HCCO Radicals Research Articles

Rate Constants for Reactions of HCCO and HCCCO Radicals with O2 over the Temperature Range 243-423 K.

A pulsed laser photolysis-photoionization mass spectrometer system has been employed to measure the rate constants of HCCO + O2 and HCCCO + O2 over the temperature range 243-423 K in 1.2-8.4 Torr of He or N2. Radicals of HCCO and HCCCO were produced by 193 nm ArF laser photolysis of ethyl ethynyl ether and methyl propiolate, respectively. HCCO was photoionized by a Kr resonance lamp with a CaF2 window (10.03 eV), and HCCCO was ionized by a Xe lamp with a sapphire window (8.44 eV). Both ions were detected as parent ions in a quadrupole mass spectrometer. From analysis of the time profiles of the ion signals for various O2 concentrations, the overall rate constants at 298 K are represented by the values k2 = (6.3 ± 1.0) × 10-13 for HCCO + O2 and k5 = (5.7 ± 0.6) × 10-12 for HCCCO + O2 in the units cm3 molecule-1 s-1. The rate coefficients for the two reactions can be described by k2(T) = (1.5-0.7+1.5) × 10-12 exp[-(225 ± 220)/T] and k5(T) = (1.8-0.9+1.9) × 10-12 exp[(343 ± 228)/T] in the units cm3 molecule-1 s-1 over the temperature range 243-423 K.

Rotational spectroscopy of the HCCO and DCCO radicals in the millimeter and submillimeter range

Context. The ketenyl radical HCCO has recently been detected in the interstellar medium (ISM) for the first time. Further astronomical detections of HCCO will help us understand its gas-grain chemistry, and subsequently revise the oxygen-bearing chemistry towards dark clouds. Moreover, its deuterated counterpart DCCO has never been observed in the ISM. A broad spectroscopic investigation is still lacking for both HCCO and DCCO, although they exhibit a significant astrophysical relevance.Aims. In this work we aim to measure the pure rotational spectra of the ground state of HCCO and DCCO in the millimeter and submillimeter region, considerably extending the frequency range covered by previous studies.Methods. The spectral acquisition was performed using a frequency-modulation absorption spectrometer between 170 and 650 GHz. The radicals were produced in a low-density plasma generated from a select mixture of gaseous precursors. We were able to detect and assign more than 100 rotational lines for each isotopolog.Results. The new lines have significantly enhanced the previous data set allowing the determination of highly precise rotational and centrifugal distortion parameters. In our analysis we took into account the interaction between the ground electronic state and a low-lying excited state (Renner–Teller pair) which enables the prediction and assignment of rotational transitions withKaup to 4.Conclusions. The present set of spectroscopic parameters provides highly accurate, millimeter, and submillimeter rest-frequencies of HCCO and DCCO for future astronomical observations. We also show that towards the pre-stellar core L1544, ketenyl peaks in the region wherec-C3H2peaks, suggesting that HCCO follows a predominant hydrocarbon chemistry, as already proposed by recent gas-grain chemical models.

Quantitative NH measurements by using laser-based diagnostics in low-pressure flames

NH is a key short-lived radical involved in the prompt-NO formation. Quantification of NH is thus particularly important for testing the NO kinetic mechanisms. However, quantitative measurements of native NH in hydrocarbon/oxygen/nitrogen flames remain very scarce. Therefore, in this work, the mole fractions of native NH were obtained using a combination of laser-based diagnostics; Laser Induced Fluorescence (LIF) and Cavity Ring-Down Spectroscopy (CRDS). The NH species was probed after exciting the transition R1(6) in the A3Π-X3Σ− (0-0) system at 333.9 nm. The mole fraction profiles of NH were successfully obtained in premixed low-pressure flames of CH4/O2/N2 and C2H2/O2/N2 at two equivalence ratios of 1.00 and 1.25. The estimated detection limit for the NH radical was around 4.5 × 108 molecule cm−3 (i.e. 2 ppb in mole fraction at 1600 K), which is nearly 2 orders of magnitude lower than previous values reported in the literature. These new experimental results were compared with predictions by a recently developed NO model (namely NOMecha2.0). In the case of the CH4 flames, a satisfying agreement between the experiment and model was observed. However, in the case of the C2H2 flames, some discrepancies were observed. Model analysis has highlighted the importance of the HCCO radicals in the NH formation through the HCNO→HNCO→NH2 reactions pathway. Modification of the rate constant values of the reactions C2H2 + O and HCCO + O2, which are key reactions for both the acetylene laminar flame speed and the HCCO predictions, has enabled the model to satisfactorily predict the experimental NH and NO profiles also in the C2H2 flames.

