSD(T)/6-311++G**

ΔEa

CO2 þ NO þ CNðP1Þ

411.325934

0.015015

410.3858246

35.4

NCO þ CO þ NOðP2Þ

411.325934

0.015015

410.4003876

44.5

N2 O þ 2COðP3Þ

411.396567

0.014722

410.4912373

101.7

CO2 þ CO þ N2 ðP4Þ

411.533363

0.022296

410.6321870

185.4

M1a

411.352232

0.027209

410.4406359

62.1

M1b

411.348894

0.027216

410.4368720

59.7

M1c

411.340628

0.026987

410.4282457

54.5

M1d

411.347119

0.027180

410.4358430

59.1

M2

411.320908

0.028965

410.4085195

40.9

M3a

411.346872

0.023786

410.4317254

58.7

M3b M4

411.313887 411.270982

0.024395 0.027107

410.4050936 410.3515182

41.6 6.2

Minimum

M5

411.211271

0.027523

410.2958855

28.9

M6

411.2719

0.024782

410.3529931

8.6

M7

411.272579

0.025906

410.3629763

14.2

M8

411.271907

0.024777

410.3529924

8.6

CO + M9a

411.301068

0.024751

410.3928044

33.6

CO + M9b

411.25507

0.02333

410.3436679

3.7

CO + M9c

411.335503

0.022854

410.4229232

53.7

Transition State

a

T1a/1b

411.343713

0.026982

410.430152

55.7

T1a/1d

411.346804

0.027007

410.4363271

59.5

T1a/8

411.228629

0.023379

410.3100806

17.4

T1b/1c

411.339833

0.026674

410.4278905

54.4

T1b/3a

411.316624

0.025146

410.39892

37.2

T1c/3b

411.298271

0.024472

410.3832574

27.8

T1d/7

411.234966

0.023113

410.3278817

6.1

T2/4 T2/5

411.250221 411.162937

0.025816 0.024995

410.3215775 410.2443804

11.7 59.7

T4/(CO+9a)

411.270863

0.026135

410.3550867

9.1

T5/6

411.179271

0.025528

410.2641337

47.6

T6/P2

411.272828

0.023733

410.3505806

7.8

T7/P2

411.267171

0.023334

410.3464365

5.4

T8/P2

411.272826

0.023740

410.3505903

7.8

CO + Ta/b

411.229652

0.022186

410.3163799

12.7

CO + Ta/c

411.215293

0.021453

410.3030962

20.6

Energy (kcal/mol) relative to reactants and corrected for ZPE, based on the DFT-B3LYP/6-311++G** vibrational frequencies.

One further experiment provides indirect evidence of the existence of CN as a product of the title reaction. If we detect CN radicals by infrared absorption, we can measure the decay rate and apply standard first order decay kinetics. Figure 9 shows the measured pseudofirst order decay rate of CN as a function of NO2, upon photolysis of MCF (0.1 Torr)/NO2 (variable) mixtures. Following a standard kinetics treatment, we obtain a rate constant of the CN + NO2 reaction from the slope of this plot, obtaining k8 = (1.33 ( 0.08) 1011 cm3 molecule1 s1 at 298 K (1σ error bars). This value is in substantial disagreement with several previous reports that have indicated a much

higher value of k8 = (7.28.9) 1011 cm3 molecule1 s1 at 298 K.27,31,32 There are many ways in which an experimentally measured rate constant may be erroneously high (such as contributions from secondary reactions, reactive impurities, etc.); however, there are fewer sources of error that would cause a measurement to be too low. The most likely explanation is that, in addition to photolytically produced CN radicals, the NCCO + NO2 reaction is generating CN radicals via eq 1a. This regeneration of CN radicals over the ∼10 μs time scale of the CN decay measurement causes the decay rate to be slower than expected on the basis of eq 8 alone. Based on the above 12178



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Table 3. Total Energies (hartree), Zero-Point Energies (ZPE, hartree), and Relative Energies (ΔE, kcal/mol) for Species on the Triplet Surface Relevant to NCCO + NO2 Reaction Obtained at DFT and CCST(T) Levels of Theory geometry optimization

