8030
J. Phys. Chem. 1994,98, 8030-8035
Theoretical Characterization of the Reaction between Nitric Oxide and Ketenyl Radicals (HCCO NO): CO versus C02 Loss
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Minh Tho Nguyen, Werner Boullart, and Jozef Peeters’ Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B- 3001 Leuven, Belgium Received: April 8, 1994; In Final Form: May 12, 1994’
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Ab initio molecular orbital theory was applied to the HCCO N O reaction in light of recent experimental studies which point to a predominance of C O plus a (CHNO) isomer as products over C02 plus HCN. Geometries of stationary points on the [C2HN02] potential energy surface were optimized using HF/6-31G(d,p) and MP2/6-3 1G(d,p) calculations, and relative energies were estimated using MP4SDTQ and QCISD(T) calculations and the 6-3 1lG(d,p) basis set and corrected for zero-point energies. Six reaction paths were (partly) characterized, all proceeding through intermediate structures, yielding either C 0 2 plus H C N or C O plus a (CHNO) isomer. The formation of CO2 is possible by two channels: a three-step process involving cis-nitrosoketene and a four-membered ring as intermediates and a direct fragmentation from formyl isocyanate. The C O formation is possible via four distinct reaction paths. Only one of the latter, involving an aziridine derivative, has an intermediate transition structure lower than those for the CO2-formation paths. The CO-forming channel via formyl isocyanate is entropically favored over the CO2-elimination path. Overall, calculated results are only in agreement with experimental observation if formyl isocyanate is invoked as reaction intermediate. Nevertheless the formation pathway (HCCO N O formyl isocyanate), which is the most exothermic process, could not be identified yet. Structure and properties of the ketenyl radical have also been analyzed.
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1. Introduction
2. Details of Calculations
Nitric oxide has recently been named as “The Molecule of the Year 1992” by Science Magazine for its many surprisingly beneficial roles in neuroscience, physiology, and immunology.’ Nevertheless, this diatomic radical remains in the atmosphere one of the most toxic pollutants and is well-known as a destroyer of stratospheric ozone and a precursor of tropospheric oxidants and of acid rain. Considerable effort has therefore been devoted to the control of the undesirable formation of NO during the combustion of various fuels. In attempts to understand the complex combustion chemistry of NO, several model kinetic studies have been undertaken on the reactions of NO with various small radicals and molecules present in flames.2 On the other hand, it is also well established that, in flames, the formation of the ketenyl radical is a dominant channel of the acetylene ~xidation:~
Ab initio molecular orbital calculations were performed with the aid of a local version of the Gaussian 90 set of programs.’ The stationary points of the [CzHNOz] potential energy surface was initiallylocated at the Hartree-Fock(HF) level with the polarized 6-3 lG(d,p) basis set and characterized by harmonic vibrational frequencies at this level. Geometrical parameters were then refined at the second-order Morller-Plesset perturbation theory (MP2/6-3 lG(d,p)) level. Relative energies between stationary points were estimated using total energies computed at the full fourth-order perturbation theory (MP4SDTQ) and quadratic configuration interaction (QCISD(T)) levels with the larger 6-3 1lG(d,p) basis set7using HF- and MP2-optimized geometries. For the ketenyl radical, other computational methods have also been employed and will be detailed in the following section. In this paper, bond distances are given in angstroms, bond angles in degrees, total energies in hartrees, zero-point and relative energies in kilocalories per mole, and vibrational wavenumbers in cm-l.
0 + C,H2
-.
HCCO
+H
(1)
In this context, a recombination reaction between ketenyl and nitric oxide radicals is possible during the combustion of hydrocarbon fuels; in view of the probable formation of HCN plus C02 and/or a (CHNO) isomer plus CO, the reaction is likely a key step in NO “reburn” mechanisms. The HCCO NO reaction has recently been studied by Unfried et ala4using infrared diode laser spectroscopy and by Temps and co-workers5 using the far infrared laser magnetic resonance (FIR-LMR) method. Although the overall rate constant of the reaction has been measured at room temperature, these authors4v5have not been able to identify the major products. In this work, we attempt todetermine the product distributionoftheHCCO NO reaction, making use of both experimental and theoretical techniques. Results of the experimental study employing the discharge-flowmolecular beam mass spectrometric technqiues (DF-MBMS) will be reported in the accompanying paper.6 In this paper, we report only theoretical results obtained from molecular orbital calculations.
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a Abstract
published in Advance ACS Abstracts, July 1, 1994.
