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The kinetics of 16 H-atom abstraction reactions of C 1 -C 4 alkanes by ketenyl radical (HCCO) are studied to enhance the existing combustion mechanisms. Firstly, the obtain the lowest-energy conformer, vibration frequencies and single point energies for the reactants, prepolymers, transition states, and products are determined using the CCSD(T)/cc-pVQZ//B3LYP/6-311G+(d,p) theory level. Subsequently, temperature-dependent reaction rate constants for the major channels are predicted by the TST method. Finally, the H-atom abstraction reactions are validated under multiple temperature conditions, and the effects of the reactions on auto-ignition process and CO emission characteristics of n-heptane/NG mixture are researched. The results indicate that the H-atom abstraction reactions by Cα-atom play a significant role for the diesel/NG combustion. To combine the reaction pathways and rate constants into the existing kinetic mechanisms resulted in more accurate predictions of the ignition delay time. Furthermore, these kinetics data are valuable for revealing the CO emissions of the dual-fuel engines. • H-atom abstraction by Cα of HCCO is dominant at low temperatures. • Hydrogen abstraction reactions caused by O-atoms hardly occur below 1200 K. • The highly symmetrical structure of methane causes the rate of H-atom abstraction to be very sensitive to temperature change. • The addition of the H-atom abstraction reactions can improve the calculation accuracy of the ignition delay time up 9.3%. • The H-atom abstraction reactions inhibit the consumption of CO and enhance branch ratio of CH 2 CO.→CO [ABSTRACT FROM AUTHOR]

The selenoketyl (HCCSe) radical has been positively identified for the first time as a product of an electric discharge through selenophene vapor. Laser-induced fluorescence, wavelength resolved emission, and fluorescence decay studies of jet-cooled HCCSe and DCCSe have given a detailed picture of the ground and excited state. The 418–400 nm band system of the HCCSe radical is assigned as à 2Πi-X 2Πi and the available evidence suggests that the radical is linear in the ground state and quasilinear in the excited state. The fluorescence decays of some upper state rotational levels show field-free molecular quantum beats, ascribed to an internal conversion interaction with high vibrational levels of the ground state. A comparison of the molecular structures and bonding in the HCCX (X=O,S,Se) free radicals shows that nonlinear ground state HCCO is best described as the ketenyl radical (H[Single_Bond]C==C==O) with the unpaired electron on the terminal carbon atom, whereas HCCS and HCCSe have linear ground state acetylenic (H[Single_Bond]C=C[Single_Bond]X) structures with the unpaired electron on the heteroatom. On electronic excitation, B 2Π HCCO reverts to the linear acetylenic structure, and à 2Π HCCS and HCCSe become quasilinear with the allenic structure. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The work presented here investigates the dynamics of the photodissociation of ethyl ethynyl ether at 193.3 nm with photofragment translational spectroscopy and laser-induced fluorescence. The data from two crossed laser-molecular beam apparatuses, one with vacuum ultraviolet photoionization detection and one with electron bombardment detection, showed that only cleavage of the C-O bond to form a C[SUB2]HO radical and a C[SUB2]H[SUB5] (ethyl) radical occurs. We observed neither cleavage of the other C-O bond nor molecular elimination to form C[SUB2]H[SUB4] + CH[SUB2]CO (ketene). The C[SUB2]HO radical is formed in two distinct product channels, with 37% of the radicals formed from a channel with recoil kinetic energies extending from about 10 to 70 kcal/mole and the other 63% formed from a channel with lower average recoil energies ranging from 0 to 40 kcal/mole. The measurements using photoionization detection reveal that the C[SUB2]HO radical formed in the higher recoil kinetic-energy channel has a larger ionization cross section for photon energies between 10.3 and 11.3 eV than the radical formed in the lower recoil kinetic-energy channel, and that the transition to the ion is more vertical. The radicals formed in the higher recoil kinetic-energy channel could be either &Xtilde;([SUP2]A") or Ã([SUP2]A') state ketenyl (HCCO) product and the shape of the recoil kinetic-energy distribution fitting this data does not vary with ionization energy between 10.3 and 11.3 eV. The C[SUB2]HO formed in the channel with the lower kinetic-energy release is likely the spin forbidden ã([SUP4]A") state of the ketenyl radical, reached through intersystem crossing. The &Btilde; state of ketenyl is energetically inaccessible. We also consider the possibility that the lower kinetic-energy channel forms two other C[SUB2]HO isomers, the CCOH (hydroxyethynyl) radical or the cyclic oxiryl radical. Signal from laser-induced fluorescence of the HCCO photofragment was... [ABSTRACT FROM AUTHOR]

