Product Channels of the CN + HCNO Reaction - The Journal of

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Product Channels of the CN + HCNO Reaction Wenhui Feng and John F. Hershberger* Department of Chemistry and Biochemistry, North Dakota State University, Department 2735, P.O. Box 6050, Fargo, North Dakota 58108-6050, United States ABSTRACT: The kinetics of the reaction of CN radical with fulminic acid (HCNO) was studied by transient infrared absorption spectroscopy with the primary goal of resolving whether the dominant product channel is NO + HCCN (1a) or HCN + NCO (1b). HCN, HCCN, and NO reaction products were directly detected. In some experiments, 15N18O reagent was included in the reaction mixtures in order to suppress possible secondary chemistry due to NCO radicals. Several other possible secondary reactions were also investigated and found to be very slow. The resulting product branching fractions of φ1a = 0.98 ± 0.07 for NO + HCCN and φ1b ≤ 0.07 for HCN + NCO, respectively, were obtained at 298 K. The potential energy surface (PES) of the reaction was calculated by ab initio methods at several levels of coupled-cluster theory. The calculations show pathways to channels (1a) and (1b) with nearly identical energetics and a substantial dependence on the level of theory used, suggesting that multireference calculations are needed to accurately predict the experimental results.

1. INTRODUCTION Fulminic acid, HCNO, is an important intermediate in NOreburning processes for the reduction of NOx pollutant emissions from fossil-fuel combustion.1 HCNO is formed in combustion primarily by the CH2 + NO2−5 and HCCO + NO6−10 reactions. Several experimental11−14 and computational studies15−23 on the reaction kinetics of HCNO have been reported, due to its interest in the overall NO-reburning mechanism. In a previous publication from our laboratory,12 we reported an experimental study of the kinetics of the CN + HCNO reaction using diode laser infrared (IR) spectroscopy. The results show this is a fast reaction with rate constant k(T) = (3.95 ± 0.53) × 10−11 exp[(287.1 ± 44.5)/T)] cm3 molecule−1 s−1 over the temperature range 298−388 K. At 298 K, k = (1.04 ± 0.1) × 10−10 cm3 molecule−1 s−1. Many product channels are thermodynamically possible:

Thermochemical information has been obtained from standard tables24 as well as other literature for the heats of formation of HCNO, NCO,25 HCNN,26 HCCO,27 NCCO,28 and CCO.29 In our previous experiment,12 we detected NO products, presumably from channel 1a, and upon the addition of NO reagent, we detected CO, CO2, and N2O secondary products, which we attributed to the following secondary chemistry: NCO + NO → N2O + CO → N2 + CO2

ΔH °298 = −32.4 kJ/mol

(1a)

→ NCO + HCN

ΔH °298 = −343.5 kJ/mol

(1b)

→ CNO + HCN

ΔH °298 = −343.5 kJ/mol

(1c)

(2b)

Where NCO originated either from channel 1b or from channel 1a followed by (3)

HCCN + NO → HCN + NCO

In that study, we determined that channels 1a and/or 1b dominated the reaction and that other product channels are minor. In order to distinguish between 1a and 1b, we replaced the NO reagent with 15N18O, resulting in the following expected secondary chemistry:

CN + HCNO → NO + HCCN

(2a)

NCO (from (1b)) + 15 N18O → N15 N18O + CO

(4a)

→ N15 N + OC18O

(4b)

→ C2N2 + OH

ΔH °298 = − 258.1 kJ/mol

→ HCCO + N2

ΔH °298 = −428.9 kJ/mol

(1e)

HCCN (from (1a)) + 15 N18O → HC15 N + NC18O

→ HCO + NCN

ΔH °298 = −89.6 kJ/mol

(1f)

NC18O + 15 N18O → N15 N18O + C18O

(1d)

15

→ NH + NCCO

ΔH °298 = −19.4 kJ/mol

(1g)

→ CO + HCNN

ΔH °298 = −256.9 kJ/mol

(1h)

ΔH °298 = − 7.2 kJ/mol

(1i)

