J. Phys. Chem. 1996, 100, 2125-2128
2125
Propargyl from the Reaction of Singlet Methylene with Acetylene J. D. Adamson, C. L. Morter, J. D. DeSain, G. P. Glass,* and R. F. Curl* Department of Chemistry and Rice Quantum Institute, Rice UniVersity, Houston, Texas 77251 ReceiVed: July 25, 1995; In Final Form: October 30, 1995X
The technique of infrared kinetic spectroscopy has been used to study the production of propargyl radical from the reaction of singlet methylene with acetylene. The rate constant for this product channel was determined to be (3.5 ( 0.7) × 10-10 cm3 molecule-1 s-1 at 295 K, measured relative to the known rate for 1CH with H or CH . Methylene was produced in the singlet state by excimer laser photolysis of ketene at 2 2 4 308 nm in the presence of acetylene and either H2 or CH4. Reaction of 1CH2 with acetylene produces propargyl, and reaction of 1CH2 with either H2 or CH4 produces CH3. The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rate of reaction of singlet methylene with H2 (or CH4) to produce CH3. The relative peak infrared absorption cross sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations.
Introduction Soot formation in combustion appears to be tied closely to the formation of polycyclic aromatic hydrocarbons (PAH). If aromatic rings are already present, plausible mechanisms for the addition of carbon species to a ring resulting in new rings and the growth of PAH have been proposed. However, for a flame burning an aliphatic fuel, the mechanism by which the initial aromatic ring is formed is still unclear. The current situation concerning this issue is well summarized in a recent paper by Miller and Melius.1 Flame studies undertaken during the mid-1980s led several groups2-5 to propose that benzene is formed by either the reaction of C4H5 with acetylene2 or the reaction of C4H3 with acetylene3 or both.4,5 These mechanisms have been summarized and extended by Westmoreland et al.6 However, Kern and coworkers7 have suggested that benzene produced in the pyrolysis of allene and 1,3-butadiene is formed by the recombination of propargyl radicals, and Alkemade and Homann8 have studied propargyl recombination directly and observed formation of several C6H6 isomers, one of which is benzene. More recent studies by Kern and co-workers9 on benzene production in the high temperature decomposition of propargyl bromide conclude that it is the result of a sequence of reactions initiated by the recombination of C3 species. In their analysis of likely mechanisms for soot formation in flames, Miller and Melius1 make the point that at high temperatures rapid isomerization can occur as a result of hydrogen atom exchange reactions, and thus chemical equilibrium between the isomers of C4H5 and C4H3 is rapidly established. The isomers that can form strong bonds upon condensation with acetylene are the less stable normal radicals,
HCtCsCHdCH (n-C4H3) H2CdCsCtCH T H2CdCdCdCH (i-C4H3) H2CdCHsCHdCH (n-C4H5) H2CdCsCHdCH2 T H2CdCdCHsCH2 (i-C4H5) but the resonantly stabilized iso radicals which form only weakly X
Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2125$12.00/0
bonded complexes with acetylene should dominate the equilibrium. Thus, they1 argue that propargyl recombination is the more plausible mechanism for aromatic ring formation. They propose that propargyl is formed in flames by the reaction sequence:
1
C2H2 + O f HCCO + H
(R1)
HCCO + H f 1CH2 + CO
(R2)
CH2 + C2H2 f C3H3 (propargyl) + H
(R3)
In experimental studies, Westmoreland et al.6 have shown that sufficient C3 species are found in acetylene flames to account for aromatic ring formation (the C3’s are presumably formed by the mechanism above) but that this is not the case for 1,3butadiene flames. They suggest that both these C3 and C4 mechanisms occur. Previous studies on the reaction of singlet methylene with acetylene have determined overall rate constants for the reaction10 of (3.7 ( 0.3) × 10-10 cm3 molecule-1 s-1 and11 (2.93 ( 0.19) x 10-10 cm3 molecule-1 s-1, but the branching ratio into the propargyl product channel has not previously been determined. In this work, the propargyl radical was directly observed via infrared kinetic spectroscopy as a product of the reaction of 1CH2 with C2H2 and the rate constant for reaction 3 determined. Experimental Section The infrared kinetic spectrometer has been previously described.12 Briefly, the unfocused beam of an excimer laser is overlapped with an IR color center laser probe beam in a multipass (White) flow cell. For rate determinations, singlet methylene was produced by photolysis of ketene at 308 nm using a XeCl excimer laser operated at 10 Hz with 108 mJ/ pulse. At this wavelength, the photolysis products have been demonstrated to be predominantly 1CH2 and CO.13, 14 In the experiment, diketene (Aldrich) was entrained in a flow of helium and ketene produced by pyrolyzing the diketene stream in a quartz tube heated to 600 °C which was then directly introduced into the flow cell as the impurities were determined not to affect our experiments. Methane (99.9%), hydrogen (99.999%), and helium buffer gas (99.995%) were used without further purification. Partial pressures of various reagents are given in Table 1. Acetone was removed from acetylene by passage through a © 1996 American Chemical Society
2126 J. Phys. Chem., Vol. 100, No. 6, 1996
Adamson et al.
TABLE 1: Partial Pressures in the Flow Cell during Kinetics Measurements species
partial pressure (Torr)
C2H2 CH4 or H2 CH2CO He total
1.25-1.34 1.21-4.32 1-2 10.0 14.3-16.4
copper coil immersed in a dry ice/acetone bath. The methyl signal15 was monitored at 3154.7468 cm-1 (N′ ) 2, K′ ) 1 r N ) 2, K ) 0), and the propargyl R(8) transition of the ν1 CH stretch16 was monitored at 3327.8484 cm-1. For calibration purposes, propargyl and methyl were produced by photolysis of crotyl bromide (Aldrich) at 193 nm using an ArF excimer laser operated at 10 Hz with 100 mJ/pulse under the conditions used in the kinetics experiments. In order to avoid interferences, as will be described later in this paper, d2acetylene was synthesized from calcium carbide (Aldrich) and deuterium oxide (99.9% Aldrich), acetylene was synthesized from calcium carbide and water, and d4-diketene was synthesized from d6-acetone (99.9% Cambridge Isotopes).17,18 The purity and identity of the d4-diketene were checked via 1H NMR and 13C NMR.
Figure 1. Time profile of CH3 at 3154.7468 cm-1 with (solid line) and without (dashed line) acetylene in the presence of CH4 in a ratio of CH4:C2H2 of 2:1. The spike at t ) 0 is an artifact associated with the excimer laser discharge.
Kinetics Singlet methylene reacts too quickly (