single point energy

species

B3LYP/6-311++G**

ZPE

CCSD(T)/6-311++G**

ΔEa

CO2 þ NO þ CNðP1Þ

411.325934

0.015015

410.3858246

35.4

NCO þ CO þ NOðP2Þ

411.325934

0.015015

410.4003876

44.5

Minimum M1

411.272429

0.026219

410.348579

5

M2

411.245998

0.025551

410.313942

16.4

M4

411.235056

0.02714

410.306811

21.8

M9

411.236378

0.027442

410.291522

31.6

M10 M11

411.240436 411.251965

0.024139 0.026504

410.315615 410.321832

14.4 12

M12

411.254912

0.026579

410.323181

11.2

Transition State

a

TR/1

411.243157

0.024273

410.305727

20.7

TR/2

411.212431

0.022251

410.283487

33.4

TR/4

411.228284

0.024432

410.290672

30.3

TR/9

411.23598

0.02587

410.289305

32

TR/10 T2/11

411.207702 411.228859

0.021588 0.025795

410.278223 410.290969

36.3 30.9

T11/P2

411.251405

0.026653

410.319059

13.8

T4/12

411.226793

0.026608

410.293044

30.1

T12/P1

411.248156

0.024428

410.311181

17.4

Energy (kcal/mol) relative to reactants and corrected for ZPE, based on the DFT-B3LYP/6-311++G** vibrational frequencies.

evidence, we conclude that eq 1a is the dominant product channel of the title reaction.

4. POTENTIAL ENERGY SURFACE The Gaussian 03 set of programs34 was used to characterize the singlet and triplet potential energy surfaces. Geometry optimization at the DFT-B3LYP/6-311++G** level was used to obtain geometries of reactants, products, minima, and transition states through the reaction. Vibrational analysis was used to identify minima and transition states according to the number of vibrational imaginary frequencies. Intrinsic reaction coordinate (IRC) calculations were used to link each transition state structure with corresponding minimum geometries to develop an energy profile. Following geometry optimization, single point energies were computed at the CCSD(T)/6-311++G** level of theory. Values from these calculations were combined with zero point energy (ZPE) corrections, determined using force constants calculated at the DFT-B3LYP/6-311++G** level, to give a total energy. These values were compared to the total energy of the reactants, NCCO + NO2, to give the relative energies shown in Tables 2 and 3. The optimized structure of minimum and saddle points for the singlet state are shown in Figure 10. The optimized structures for the triplet states are shown in Figure 11. 4.1. Singlet Surface. A number of minima and transition states (Figure 10) were found on the singlet surface. The singlet potential energy surface is shown in Figure 12. As shown in Figure 12a, M1a is a direct adduct product from the reactants. It is formed by α-C atom NCCO attack onto an O atom of NO2, giving a minimum with an energy of 62.1 kcal/mol relative to the reactants.

M1b, M1c, and M1d are structures similar to M1a, except for different orientations of the NO bond. However, careful potential energy surface scan shows they are not direct adducts of the reactants. M1b is formed from M1a via small barrier associated with the transition state T1a/1b, which involves an out-of plane rotation of the NO moiety, energetically lying at 55.7 kcal/mol below the reactants. M1c and M1d are formed from M1a and M1b through out-of-plane rotation of O atom via transition state T1b/1c, and T1a/1d, respectively. The barrier heights for these transformations are about 5.3 and 2.6 kcal/mol. M1b can evolve to M3a via the transition state T1b/3a which involves concurrent stretching of the CC and NO bonds. T1b/3a, lying 37.2 kcal/mol below the reactants represents a barrier of 22.5 kcal/mol from M1b. M3a is a weak bound complex of CO2 and CNNO, which are very close to their separate stable structures, and can readily decompose to CO2 + NO + CN products. The dissociation energy for the process is 23.3 kcal/mol, which can be available from the previous formation energy of M1a. The overall pathway for formation of the CO2 + NO + CN product channel is therefore R f M1a f T1a=1b ð 55:7Þ f M1b f T1b=3a ð 37:2Þ f M3a f CO2 þ NO þ CNðP1Þ ðiÞ (The energy values of the transition states, relative to the reactants, are shown in parentheses in kcal/mol). The highly lying transition state T1b/3a is lower than the energy of P1, so the key step is the dissociation of M3a. 12179



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Figure 10. DFT-B3LYP/6-311++G** optimized geometries of the minima and the transition states on the singlet state potential energy surface in the NCCO + NO2 reaction. Bond lengths are given in Å and bond angles in degrees. 12180



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Figure 11. DFT-B3LYP/6-311++G** optimized geometries of the minima and the transition states on the triplet state potential energy surface in the NCCO + NO2 reaction. Bond lengths are given in Å and bond angles in degrees.