→ CCO + N2 + H

© 2012 American Chemical Society

or

18

18

→ N N + OC O

(5)

(6a) (6b)

Received: July 9, 2012 Revised: September 21, 2012 Published: September 22, 2012 10285

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By infrared detection and quantification of 16OC18O and 18 OC18O, we concluded that most of the NCO originates from the HCCN + NO reaction, and therefore, channel 1a of the title reaction dominates. Subsequent to our study, Pang et al. reported ab initio calculations of the potential energy surface of the reaction at the G3B3 and CCSD(T)/aug-cc-pVTZ levels.21 Their results agree that channels other than 1a and 1b are minor, but they found a slightly lower barrier to HCN + NCO than to HCCN + NO and therefore suggested that channel 1b dominates and that NO molecules detected in our experiment might originate from secondary reactions such as NCO + HCNO. The NCO + HCNO reaction has been studied by several groups13,17−19 and established to be a fast reaction with the rate constant of k = 1.58 × 10−11 cm3 molecule−1 s−1 at 298 K and the primary product channel to be CO + NO + HCN. This reaction can therefore produce NO molecules and thus affect the product detection of NO from channel 1a, especially if HCN + NCO is a significant or main product channel of the title reaction, as suggested by Pang et al.21 This still does not explain our OC18O/18OC18O data, but we concede that the OC18O signals in that experiment did have a substantial background, presumably due to NCO production from HCNO photolysis. In this article, we report new experimental measurements on the title reaction aimed at resolving the question of whether channel 1a or 1b dominates. These data support our earlier conclusion that channel 1a is the primary reaction product. We also report ab initio calculations, which, while not disputing the results of Pang et al, suggest that the relative energetics of the HCCN + NO products and the barrier to HCN + NCO formation are close and sufficiently dependent on the choice of theoretical method and basis set to make this system too close to call by standard ab initio methods. NH, HCCO, and HCO are possible minor products of the title reaction, and HNCO is a common trace impurity of HCNO reagent in these experiments. Secondary reactions such as NH + HCNO, HCCO + HCNO, HCO + HCNO, and CN + HNCO could possibly have also made a minor contribution to the products detected in our previous experiments. To investigate this possibility, we report measurements of the rate constants of these reactions.

characterized by Fourier transform (FT)-IR spectroscopy and was typically 95% pure or better, with only small CO2 and HNCO impurities. Because HCNO has poor long-term stability, samples were kept at 77 K except when filling the reaction cell. In general, HCNO could be allowed to stand at room temperature for ∼5 min in our Pyrex absorption cell with minimal decomposition. ICN (Aldrich) was purified by vacuum sublimation to remove dissolved air. SF6 (Matheson) was purified by repeated freeze−pump−thaw cycles at 77 K and by passing through an Ascarite II column to remove traces of CO2. 15N18O (Isotec) were purified by repeated freeze−pump−thaw cycles at 153 K. The following molecules were probed using infrared diode laser absorption spectroscopy: NO(v=1←v=0) HCN(v=1←v=0)

P(19) at 3251.823 cm−1

HC15N(ν1, v=1←v=0)

P(19) at 3252.206 cm−1

HCCN(ν1 , v=1←v=0)

P(25) at 3327.805 cm−1

DCO(ν1 , v=1←v=0)

at ∼ 1909.8 cm−1

HCCO(ν2 , K =0, v=1←v=0) CN(v=1←v=0)

R(7) at 2028.25 cm−1

at 2067.91 cm−1

The HITRAN molecular database was used to locate and identify the spectral lines of NO and HCN product molecules.33 Other published spectral data were used to locate and identify HC15N,34 DCO,35 and HCCO36 lines. Typical experimental conditions were P(ICN) = 0.1 Torr, P(HCNO) = 0.2 Torr, P(SF6) = 1.00 Torr, P(15N18O) = 0−1.0 Torr, and UV laser pulse energies of 12 mJ. The choice of buffer gas SF6 was motivated by the desire to relax any nascent vibrationally excited product molecules to a Boltzmann distribution.37,38 The measurement of the NH + HCNO rate constant was performed using 266 nm Nd:YAG laser photolysis of HN3 to produce NH radicals and a YAG-pumped dye laser at 336 nm to detect NH by laser-induced fluorescence, under pseudofirst order conditions. Typical conditions were P(HN3) = 0.5 Torr and 266 nm photolysis energy of 2.5 mJ/pulse.

