The propargyl (HC⋮CCH2) radical is among the critical intermediates in hydrocarbon reaction systems pertinent to both the high temperatures of combustion systems and the low temperatures of planetary atmospheres. This work reports experimental results on the nature and the relative yields of the final products of the propargyl combination reaction, C3H3 + C3H3 → C6H6 (1). Propargyl radicals, for most experiments, were generated by the 248 nm excimer laser photolysis of propargyl bromide. The 193 nm photolysis of propargyl chloride and of allene were also used in a number of experiments, particularly at higher temperatures. Product studies were performed at a pressure range of 27 mbar (20 Torr) to 933 mbar (700 Torr) and at a temperature range of 295−623 K. Final reaction products were separated, identified, and quantified using an on-line gas chromatograph/mass spectrometer system. Five isomeric C6H6 final products were detected including 1,5-hexadiyne, fulvene, dimethylenecyclobutene, and benzene. The relative yields of the major reaction products showed significant pressure and temperature dependencies. Under high-pressure conditions 1,5-hexadiyne is a major product with a relative yield of 51 % at P = 933 mbar and T = 295 K. However, its yield decreases to 27 % at P = 933 mbar and T = 623 K and to 1 % at P = 27 mbar, T = 623 K. Dimethylenecyclobutene, has a relative yield of 6% at 295 K and 933 mbar. It becomes the most abundant product with a relative yield of nearly 90% at 623 K and 133 mbar. Fulvene appears to be a minor product at all conditions of this study and its relative yield (∼1.5% or less) is significantly lower than the reported computational predictions. Interestingly, an appreciable amount of benzene is also formed, particularly at lower pressures with relative yields considerably higher (43% at 27 mbar and 623 K) than those predicted computationally (∼3% at P = 27 mbar T = 650 K). The results of this work suggest that the formation of dimethylenecyclobutene and benzene from the propargyl combination reaction is significantly more efficient than previously predicted. Implications of these results on modeling of combustion processes as well as planetary atmospheric processes are discussed.

Submonolayer formaldehyde (H 2 CO) has been photodissociated on Ag(111) at 35 K using nanosecond 287 nm laser pulses. The low temperature prohibits further bimolecular reactions of the dissociation products and allows their analysis by electron energy loss spectroscopy. One of the dissociation products has been identified as the radical species methylene (CH 2 ). The dissociation is induced by photoexcited substrate electrons attached to formaldehyde, forming a transient formaldehyde negative ion that dissociates. Due to the influence of adsorbate-substrate bonding on reaction energetics, the dissociation channel of H 2 COon Ag appears to be different from that of the gaseous negative ion.

This work reports measurements of absolute rate coefficients and Rice-Ramsperger-Kassel-Marcus (RRKM) master equation simulations of the C2H3+C2H4 reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 20 and 133 mbar. Vinyl radicals (H2C=CH) were generated by laser photolysis of vinyl iodide (C2H3I) at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. Measurements at 20 mbar are in good agreement with previous determinations at higher temperature. A weighted three-parameter Arrhenius fit to the experimental rate constant at 133 mbar, with the temperature exponent fixed, gives k=(7+/-1)x10(-14) cm3 molecule(-1) s(-1) (T/298 K)2 exp[-(1430+/-70) K/T]. RRKM master equation simulations, based on G3 calculations of stationary points on the C4H7 potential energy surface, were carried out to predict rate coefficients and product branching fractions. The predicted branching to 1-methylallyl product is relatively small under the conditions of the present experiments but increases as the pressure is lowered. Analysis of end products of 248 nm photolysis of vinyl iodide/ethylene mixtures at total pressures between 27 and 933 mbar provides no direct evidence for participation of 1-methylallyl.