Askar Fahr

Synchrotron Photoionization Mass Spectrometry Measurements of Kinetics and Product Formation in the Allyl Radical (H2CCHCH2) Self-Reaction†

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

Temperature-dependent kinetics of the vinyl radical (C2H3) self-reaction.

The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.

PRESSURE EFFECT ON CH3 AND C2H3 CROSS-RADICAL REACTIONS

The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH{sub 3}COC{sub 2}H{sub 3}) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C{sub 3}H{sub 6}), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C{sub 2}H{sub 6}), the methyl radical combination product. At 27 kPa [C{sub 3}H{sub 6}]/[C{sub 2}H{sub 6}] = 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to bout 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C{sub 2}H{sub 3} + CH{sub 2}+ M {r{underscore}arrow} C{sub 3}H{sub 6} + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C{sub 2}H{sub 3} + CH{sub 3} {r{underscore}arrow}C{sub 3}H{sub 5} + H, becomes available, producing a host of allyl radicalmorexa0» reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C{sub 3}H{sub 6}. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C-C bond as contrasted to the weaker C-C bond formed from combination of saturated hydrocarbon radicals. This C-C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C-H bond or isomerize to cyclopropane.«xa0less

Pressure Effect on CH3 and C2H3 Cross-Radical Reactions

The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH{sub 3}COC{sub 2}H{sub 3}) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C{sub 3}H{sub 6}), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C{sub 2}H{sub 6}), the methyl radical combination product. At 27 kPa [C{sub 3}H{sub 6}]/[C{sub 2}H{sub 6}] = 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to bout 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C{sub 2}H{sub 3} + CH{sub 2}+ M {r{underscore}arrow} C{sub 3}H{sub 6} + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C{sub 2}H{sub 3} + CH{sub 3} {r{underscore}arrow}C{sub 3}H{sub 5} + H, becomes available, producing a host of allyl radicalmorexa0» reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C{sub 3}H{sub 6}. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C-C bond as contrasted to the weaker C-C bond formed from combination of saturated hydrocarbon radicals. This C-C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C-H bond or isomerize to cyclopropane.«xa0less

Rate Coefficients and Products of Ethyl and Vinyl Cross-Radical Reactions

Rate constants and products for cross-radical reactions of vinyl (C2H3) and ethyl (C2H5) radicals have been determined at T = 298 K and a total pressure (predominately helium) of 93.3 kPa (700 Torr). C2H3 and C2H5 were produced simultaneously through the 193-nm excimer laser photolysis of dilute mixtures of C2H5COC2H3 (EVK)/He. This is the first report of direct rate determination for the C2H5 + C2H5 reaction and using the photolysis of EVK as a precursor for producing a nearly 1:1 ratio of C2H5/C2H3. Time-resolved UV absorption spectroscopy and gas chromatographic/mass spectroscopic (GC/MS) product analysis methods were employed for kinetics and product studies. Major reaction products consisted of n-butane, 1,3-butadiene, and 1-butene which are formed, respectively, through the combination reactions:u2009 C2H5 + C2H5 → n-butane (1c), C2H3 + C2H3 → 1,3-butadiene (2c,) and C2H5 + C2H3 → 1-butene (3c). Minor products, ethane, ethylene, and acetylene, result from disproportionation reactions. Analysis of the te...