0022-3654/94/2098-8030$04.50/0
3. Results and Discussion 3.1. Ketenyl Radical. There have been some previous e~perimental~J’~ and theoretical”J-12studies of the ketenyl radical. The rotational spectrum observed by Endo and Hirotas indicates that the radical has a planar and bent (quasilinear) equilibrium structure. The quasilinearity is a consequence of the RennerTeller interaction of its ground electronic state with a low-lying excited state. As the molecular skeletonbends, mostly along the HCC bending mode, the orbital degeneracy is lifted and the 2 I I electronic state is split into two distinct states having 2A’ and ZA” symmetry of a C, structure. Earlier theoretical studies1”12 emphasize the dependence of the calculated 2Ar’ structure on the method employed. Our results, summarized in Table 1, concur with this finding. Employing the unrestricted Hartree-Fock method, geometry optimization with the 6-3 lG(d,p) basis of both ZArand 2A’rstates results in two degenerate linear structures having the same geometrical parameters and total energies. Nevertheless 0 1994 American Chemical Society
Theoretical Study of the HCCO + NO
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8031
TABLE 1: Geometries of Both 2A” and 2A’ Electronic States of HCCO at Different Levels of Theory method basis set state C-0 C-C C-H cco HCC UHF 6-3lG(d,~)~ 2Aff 1.172 1.247 1.055 180.0 180.0 2A‘
’ROHF‘
6-31G(d,p)
UHF
UMPZ
6-3 1IG(d,p) 6-31 l++G(d,p) DZP 6-311G(d,p) 6-31G(d,p)
UMPZ
6-311G(d,p)
UMPZ
6-31l++G(d,p)
ROHF‘
ZAff ZA’ ZAtf 2Aff ZA“ 2Aft 2Aff
2Af ZAff ZA’ ZAfJ
ZA‘
1.172 1.147 1.157 1.161 1.161 1.170 1.136 1.171 1.177 1.162 1.167 1.162 1.168
1.247 1.290 1.258 1.247 1.247 1.259 1.293 1.320 1.272 1.321 1.272 1.323 1.273
1.055 1.062 1.053 1.054 1.054 1.059 1.063 1.074 1.058 1.078 1.061 1.079 1.062
180.0 171.6 180.0 178.0 178.5 175.1 171.9 169.5 180. 169.5 180. 169.5 180.
180.0 138.4 180.0 171.0 173.7 159.4 135.8 126.2 180. 126.9 180. 126.2 180.
E” -151.084 33 -151.084 33 -151.076 65 -151.075 50 -151.123 64 -151.126 53 -151.112 52 -151.116 23 -151.499 61 -151.49273 -151.609 39 -151.604 19 -151.614 89 -151.609 66
AEb
0.0 0.7
3.7 3.3 3.3
Total energies. The (S2) of UHF wave functions are 0.82 for both states. Energy differences between 2A’ and ZA” states at the level of theory indicated. e Restricted open-shell HF method. Harmonic vibrational wavenumbers at this level are (cm-l): 2Aff,3631, 2092, 1396, 619, 546 (a”), and 64; ZA‘, 3631, 2092, 1396, 616 (a”), 551, and 368, calculated under C, symmetry. a
TABLE 2 Rotational Constants (GH.) of HCCO and DCCO constant“ HCCOb DCCOb A
B C
1201.0 (1243.0) 10.804 (10.897) 10.708 (10.766)
681.7 (652.1) 9.847 (9.927) 9.707 (9.755)
4 Using the proposed geometry: r(C-O) = 1.17 A, r(C=C) = 1.305 A, r(C-H) = 1.056 A, LCCO = 177.0’, and LHCC = 137.5’. In parentheses are experimental values from ref 8.
TABLE 3
Isotopic Wavenumber Shifts in HCCO isotopic shifts‘ vr(HCC0)” DCCO H W 3 C 0 HCC1*O DCC180
mode C-H stret 3 166 -801 -11 0 -799 C=Ostret 2262 (2023)b -36 -63 -12 -67 C e s t r e t 1182 -42 -26 -23 -65 -47 -5 CCHbend 623 0 -5 1 CCObend 567 -102 -17 -5 -104 -14 -4 -25 -21 torsion (a”) 503 a Vibrational wavenumberscalculated at the UMP2/6-31 l++G(d,p) level and scaled by 0.95. The isotopesare lH12C12C160unless otherwise indicated. b Experimentalvalue? e Isotopicshiftsrelative to lH12CIXW.
