Product Branching Ratio of the HCCO + NO Reaction Kwang Taeg Rim † and John F. Hershberger* Department of Chemistry, North Dakota State UniVersity, Fargo, North Dakota 58105 ReceiVed: June 29, 1999; In Final Form: October 29, 1999 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 2 product molecules. After quantification of product yields and consideration of secondary chemistry, we obtain the following product branching ratios (1σ error bars) at 296 K: 0.12 ( 0.04 for CO 2 + (HCN) and 0.88 ( 0.04 for CO + (HCNO). In addition, we estimate a relative quantum yield for HCCO production in the 193 nm photolysis of CH 2 CO to be 0.17 ( 0.02. Introduction The spectroscopy 1-4 and kinetics 4-8 of the ketenyl radical (HCCO) is of recent interest, partly because of the role this species plays in combustion chemistry. HCCO is formed in flames primarily by the oxidation of acetylene. 9,10 It has been observed in laboratory studies by infrared absorption 4 and laser- induced fluorescence spectroscopy. 1,2 Several kinetic studies involving HCCO have appeared recently, with total rate constant measurements reported for reactions with NO, 4-7 NO 2 , 5 O 2 , 5 O, 8 and C 2 H 2 . 5 The reaction with NO is of particular interest because of the role it plays in NO-reburning mechanisms 11-15 for the reduction of NO x emissions from fossil-fuel combustion processes. Several product channels are possible: The thermochemistry shown is for HCN and HCNO, but several other isomers of these species represent possible, albeit unlikely, product channels as well. Several reports of the total rate constant of this reaction have appeared. Unfried et al. used infrared absorption near 2023 cm -1 to detect HCCO and reported k 1 ) (3.9 ( 0.5) × 10 -11 cm 3 molecule -1 s -1 at 298 K. 4 Temps et al. used far-infrared laser magnetic resonance to detect HCCO and obtained (2.2 ( 0.6) × 10 -11 cm 3 molecule -1 s -1 at 298 K. 5 Boullart et al. used discharge flow-mass spectrometry to obtain k 1 ) (1.0 ( 0.3) × 10 -10 exp [-350 ( 150)/T] cm 3 molecule -1 s -1 over the temperature range 290- 670 K and also reported the following product branching ratios at 700 K: φ 1a ) 0.23 ( 0.09, φ 1b ) 0.77 ( 0.09. 6 These data are contradicted by recent calculations and flow reactor studies. Miller et al. used statistical theories and a QCISD potential surface of Nguyen et al. 7 to predict φ 1a ) 0.81 at 300 K, with a moderate temperature dependence, decreasing to about 0.32 at 2000 K. 12 Kinetic modeling of flow reactor experiments at 1100-1400 K were best fit by a branching ratio of φ 1a ) 0.65. 13 Measurement of the product branching ratio of the title reaction represents a challenge because no ideal photolytic source of HCCO is known. Typically, previous experiments have used the reaction of oxygen atoms with acetylene to form HCCO: This approach has two problems: reaction 2 is quite slow at moderate temperatures, 8,9 and the presence of channel 2b complicates quantification of product yields. Unfried et al., however, reported that direct photolysis of ketene at 193 nm produces ketenyl radicals as well as the well-known methylene + CO channel: 4 We have chosen this approach in the experiments reported here. Reaction 3b represents a faster source of HCCO than (2a), although we must still contend with the production of CH 2 + CO in channel 3a. Experimental Section The experimental procedure is similar to that described in previous publications. 16,17 A schematic of the experimental apparatus is shown in Figure 1. Photolysis light of 193 nm was provided by an excimer laser (Lambda Physik, Compex 200). Several lead salt diode lasers (Laser Photonics, Analytics Division) 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 beams were copropagated through a 1.46 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 LeCroy 9310A digital oscilloscope and transferred to a computer for analysis. † Current address: Department of Chemistry, Columbia University, 3000 Broadway MS 3109, New York, NY 10027. O + C 2 H 2 f HCCO + H (2a) f CH 2 + CO (2b) CH 2 CO + hν (193 nm) f CH 2 + CO (3a) f HCCO + H (3b) HCCO + NO f CO 2 + (HCN) ΔH° )-527.2 kJ/mol (1a) f CO + (HCNO) ∆H° )-200.8 kJ/mol (1b) f NCO + HCO ∆H° )-71.1 kJ/mol (1c) 293 J. Phys. Chem. A 2000, 104, 293-296 10.1021/jp9922209 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/07/1999