Absolute Rate Coefficient and Mechanism of Gas Phase Reaction of Ketenyl Radical and SO2

The kinetics of the gas phase reaction of the ketenyl radical with SO(2) was investigated over the temperature range 296-568 K using a laser-photofragment/laser-induced fluorescence technique (LP/LIF). The reactor pressure was 10 Torr N(2) or He. Pulsed photolysis of ketene (CH(2)CO) at 193 nm was used as the source of HCCO radicals. The rate coefficient for the title reaction was determined to be described by k(T) = (1.05 ± 0.33) × 10(-12) exp[(690 ± 98)K/T] cm(3) s(-1) molecule(-1) (2σ error). We applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. The dominant reaction pathway begins with a barrierless association of the C of the CH group of HCCO and the O atom of SO(2).

Trajectory calculations of OH radical- and Cl atom-initiated reaction of glyoxal: atmospheric chemistry of the HC(O)CO radical

On-the-fly quasi-classical trajectory calculations using the density functional method were carried out to investigate the dynamics of the HC(O)CO radical, formed by OH radical- and Cl atom-initiated reactions of glyoxal at 298 K. The energy difference between the A' HC(O)CO radical, formed immediately after H atom abstraction, and the most stable A″ HC(O)CO radical is estimated to be 6.0 kcal mol(-1). The surplus energy followed by relaxation from A' HC(O)CO to A″ HC(O)CO goes to internal energy of the nascent HC(O)CO radicals and causes prompt decomposition into HCO + CO. The average internal energy partitioned into the HC(O)CO radical is higher in the OH reaction than in the Cl reaction, in accordance with exothermicity of the reactions. A fraction of the nascent HC(O)CO radicals (91% for the OH reaction and 47% for the Cl reaction) promptly decomposes into HCO and CO within 2.5 ps. The remaining HC(O)CO radicals, which do not undergo prompt decomposition, decompose thermally or add with O(2) in the presence of O(2). I re-evaluated the previous two experiment results of the product yield ratio [CO]/[CO(2)] vs. [O(2)](-1) in the Cl atom-initiated reaction, in light of the reaction mechanism involving prompt decomposition. The two results give 9.5 × 10(6) s(-1) and 1.08 × 10(7) s(-1) for the thermal decomposition rate and 47% and 41% for the fraction of prompt decomposition in the Cl atom-initiated reaction, in good agreement with the present trajectory calculation.

Analysis of Nitrogen-Oxide Formation Mechanisms in Filtration Combustion of Methane–Air Mixtures

The kinetics of nitrogen‐oxide formation under conditions of filtration combustion of lean and rich methane–air mixtures is analyzed. Intense interphase heat exchange in the wave of filtration combustion of gases leads to a drastic decrease in the gas residence time in the high‐temperature region and also to a decrease in the maximum temperatures in the reaction zone. As a result, the processes of filtration combustion of gases are characterized by reduced emission index of nitrogen oxides as compared to diffusion flames. Under these conditions, the thermal mechanism does not make any significant contribution to NO formation, and the dominating channels are the NNH channels for lean mixtures and the Fenimore mechanism for rich mixtures. Nitrogen oxides in lean mixtures are mainly represented by NO and N2O, whereas nitrogen‐containing components HCN and NH3 are typical of rich mixtures. NO decomposition in rich mixtures occurs in “afterburning” reactions with participation of HCCO, CH3, and CH2 radicals.