2. EXPERIMENTAL SECTION CN radicals were created by photolysis of ICN using the fourth harmonic of an Nd:YAG laser at 266 nm: ICN + hν (266 nm) → CN + I

R(8.5e) at 1903 cm−1

(7)

3. RESULTS We concentrate on the proposed product channels 1a and 1b in this discussion. In our previous experiment,12 we detected large yield of NO product upon 248 nm laser photolysis of an ICN/ HCNO/SF6 mixture, which was attributed to the product channel 1a. At that time, we ignored the possible interference from the secondary reaction of NCO with HCNO, where NCO may originate from photolysis of HCNO and/or from channel 1b:

Reaction products were detected by infrared diode laser absorption spectroscopy. Several lead salt diode lasers (Laser Components) operating in the 80−110 K temperature range were used to provide tunable infrared probe laser light. The IR beam was collimated by a lens and combined with the UV light by means of a dichroic mirror, and both 0.6 cm diameter beams were copropagated though a 1.43 m absorption cell. After the UV light was removed by a second dichroic mirror, the infrared beam was then passed into a 1/4 m monochromator and focused onto a 1 mm InSb detector (Cincinnati Electronics, ∼1 μs response time). Transient infrared absorption signals were recorded on a digital oscilloscope and transferred to a computer for analysis. All experiments were performed at 298 K. HCNO samples were synthesized as previously described30−32 by flash vacuum pyrolysis of 3-phenyl-4-oximinoisoxazol-5(4H)-one. The purity of the HCNO samples was

NCO + HCNO → NO + CO + HCN

(8)

Our approach here is to add a reagent that will quickly remove NCO and therefore suppress reaction 8, without removing CN radicals or producing the detected NO products. An ideal reagent for this purpose is the double labeled isotope 15N18O, first used for this purpose in our previous study of the CN + O2 reaction.39 This reagent quickly removes NCO: 10286

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NCO + 15 N18O → N15 N18O + CO

(9a)

→ N15 N + CO18O

(9b)

15

HCNO and subsequent secondary chemistry. We note that, compared to our previous experiments,12 which were performed using 248 nm photolysis light, the absorption of HCNO at 266 nm is somewhat smaller, resulting in smaller background signals. These background signals were subtracted from the signals obtained with ICN to obtain the signals that would be expected in the absence of HCNO photolysis. Although this procedure is only approximate, it is sufficient when the correction is small, as in Figure 1a. The resulting signals were then converted to absolute concentrations using the peak−peak amplitudes and HITRAN line strengths.38 In Figure 2, we show the yield of unlabeled 14N16O molecules as a

18

The CN + N O reaction is much slower, with (unlabeled version) k = 1.25 × 10−13 cm3 molecule−1 s−1 at 298 K and 2.0 Torr.40 This reagent therefore suppresses NCO secondary chemistry without affecting the title reaction, and unlabeled 14 16 N O products can be detected with virtually no interference. Figure 1 shows transient infrared absorption signals for unlabeled 14N16O and HCN molecules. Signals such as the NO

Figure 2. Product yield of NO as a function of 15N18O pressure. Reaction condition: P(ICN) = 0.1 Torr, P(HCNO) = 0.2 Torr, P(SF6) = 1.0 Torr, P(15N18O) = variable, and 266 nm photolysis laser pulse energy = 12 mJ.