An alternative pathway to the same products involves transformation of M1b into M1c, as mentioned above. M1c can then form M3b via the transition state T1c/3b, which involves stretching of the CC and NO bonds. T1c/3b lies at 27.8 kcal/mol below the reactants. M3b is a weakly bound complex of CO2 and NOCN, which can dissociate to CO2 + NO + CN. The dissociation energy is 19.1 kcal/mol. The secondary pathway of P1 is therefore R f ... f M1b f T1b=1c f M1c f T1c=3b f M3b f CO2 þ NO þ CNðP1Þ

ðiiÞ

T1c/3b is identified as the key transition state of the pathway, which is higher than the energy of P1. The pathway i is more favorable than pathway ii for the formation of P1, but both pathways are energetically accessible with no barriers higher than the original reactants. There are five pathways leading to product NCO + CO + NO found on the singlet PES. Two are shown in Figure 12a: R f M1a f T1a=1d f M1d f T1d=7 f M7 f T7=P2 f NCO þ CO þ NOðP2Þ R f M1a f T1a=8 f M8 f T8=P2 f NCO þ CO þ NOðP2Þ

þ CO þ NOðP2Þ R f ... f CO þ M9a f CO þ Ta=b f CO þ M9b f ... f NCO þ NO þ COðP2Þ

R f ... f CO þ M9a f ... f CO þ M9b f ... f 2CO þ N2 OðP3Þ

ðivÞ

R f ... f CO þ M9a f ... f CO þ M9b f ... f CO þ CO2 þ N2 OðP4Þ

R f M2 f T2=5 f M5 f T5=6 f M6 f T6=P2 ðvÞ

ðviÞ

ðviiÞ

In pathway vii, M9a isomerizes to M9b via the transition state Ta/b. The further isomerization of M9b involves many minima and transition states, which have been studied in detail by Zhang et al.35 We simply note that CO + M9b can finally proceed to product NCO + NO + CO. These five pathways all involve one or more transition states which have energies above the reactants, such as T1d/7 (6.1 kcal/mol) for iii, T1a/8 (17.4 kcal/mol) for iv, T2/5 (59.7 kcal/mol) for v, and CO+Ta/c (20.6 kcal/mol) for pathways vi and vii. These pathways are, therefore, kinetically unfavorable. In addition to the isomerization of M9b,35 CO + M9b can also proceed to produce 2CO + N2O (P3) and CO + CO2 + N2O (P4):

ðiiiÞ

The other three are given in Figure 12b:

f NCO þ CO þ NOðP2Þ

R f M2 f T2=4 f M4 f T4=ðCO þ 9aÞ f CO þ M9a f CO þ Ta=c f CO þ M9c f NCO

ðviiiÞ

ðixÞ

These two involve multiples high transition states in common with pathways vi and vii, so they are also unfavorable. 4.2. Triplet Surface. Figure 13 shows that M1, M2, M4, M9, and M10 are intermediates formed from the reactants via high 12181



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We note that all the transition states and minima (except M1) are above the reactants. Furthermore, formation of the M1 involves a large barrier. We, therefore, conclude that the triplet potential surface does not play an important role in this reaction, at least at moderate temperatures. 4. 3. Reaction Mechanism. In summary, ab initio calculations indicate that the reaction takes place primarily on the singlet potential energy surface. The calculations indicate that CO2 + NO + CN is the dominant product channel, in agreement with the experimental observations. This channel may be formed by two different pathways, i or ii. Pathway i is probably the dominant mechanism because its key transition state T1b/3a is lower than T1c/3b in pathway ii.

5. CONCLUSIONS The kinetics of the NCCO + NO2 was studied using transient infrared laser spectroscopy. This reaction is fast, with a rate constant k1 = (2.1 ( 0.1) 1011 cm3 molecule1 s1 at 298 K. Product detection and consideration of likely secondary chemistry leads to the conclusion that the primary product channel is a CO2 producing channel, most likely CO2 + NO + CN. Ab initio calculations show that this channel is the most readily accessible via a low-barrier pathway, in agreement with the experimental results.

Figure 12. Singlet potential energy surface of the NCCO + NO2 reaction. Relative energies are taken from the CCSD(T)/6-311++G** values, on base of optimized geometries and zero-point energy corrections calculated at the B3LYP/6-311++ G** level.

Figure 13. Triplet potential energy surface of the NCCO + NO2 reaction. Relative energies are taken from the CCSD(T)/6-311++G** values, on the basis of optimized geometries and zero-point energy corrections calculated at the B3LYP/6-311++ G** level.

transition states TR/1, TR/2, TR/4, TR/9, TR/10. M2 and M4 can further isomerizes, leading to P1 and P2 products: R f TR=4 f M4 f T4=12 f M12 f T12=P1 f CO2 þ NO þ CNðP1Þ ðxÞ R f TR=2 f M2 f T2=11 f M11 f T11=P2 f NCO þ CO þ NOðP2Þ ðxiÞ

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Kinetics of the NCCO+ NO2 Reaction

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