Experimental determination of the temperature dependence of the absolute rate coefficients of the HCCO + NO2and HCCO + H2reactions

The absolute rate coefficients of the gas-phase reactions HCCO + NO2 and HCCO + H2 were experimentally determined for the first time over extended temperature ranges: 293 K to 769 K and 438 K to 761 K, respectively. HCCO radicals were generated by pulsed-laser photolysis of CH2CO at 193 nm. Their subsequent decay, under pseudo-first-order conditions, was monitored in real-time using a laser-photofragment/laser-induced fluorescence technique. The rate coefficient of HCCO + NO2 exhibits a negative temperature dependence similar to that of the HCCO + NO reaction, but the Arrhenius A-factor is 1.4 times larger; k(T)(HCCO+NO2) = (2.3 ± 0.4) × 10−11 exp (340 ± 40) K/T) cm3 s−1. It is argued that, if the major product channels yield N, NH or NCO, the HCCO + NO2 reaction should be a significant removal route of NOx in stationary combustion systems under fuel-rich conditions at temperatures below ca. 1300 K. The rate coefficient for the HCCO + H2 reaction was determined as k(T)(HCCO+H2) = (2.2 ± 1.4) × 10−11 exp(−2000 ± 400)K/T). In fuel-rich combustion environments, given the high concentrations of H2, this reaction is likely to be a significant loss process for HCCO radicals: k(1500 K)HCCO+H2 = (6+0.4−0.2) × 10−12 cm−3 s−1, a factor of three greater than k(1500 K)HCCO+O2.

Absolute Rate Coefficient of the HCCO + NO Reaction over the Range T = 297−802 K

The absolute rate coefficient of the gas-phase reaction HCCO + NO was experimentally determined for the first time over an extended temperature range, 297−802 K. HCCO radicals were generated by pulsed-laser photolysis of CH2CO at 193 nm. Their subsequent decay, under pseudo-first-order conditions, was monitored in real time using a newly developed laser-photofragment/laser-induced fluorescence technique (Carl, S. A.; Sun, Q.; Peeters, J. J. Chem. Phys. 2000, 114, 10332) that involved pulsed-laser photodissociation of HCCO at 266 nm and laser-induced fluorescence at ca. 430 nm of the resulting nascent rotationally excited CH(X2Π) photofragment. The rate coefficient of the title reaction was found to exhibit a negative temperature dependence described by k5(T)(HCCO+NO) = (1.6 ± 0.2) × 10-11 exp(340 ± 30 K/T) cm3 s-1 molecule-1 (2σ errors). In combination with the recent theoretically determined branching ratios for this reaction of this laboratory (Vereecken, L.; Sumathy, R.; Carl, S. A.; Peeters, J. Chem. Phys. Lett. 2001, 344, 400), the temperature dependencies of the two dominant product channels, HCN + CO2 and HCNO + CO, may be described by k5a = (3.7 ± 0.3) × 10-10T-0.72±0.02 exp(200 ± 30 K/T) cm3 s-1 molecule-1 and k5b = (1.4 ± 0.2) × 10-11 exp(320 ± 30 K/T) cm3 s-1 molecule-1, respectively, where the given (2σ) error limits are derived from those of the present experimental work only.

Laser-induced fluorescence of nascent CH from ultraviolet photodissociation of HCCO and the absolute rate coefficient of the HCCO+O2 reaction over the range T=296–839 K

The absolute rate coefficient of the gas-phase reaction HCCO+O2 was determined over the temperature range 296–839 K and at a pressure 7±1 Torr helium. The experiments were performed in a slow-flow kinetic apparatus employing pulsed photolysis of CH2CO at 193 nm as a source of HCCO radicals. Reaction time profiles of [HCCO] were constructed using a newly developed, sensitive spectroscopic technique in the visible spectral region to detect this radical: laser—induced fluorescence of nascent CH(X 2Π) photofragments following HCCO photodissociation at 266 nm. Photodissociation of HCCO at this wavelength was found to produce rotationally excited CH(X) populated to N″⩾26. The rate coefficient for the title reaction was found to be described by k(T)(HCCO+O2)=(2.6±0.3)×10−12 exp[−(325±80)K/T] cm3 s−1 molecule−1 (2σ errors). The absorption cross section of HCCO at 266 nm, σHCCO(266 nm), was also determined relative to that of CH2CO at 193 nm as σHCCO(266 nm)=0.07−0.05+0.20σCH2CO(193 nm).