The results of ab initio calculations of the vibronic spectra involving both lowest-lying states X[SUP2]A" and A[SUP2]A′ of the ketenyl radical HCCO are presented. Potential energy surfaces are computed by means of the coupled cluster method with perturbative triple corrections [CCSD(T)]. A recently developed ab initio approach for the treatment of the Renner-Teller effect in tetra-atomic asymmetric molecules is applied to calculate the vibronic energy levels. [ABSTRACT FROM AUTHOR]

Investigates the electronic spectroscopy of the band system of ketenyl radical compound using laser-induced fluorescence in a free-jet environment. Method used for obtaining vibrationally resolved excitation spectra for HCCO; Parallel transitions observed after applying the method.

Reports the first laser-induced fluorescence (LIF) excitation spectrum of the ketenyl radical, HCCO, which is produced by the 193 nanometer photolysis of ketene in a free jet expansion. Estimation of the fluorescence quantum yield; Vibronic structure of the photofragment yield spectrum.

Ketenyl radical (HCCO) is an important hydrocarbon combustion intermediate. The mechanisms and kinetics for the reaction of HCCO (X2A″) with H(2S) occurring on both singlet and triplet surfaces have been studied by a combination of ab initio calculations and rate constant predictions at the CCSD(T)/6-311++G(3df,2p)//CCSD/6-311++G(d,p) level of theory. The kinetics and product branching ratios have been investigated in the temperature range of 297–3000 K by variational transition state and Rice–Ramsperger–Kassel–Marcus (RRKM) theories for the production of CH2(a1A1) + CO(X1Σ+) and CH2(X3B1) + CO(X1Σ+). Our prediction for the primary product CH2(a1A1) + CO(X1Σ+) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed byk1(T) = 8.62 × 10–11T0.16exp(–20/T) cm3molecule–1s–1. For the production of CH2(X3B1) + CO(X1Σ+), the rate constantk2can be represented ask2(T) = 7.63 × 10–16T1.56exp(–386/T) cm3molecule–1s–1. The predicted product branching ratios for the reaction are in close agreement with experimental data as well. We also predicted the heat of formation at 0 K for2HCCO,3CCO, and1CCO by CCSD(T)/6-311++G(3df,2p), CBS-QB3, and G2M; the values are in good agreement among one another. Specifically, the CCSD(T) values are: ΔfH°(HCCO, X2A″) = 42.52 ± 0.70; ΔfH°(CCO, X3Σg) = 91.50 ± 0.54; and ΔfH°(CCO, a1Δ) = 110.22 ± 0.54 kcal/mol. [ABSTRACT FROM PUBLISHER]

We conducted radioastronomical observations of 9 dark clouds with the IRAM 30 m telescope. We present the first identification in space of the ketenyl radical (HCCO) toward the starless core Lupus-1A and the molecular cloud L483 and the detection of the related molecules ketene (H2CCO) and acetaldehyde (CH3CHO) in these two sources and 3 additional dark clouds. We also report the detection of the formyl radical (HCO) in the 9 targeted sources and of propylene (CH2CHCH3) in 4 of the observed sources, which significantly extends the number of dark clouds where these molecules are known to be present. We have derived a beam-averaged column density of HCCO of ~5 × 1011 cm-2 in both Lupus-1A and L483, which means that the ketenyl radical is just ~10 times less abundant than ketene in these sources. The non-negligible abundance of HCCO found implies that there must be a powerful formation mechanism able to counterbalance the efficient destruction of this radical through reactions with neutral atoms. The column densities derived for HCO, (0.5-2.7) × 1012 cm-2, and CH2CHCH3, (1.9-4-2) × 1013 cm-2, are remarkably uniform across the sources where these species are detected, confirming their ubiquity in dark clouds. Gas phase chemical models of cold dark clouds can reproduce the observed abundances of HCO, but cannot explain the presence of HCCO in Lupus-1A and L483 and the high abundances derived for propylene. The chemistry of cold dark clouds needs to be revised in light of these new observational results. [ABSTRACT FROM AUTHOR]