function of added 15N18O reagent. The key point is that, if reaction 8 were contributing to the 14N16O yield, we would expect the data in Figure 2 to show a rapid decay with increasing [15N18O]; at 0.4 Torr of 15N18O, reaction 8 should be largely suppressed. Figure 2, however, shows that the 14N16O yield is independent of added 15N18O, verifying that the detected 14N16O products were not produced by secondary chemistry from NCO or any other radical such as NH, HCCO, etc., that reacts quickly with NO. Assuming that all of the detected 14N16O (except for the HCNO photolysis background) originates from channel 1a, we can estimate a branching fraction into 1a by comparing the yield of 14N16O with the initial number of CN radicals produced in the ICN photolysis. After subtracting the photolysis background, we estimated [NO] = 4.86 × 1013 molecules cm−3 under the conditions of Figure 1. Using the 266 nm absorption coefficient of ICN (α = 0.009 Torr−1 cm−1) and a quantum yield of unity, we estimate [CN]0 = 4.95 × 1013 molecules cm−3 in the 0.6 cm diameter cylindrical reaction volume. The ratio of NO yield to initial CN concentration gives a branching fraction into channel 1a of φ1a = 0.98 ± 0.07. This result strongly suggests 1a as the dominant product channel. In a second series of experiments, we attempted to detect HCN products from channel 1b, using a newly acquired laser diode at ∼3250 cm−1. The lower panel of Figure 1 shows HCN transient signals with and without the ICN precursor. As shown, the yields of HCN produced with and without ICN are nearly identical, indicating that this HCN is primarily produced by photolysis of HCNO. By subtracting these signals and

Figure 1. Transient infrared absorption signals of NO and HCN. Reaction conditions: P(ICN) = 0.1 Torr (upper traces only), P(HCNO) = 0.2 Torr, P(15N18O) = 0.4 Torr, P(SF6) = 1.0 Torr, and 266 nm photolysis laser pulse energy = 12 mJ.

traces shown in Figure 1a typically display a fast rise time, which is attributed to formation of product molecules by the title reaction as well as possible secondary chemistry, discussed below. The rise times for these signals is often slower than predicted based on chemical kinetic considerations alone because the rise times are also affected by the kinetics of vibrational relaxation of an excited nascent product distribution to a Boltzmann distribution. This relaxation typically takes place on a time scale of ∼10−100 μs, depending on the detected molecule and choice and pressure of buffer gas. Results from a previous publication39 have suggested that 15 18 N O is an effective relaxer of 14N16O vibrational excitation. The transient signals also typically display a slow decay on a ∼millisecond time scale, which is attributed to diffusion of detected molecules out of the probed cylindrical volume. As shown in Figure 1, signals were collected with and without the ICN precursor molecules. The background signals obtained in the absence of ICN are attributed to photolysis of 10287

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converting to absolute concentrations and comparing to [CN]0 as described above, we estimate an upper limit for HCN production from 1b of φ1b < 0.07. In a third experiment, we directly detect HCCN, using the spectral data of Curl et al.41 Figure 3 shows a transient signal

Figure 4. Transient infrared absorption signal of HC15N. Reaction condition: P(ICN) = 0.1 Torr, P(HCNO) = 0.2 Torr, P(15N18O) = 1.0 Torr, P(SF6) = 1.0 Torr, and 266 nm photolysis laser pulse energy = 11.2 mJ.

expect the labeled nitrogen to appear in the HCN product, as shown above in reaction 5. Production of HCN + 15NC18O would appear to be far less likely, and this assertion is supported by the lack of unlabeled HCN detection shown in Figure 1b. Note that no secondary chemistry of any NCO potentially formed in 1b would lead to HC15N products; we are not aware of any route to HC15N in our experiment other than reaction 5. By converting the HC15N absorption signal into an absolute concentration, we obtain [HC15N] ≈ 2.2 × 1013 molecules cm−3, under conditions of initial [CN]0 ≈ 4.6 × 1013 molecules cm−3. We find no significant dependence of [HC15N] on 15N18O pressure over the range 0.4−2.0 Torr. This would imply a branching fraction of only ∼0.48; however, that assumes that HCN is the only product channel of reaction 3. In their computational study, Chen et al.43 suggested that adduct formation competes with HCN + NCO formation in reaction 3, with the details both temperature and pressure dependent. Our HC15N detection experiment therefore is qualitative in nature but definitely provides further evidence of channel 1a. In a further series of experiment, we explored several possible minor secondary reactions:

Figure 3. Transient infrared absorption signal of HCCN (ν1, P(25) line at 3327.805 cm−1). Reaction condition: P(ICN) = 0.2 Torr, P(HCNO) = 0.2 Torr, P(SF6) = 1.0 Torr, and 266 nm photolysis laser pulse energy = 10 mJ. Signal is an average of 9 laser shots.

for HCCN at 3327.805 cm−1, which is assigned to the P(25) transition of the ν1 vibrational mode. A similar transient was also detected at 3241.489 cm−1, assigned to P(7). These particular transitions were chosen partly for good infrared power but partly because they do not have spectral overlap with any HCN or HCNO lines. These signals were unchanged by varying the SF6 buffer gas pressure over the range 1.0−3.0 Torr. No HCCN signal was detected from photolysis of HCNO (i.e., without the ICN precursor). Although these signals are small, involving absorption of only ∼0.7% of the incident infrared light, they do demonstrate that HCCN is a product of the title reaction. Unfortunately, line strengths are unknown, so we have no way of quantifying the yield from the magnitude of the signals. However, the amplitude of the HCCN signals are comparable with those obtained from 193 nm photolysis of 0.2 Torr dibromoacetonitrile (Br2HCCN) at similar laser pulse energies; this was the precursor that was used to produce HCCN radical in the study of the HCCN + NO reaction by Curl et al.41 In addition to direct detection of HCCN, we have additional evidence of its formation in this reaction. We have successfully detected the HC15N isotope upon photolysis of an ICN/ HCNO/15N18O/SF6 mixture, as shown in Figure 4. Note that, unlike the unlabeled HCN detection described above, there is no background signal from HCNO photolysis. We attribute the HC15N formation to channel 1a followed by the previously stated reaction 5:

NH + HCNO → products

(10)

DCO + HCNO → products

(11)

HCCO + HCNO → products

(12)

CN + HNCO → products

(13)

Reactions 10−12 involve possible minor product channels of the title reaction. Reaction 13 involves HNCO, isocyanic acid, which is commonly present as a ∼5% impurity in fulminic acid samples. No experimental data has previously been reported for these reactions. We obtained upper limits for the 298 K rate constants of reactions 10−13 using standard pseudofirst order kinetics techniques, similar to our previous reports of the kinetics of HCNO with OH,11 CN,12 and NCO13 radicals. For the NH + HCNO reaction, we used laser induced fluorescence (LIF) at 336 nm to monitor the decay of NH radicals produced by 266 nm Nd:YAG laser photolysis of HN3 precursor molecules. DCO, HCCO, and CN were detected by infrared diode laser absorption spectroscopy at 1909.8, 2028.25, and

HCCN + 15 N18O → HC15 N + NC18O

The unlabeled version of this reaction (reaction 3, above) has been studied by Curl et al.42 and Chen et al,43 obtaining a rate constant of (3.5 ± 0.6) × 10−11 cm3 molecule−1 s−1, and identifying HCN + NCO as a major reaction product. If this reaction proceeds via NO attack at the C−H carbon atom of HCCN to make an HC(NO)CN intermediate, followed by a four-center transition state that dissociates to products, we 10288

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2067.91 cm−1, respectively, upon photolysis of D2CO (at 248 nm), C2H5OCCH (at 193 nm), and ICN (at 266 nm) precursor molecules, respectively. (DCO was detected rather than HCO because we did not have an appropriate laser diode for HCO). We found that these reactions are all very slow; we obtained that upper limits of their rate constants at 298 K are k10 ≤ 2.1× 10−13, k11 ≤ 1.7 × 10−13, k12 ≤ 2.2 × 10−13, and k13 ≤ 1.7 × 10−14 cm3 molecule−1 s−1, respectively. The upper limits of k(NH + HCNO) and k(CN + HNCO) are in agreement with theoretical calculations.22,23,44 We conclude that these reactions do not significantly affect our measurements of the product yields of the title reaction.