both states are distinguishable by their harmonic vibrational frequencies that differ by the bending and torsion modes (Table 1). In other words, the Renner-Teller effect is not detectable at the UHF/6-31G(d,p) level of theory. Extension of the atomic functions of 6-3 1lG(d,p) and 6-3 11++G(d,p) induces only a marginal bending of both HCC and CCO angles. When the restrictedopen-shellHartree-Fockmethod (ROHF) is employed either with the 6-31G(d,p) or 6-31 l++G(d,p) basis, the bent 2Affstructure is well reproduced. Similarly, the latter can be obtained by calculations with correlation energy at the UMPZ level, irrespective of the basis set employed. Taken together, our calculations agree with earlier studies pointing toward a 2Afr ground electronic state for HCCO. Although the rotational constants of both HCCO and DCCO species have been derived from observed rotational spectra, the full set of their geometrical parameters cannot be determined.8 Assuming a linear CCO framework,LCCO = 180° and a distance r(C=O) = 1.15 A, a value suggested by an earlier MO study,10 the three remaining parameters have been determined as follows: r(C=C) = 1.314 A, r(C-H) = 1.056 A, and LHCC = 138.7’. Accordingly, the UMP2 value of LHCC = 126.2’ is too small relative to the experimental value. In fact, it is rather hard to calculate the HCC bond angle. Recent calculations using the coupled-cluster method, CCSD and CCSD(T), also failed to describe this parameter.12 Note that the rotational constant A of HCCO is extremely sensitive to this parameter. It is however possible to refine the HCCO geometry using both available experimental and theoretical data. An empirical
refinement approach can be summarized as follows: (i) We adopt r(C-H) = 1.056 A obtained in ref 8 and r(C=O) = 1.17 A suggested in ref 12. The experimental C...O distance of 2.464 A leads to a r(C=C) = 1.305 A. (ii) Employing the threechosen distances, both HCC and CCO angles can be fitted to reproduce best the experimental rotational constants of HCCO and DCCO. While the A constant of HCCO is quite sensitive to the HCC bond angle, that of DCCO is rather dependent on the CCO bond angle. Variation of either angle induces two opposite effects on A constants of HCCO and DCCO. Thus, enlarging the HCC angle improves A(HCC0) but worsens A(DCC0) and vice versa for the CCO angle. The best fit corresponds to the values LHCC = 137.5’ and LCCO = 177’. Comparison with experimental values is summarized in Table 2. The infrared spectrum of HCCO has been recorded: but only a band at 2023 cm-I can be assigned to its C=O stretching mode. Calculated harmonic frequencies of HCCO have been reported extensively in earlier papers.1@12 In Table 3 we complement these with the isotopic shifts that have not been examined in previous studies. Although the scaled wavenumber remains large, relative to the experimental value, the isotopicwavenumber shifts derived from UMP2/6-31 l++G(d,p) calculations are expected to be much closer to the true values.I3 The shifts induced by D or 13C are significantly large. We now turn our attention to the2Afelectronicstate. Geometry optimization at all levels of theory considered starting with a strongly bent 2A’ form, results invariably in a linear structure (Table 1) whichisin fact acomponent oftheznstate. Vibrational analysis under C,symmetry shows unambiguously a (2A‘ = Z I I ) linear equilibrium structure. In ref 12, a linear HCCO structure characterized by an imaginary frequency has been reported. We found that the linear structure reported in ref 12 is a transition structure for inversion of the 2Affequilibrium structure. In contrast, the linear structure we are considering corresponds to an equilibrium structure in the 2A’ energy surface (2A’ 2n). The splitting energy 2A’- 2A’ris rather small amounting to about 3.3 kcal/molat theUMP2 level (Table 1). Separatecalculations using the configuration interaction method CISDQ/6-3 11++G(d,p) basedon the UMP2/6-31 l+G(d,p) geometry yield a smaller 2A’ - 2A‘‘ energy difference of 1.9 kcal/mol, which can be compared with the experimental estimate of 1.5 kcal/mol (540 cm-I).* The vertical ZA’ 2Afftransition energy is larger, about 10kcal/mol at the CISDQ level, reflecting an important geometry relaxation. The ketenyl radical possesses another doublet excited state, namely, a 2Z+electronicstate. TheUMP2/6-3 lG(d,p) geometry of this state is the following:
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1.059 H-C-C-0
1326
1190
thus indicating a H C d O ’ oxygen-centered radical. UMP4/
8032
Nguyen et al.
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 1425 1613 c 32%-
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