Abstract: The potential energy surface of C2H2O2 for HCCO+OH bimolecular reactions and the unimolecular decomposition of hydroxyketene (HOCH C O) are investigated employing high-level quantum chemical methods. Variable reaction coordinate transition state theory is used for the high-pressure limit rate constant calculation of the main reaction channels and RRKM-based multiwell master equation is used to calculate the pressure dependent rate constants and product branching ratios of these channels. The predicted rate constants are in good agreement with the limited experimental data available in the literature. The product distribution analysis shows that the association/decomposition of HCCO+OH to the formation of CO+ 1HCOH and CO+ 3HCOH channels are dominant in the whole temperature range of 500–2000K below 1atm, whereas at higher pressure and low temperature, the association reaction producing 3CHCOOH becomes competitive. For the unimolecular decomposition of hydroxyketene, the formation of CO+ 1HCOH channel is found to be predominant over a wide range of temperatures and pressures. Rate constants of these reactions and thermodynamic parameters for species involved in these reactions are also provided. [Copyright &y& Elsevier]

Highlights: [•] ISC rate constants measured for lowest cis-quartet-state HCCO with He, Ar, and Xe. [•] Ar and Xe induced ISC shown to exhibit an external heavy atom effect. [•] He observed to be more effective at vibrational deactivation than inducing ISC. [•] First experimental observation of nu2 mode of cis-quartet-state HCCO at 1785±36cm−1. [ABSTRACT FROM AUTHOR]

The kinetics of the gas phase reaction of the ketenylradical with SO2was investigated over the temperaturerange 296–568 K using a laser-photofragment/laser-induced fluorescencetechnique (LP/LIF). The reactor pressure was 10 Torr N2or He. Pulsed photolysis of ketene (CH2CO) at 193 nmwas used as the source of HCCO radicals. The rate coefficient forthe title reaction was determined to be described by k(T) = (1.05 ± 0.33) × 10–12exp[(690 ± 98)K/T] cm3s–1molecule–1(2σ error). We applied the coupledcluster and density functional theory to explore the mechanism ofthe title reaction. The dominant reaction pathway begins with a barrierlessassociation of the C of the CH group of HCCO and the O atom of SO2. [ABSTRACT FROM AUTHOR]

The troublesome barrier to linearity of the ketenyl radical (HCCO) is precisely determined using state-of-the-art computations within the focal point approach, by combining complete basis set extrapolation, utilizing the aug-cc-pVXZ (X= D, T, Q, 5, 6) family of basis sets, with electron correlation treatments as extensive as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]. Auxiliary terms such as diagonal Born−Oppenheimer corrections (DBOCs) and relativistic contributions are included. To gain a definitive theoretical treatment and to assess the effect of higher-order correlation on the structure of HCCO, we employ a composite approximation (c∼) to all-electron (AE) CCSDT(Q) theory at the complete basis set (CBS) limit for geometry optimizations. A final classical barrier to linearity of 630 ± 30 cm−1is obtained for reaching the 2Π Renner−Teller configuration of HCCO from the 2A′′ ground state. Additionally, we compute fundamental vibrational frequencies and other spectroscopic constants by application of second-order vibrational perturbation theory (VPT2) to the full quartic force field at the AE-CCSD(T)/aug-cc-pCVQZ level. The resulting (ν1, ν2, ν5) fundamental frequencies of (3212, 2025, 483) cm−1agree satisfactorily with the experimental values (3232, 2023, 494) cm−1. [ABSTRACT FROM AUTHOR]

Abstract: The potential energy surface for the HCCO+NO2 reaction was characterized at the B3LYP-DFT/6-311++G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, combined with high-level single point CCSD(T)/6-311++G(2d,p) calculations. Energetically accessible reaction paths leading to HCNO+CO2 and HCO+CO+NO product channels were found. Pathways to other reaction products were also characterized, but probably represent minor contributions. [Copyright &y& Elsevier]