Table 1. Total Energies (hartree), Zero-Point Energies (ZPE, hartree), and Energies (kcal/mol) Referred to the Reactants for Species Relevant to CN + HCNO Reaction Obtained at DFT and CCST(T) Levels of Theory geometry optimization

4. POTENTIAL ENERGY SURFACE The Gaussian03 set of programs45 were used to perform an investigation of the potential energy surface of the reaction. Geometry optimization at the DFT-B3LYP/6-311++G(d,p) level was used to obtain geometries of reactants, products, and stationary points through the reaction. Three critical points were reoptimized at the CCSD(T)/6-31G(d,p) level. Vibrational analysis verified the identity of each stationary point as either a minimum or transition state. IRC (intrinsic reaction coordinate) calculations were used to link each transition state structure with corresponding intermediate geometries to develop an energy profile. Following the geometries optimization, a single-point energy calculation was performed at the CCSD(T)/6-311++G(d,p) 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(d,p) level, to give a total energy. These values were compared to the total energy of the reactants, CN + HCNO, to give the relative energies shown in Table 1. The structure of the reactants, products, intermediates, and transition states calculated in the present article are shown in Figure 5. The profile of the potential energy surface for the CN + HCNO reaction at the CCSD(T)/6-311++G(d,p) level is shown in Figure 6. As shown in Figure 6, the combination of the CN and HCNO radicals proceeds without a barrier and leads to a planar HC(CN)NO intermediate M1 as the initially formed energized adduct, with an energy of −69.7 kcal/mol relative to the reactants. M1, the cis form of the adduct HC(CN)NO, can isomerize to the trans form M1′ by rotation of a C−N bond. This process is neutral-energetic but is associated with a barrier height of 14.4 kcal/mol. Given the excess available energy, M1 and M1′ are probably in equilibrium with each other. Both M1 and M1′ can directly dissociate into HCCN + NO products (P1) by simply breaking the C−N bond. The dissociation energy 54.4 kcal/mol is large but the reaction can happen with the available excess energy. M1 can form a four-membered ring isomer M2 through cycloaddition of the terminal O atom and the C atom, via a transition state T1/2. The transition state is 24.3 kcal/mol below the reactants. The cyclic isomer M2 can decompose to the products HCN + NCO (P2) by breaking C−C and N−O bonds via transition state T2/P2, which lies 13.8 kcal/mol below the reactants. M2, alternatively, can evolve into HCN + NCO products via a series of isomerization steps as M2 → T2/ 4 → M4 → T4/5 → M5 → T5/P2 → HCN + NCO(P2), but this is a higher energy pathway. From the energy profile of the reaction, we can see P3(HCN + CNO) and P4(OH + C2N2) have higher key transition states T3/7 and T1′/6, with energy to be 2.4 kcal/mol and −6.8 kcal/

single-point energy

species

ZPE

B3LYP/6-311+ +G(d, p)

CCSD(T)/6-311+ +G(d, p)

CN + HCNO (R) NO + 3HCCN (P1) NO + 1HCCN (P1) HCN + NCO (P2) HCN + CNO (P3) OH + C2N2 (P4) minima M1 M1′ M2 M3 M4 M5 M6 M7 transition state T1/1′ T1/2 T1/3 T1′/6 T2/4 T2/P2 T3/7 T4/5 T5/P2 T6/P4 T7/P3

0.024247

−261.362171

−260.7396462

0.0

0.021792

−261.369895

−260.7618877

−15.5

0.022645

−261.344088

−260.7455809

−4.7

0.026334

−261.492925

−260.8807082

−87.2

0.025119

−261.391325

−260.7796254

−24.5

0.024941

−261.445416

−260.8495178

−67.4

0.031464 0.031026 0.030438 0.032277 0.028323 0.028081 0.030011 0.029940

−261.462263 −261.462839 −261.393317 −261.384665 −261.377459 −261.376954 −261.424955 −261.412789

−260.8582158 −260.8584046 −260.7947907 −260.7875950 −260.7825879 −260.7818494 −260.8191405 −260.8008197

−69.9 −70.3 −30.7 −25.1 −24.4 −24.1 −46.3 −34.8

0.029873 0.028710 0.030634 0.023837 0.027827 0.027309 0.027857 0.027781 0.025854 0.027061 0.026754