Rate constants for the reactions of HCCCO and NCCO radicals with molecular oxygen

AbstractReactions of HCCCO and NCCO radicals with O2 have been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. HCCCO was produced by 193‐nm photolysis of methylpropiolate or 3‐butyn‐2‐one, and NCCO was formed by 193‐nm photolysis of acetylcyanide. The rate constants obtained at 298 ± 3 K were (6.5 ± 0.7) × 10−12 cm3 molecule−1 s−1 for the HCCCO + O2 reaction, and no pressure dependence was observed between 1.5 and 16 Torr of N2 as a bath gas. Because HCO and HCCO radicals were observed as reaction products, it was confirmed that the reaction proceeds by a two‐body reaction. On the other hand, the rate constants of NCCO with O2 depended on the total pressure and were (5.4–8.8) × 10−13 cm3 molecule−1 s−1 for total pressures 2.0–15.5 Torr of N2, confirming that the reaction proceeds by a three‐body process. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 440–448, 2001

Product Branching Ratio of the HCCO + NO Reaction

The reaction of HCCO radicals with NO was studied at room temperature by excimer laser photolysis of ketene precursor molecules followed by infrared absorption spectroscopic detection of CO and CO{sub 2} product molecules. After quantification of product yields and consideration of secondary chemistry, the authors obtain the following product branching ratios (1{sigma} error bars) at 296 K: 0.12 {+-} 0.04 for CO{sub 2} + (HCN) and 0.88 {+-} 0.04 for CO + (HCNO). In addition, they estimate a relative quantum yield for HCCO production in the 193 nm photolysis of CH{sub 2}CO to be 0.17 {+-} 0.02.

Theoretical prediction of the spin-orbit splitting in the NCO, NCS, HCCO and HCCS radicals

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A computational study of the HCCO and HCCS radicals

The equilibrium geometries of the lowest electronic states of the HCCO and HCCS radicals have been determined by ab initio SCF and Møller-Plesset perturbation theory calculations using split-valence plus polarization basis sets. When optimized at a correlated level the ground state, 2A″, of HCCO is bent with a linear 2Π( 2A′) excited state forming the other half of a Renner-Teller pair. In contrast, ground state HCCS is predicted to be linear at all levels of calculation considered. The UMP3/6-31 G* predictions for the geometries of HCCO are: 2A″, r e(CH) = 1.074 Å, r e(CC) = 1.301 Å, r e(CO) = 1.170 Å, α e(CCH) = 133.23°, α e(CCO) = 169.96°; and 2Π( 2A′), r e(CH) = 1.064 Å, r e(CC) = 1.264 Å and r e(CO) = 1.181 Å. The UMP3/6-3 IG* geometry optimization for HCCS predicted: 2Π, r e(CH) = 1.066 Å, r e(CC) = 1.212 Å, and r e(CS) = 1.634 Å.

The submillimeter-wave spectrum of the HCCO radical

The pure rotational spectra of the HCCO and DCCO radicals were observed for the first time in the submillimeter-wave region. The HCCO radical was generated by either the C2H2+O reaction or the H2CCO+F reaction. The former was found to be about twice as efficient as the latter, and was employed for obtaining DCCO using C2D2 as a precursor. The observed spectra were fitted to the pattern of a near prolate symmetric top with a large A rotational constant. The observed spin-rotation splittings result in an extremely large εaa constant and exhibit an anomalous K dependence, indicating that there is a low-lying excited electronic state which interacts with the ground state.