The reaction of the ketenyl radical (HCCO) with acetylene (C2H2) is very relevant to the oxygen−acetylene flames and fuel-rich combustion process for nitrogen-containing compounds. Unfortunately, except for several rate constant measurements, the mechanism is completely unknown for this reaction. In this paper, detailed theoretical investigations are performed for the HCCO + C2H2 reaction at the G3B3 level using the B3LYP/6-31G(d), B3LYP/6-311++G(d,p), and QCISD/6-31G(d) geometries. The exclusive fragmentation channel is the formation of the cyclopropenyl radical (c-C3H3) and carbon monoxide (CO) via the chainlike OCCHCHCH and three-membered ring OC−cCHCHCH intermediates. Thus, the mass spectroscopic peak of C3H3+ in a previous experiment can be explained. The calculated overall reaction barrier is 4.4, 4.4, and 5.3 kcal/mol at the G3B3//B3LYP/6-31G(d), G3B3//B3LYP/6-311++G(d,p), and G3B3//QCISD/6-31G(d) levels, respectively. The title reaction may provide an effective route for generating the long-sought cyclopropenyl radical in the laboratory, which has been the long-standing subject of numerous theoretical studies as the simplest cyclic conjugate radical, and its bulky derivatives were already known. Future experimental investigations for the HCCO + C2H2 reaction are greatly desired to test the predicted fragmentation channel. The implication of the present study in combustion and interstellar processes is discussed. [ABSTRACT FROM AUTHOR]

The radical−radical reaction between the ketenyl radical (HCCO) and nitrogen dioxide (NO2) played a very important role in atmospheric and combustion chemistry. Motivated by recent laboratory characterization about the reaction kinetics of ketenyl radical with nitrogen dioxide, in this contribution, we applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. These calculations indicate that the title reaction proceeds mostly through singlet pathways, less go through triplet pathways. It is found that the HCCO + NO2 reaction initially favors formation of adduct OCCHNO2 (1) with no barrier. Subsequently, starting from isomer 1, the most feasible pathway is ring closure of 1 to isomer O-cCCHN(O)O (2) followed by CO2 extrusion to product HCNO + CO2 (P1), which is the major product with predominant yields. Much less competitively, 1 can take the successive 1,3-H- and 1,3-OH-shift interconversion to isomer OCCNOHO (3a, 3b, 3c) and then to isomer OCOHCNO (4a, 4b), which can finally take a concerted H-shift and C−C bond fission to give HCNO + CO2 (P1). The least competitive pathway is the ring-closure of isomer 3a to form isomer O-cCCN(OH)O (5a, 5b) followed by dissociation to HONC + CO2 (P2) through the direct side CO2 elimination. Because the intermediates and transition states involved in the most favorable channel all lie below the reactants, the title reaction is expected to be rapid, as is confirmed by experiment. Therefore, it can be significant for elimination of nitrogen dioxide pollutants. The present results can lead us to a deep understanding of the mechanism of the title reaction and can be helpful for understanding NOx-combustion chemistry. [ABSTRACT FROM AUTHOR]

High level ab initio coupled cluster (CC) and equation-of-motion (EOM) CC methods have been used to characterize the lowest doublet state of the ketenyl radical (HCCO) and its four low-lying isomers: the quartet trans- and cis-states of ketenyl, the hydroxyethynyl radical, and the oxiryl radical. These four isomers are found to be minima and to lie between 50 and 70 kcal/mol higher in energy than the ketenyl radical. [Copyright &y& Elsevier]

Studies the ketenyl radical HCCO (a4A'') energy hypersurface using ab initio quantum mechanical techniques. Rate constant measurements; Attractive potential of the reaction of CH(a4 sigma -) with CO; Vanishingly small energy barrier; Energetics and geometries of reactants and products; More.

The dynamics of hyperthermal O((3)P) reactions with acetylene have been investigated with the use of crossed molecular beams techniques, employing both mass spectrometric and optical detection of products. With collision energies of 40-150 kcal mol(-1), O((3)P) + HCCH/DCCD → HCCO/DCCO + H/D may follow multiple pathways to form the ketenyl radical (HCCO or DCCO) in ground doublet states or in electronically excited quartet and doublet states. Theoretical calculations support the assignment of the various reaction pathways. The fraction of electronic excitation is substantial. At the highest collision energy studied, ∼65% of the ketenyl radical products that survive are electronically excited, with the majority of the excited products in a quartet state. In this case, a population inversion exists between the electronically excited quartet and ground doublet states of the ketenyl product. Such significant electronic excitation in products is unusual in bimolecular reactions, especially when ground-state products are accessible by spin-allowed pathways.