−261.444194 −261.386946 −261.382227 −261.359514 −261.359964 −261.368663 −261.318224 −261.373734 −261.378907 −261.418293 −261.370592

−260.8337882 −260.7828520 −260.7809631 −260.7500097 −260.7507454 −260.7646456 −260.7393602 −260.7792816 −260.7697930 −260.8069095 −260.7608742

−55.5 −24.3 −21.9 −6.8 −4.7 −13.8 2.4 −22.7 −17.9 −40.4 −11.8

Erela

a

Erel = energy (kcal/mol) relative to reactants and corrected for ZPE, based on the DFT-B3LYP/6-311++G(d,p) vibrational frequencies.

mol, respectively, so they are less competitive than P1 and P2. Obviously P1 and P2 are the two most competitive pathways for the title reaction, and the critical comparison is the relative energetics of the T2/P2 transition state versus the HCCN + NO products. Table 1 shows that the energy of HCCN + NO is lower than T2/P2 by 1.7 kcal/mol at the CCSD(T)/6-311+ +G(d,p)//DFT-B3LYP/6-311++G(d,p) level of theory. As their energies are so close, we recalculated their energy from a more accurate optimized geometry at the CCSD(T)/631G(d,p) level, as shown in Table 2. The resulting energy of HCCN + NO is lower than T2/P2 by 2.4 kcal/mol at the CCSD(T)/6-311++G(d,p)//CCSD(T)/6-31G(d,p) level or 1.3 kcal/mol at the CCSD(T)/6-31G(d,p) level alone, but higher by 4.9 kcal/mol at the CCSD(T)/aug-cc-pVTZ// CCSD(T)/6-31G(d,p) level. Pang’s calculation21 of the PES of reaction 1 shows that the energies of HCCN + NO are higher than T2/P2 at G3B3//B3LYP/6-311++G(d,p), CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p), and G3B3// QCISD/6-311++G(d,p) levels, by 5.8, 5.2, and 5.2 kcal/mol, respectively. It is not clear which method is preferable, but what is clear is that the relative energies are very close and that there 10289

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Figure 5. Geometries of the reactants, products, intermediates, and transition states relevant to the CN + HCNO reaction, optimized at the DFTB3LYP/6-311++G(d,p) level of theory. Geometries of several critical species are reoptimized at the CCSD(T)/6-31(d,p) level, shown in parentheses. Bond lengths are given in Å and bond angles in degrees.

5. CONCLUSIONS

is substantial dependence on the choice of basis set and theoretical method. The T1 diagnostic has been suggested as a measure of the single-reference character of wave functions, with values of T1 > 0.02 indicating significant nondynamical electron correction.46 We find that T1 diagnostics for HCCN, NO, and T2/P2 are typically on the order of 0.05, with minor variations depending on the basis set used; i.e., there are significant multireference effects in this system. The fact that reaction 3 is fast clearly suggests that T2/P2 cannot be much higher energy than HCCN + NO, but regarding the title reaction, entropic factors favor channel 1a over 1b if the energies are comparable. We calculated entropies of HCCN + NO and T2/P2, obtaining 108.3 and 69.5 cal mol−1 K−1, respectively, from the B3LYP calculation for geometries. This leads to a 11.6 kcal/mol difference in free energy, assuming isoenergetic values. Our overall conclusion is that, because of the substantial variations depending on choice of basis set and theoretical method, it is difficult to conclusively predict the dominant product channel of the title reaction using standard single-reference ab initio methods and that a sufficiently accurate treatment would require multireference calculations, which are beyond the scope of this study.

The product channels of the CN + HCNO reaction were investigated by IR diode laser spectroscopy. Several pieces of experimental information indicate that channel 1a, NO + HCCN, dominates this reaction. Product detection of NO provides a product branching fraction into channel 1a of 0.98 ± 0.07, and experiments performed to remove NCO radicals indicate that NCO secondary chemistry does not contribute to the yield of NO. HCN product detection indicates that HCN + NCO is at most a minor product channel with an upper limit to the branching fraction of