Pressure Effect on CH₃ and C₂H₃ Cross-Radical Reactions

Abstract

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 3 COC 2 H 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 3 H 6 ), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C 2 H 6 ), the methyl radical combination product. At 27 kPa [C 3 H 6 ]/[C 2 H 6 ] ) 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to about 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C 2 H 3 + CH 3 + M f C 3 H 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 2 H 3 + CH 3 f C 3 H 5 + H, becomes available, producing a host of allyl radical 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 3 H 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.

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…The C 3 H 6 adduct can lead to C 3 H 5 + H formation, which means that adduct stabilization and bimolecular product formation compete. The results of Stoliarov et al (2000) and Fahr et al (1999) as a function of pressure lead to lower k 0 values than k 0,CH 3 + CH 3 →C 2 H 6 (T ). We estimate k 0,CH 3 + C 2 H 3 →C 3 H 6 (T ) = 0.1 × k 0,CH 3 + CH 3 →C 2 H 6 (T ).…”

“…There is no barrier on the singlet surface, leading to the C 3 H 6 adduct (Thorn et al 2000;Qu et al 2010), and this reaction has been studied experimentally by Fahr et al (1999), Thorn et al (2000) and Stoliarov et al (2000) between 1 and 200 Torr. The H atom abstraction reaction by CH 3 is likely to be a separate channel with a rate constant that has been measured on several previous occasions (Fahr et al 1999;Stoliarov et al 2000). The C 3 H 6 adduct can lead to C 3 H 5 + H formation, which means that adduct stabilization and bimolecular product formation compete.…”

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

“…Two product branching studies of this reaction have very recently been performed by Stoliarov et al [60] and Fahr et al [61]. Low-pressure studies (P ϭ 0.9 -3.7 Torr He) at T ϭ 310 K by Stoliarov et al [60] found that the combination-stabilization channel and the disproportionation channel dominated with average yields of ϳ42% and ϳ40%, respectively, and the combination-decomposition channel was the minor channel with ϳ18% average yield.…”

“…Low-pressure studies (P ϭ 0.9 -3.7 Torr He) at T ϭ 310 K by Stoliarov et al [60] found that the combination-stabilization channel and the disproportionation channel dominated with average yields of ϳ42% and ϳ40%, respectively, and the combination-decomposition channel was the minor channel with ϳ18% average yield. At T ϭ 298 K, Fahr et al [61] measured a 39% yield for the combinationstabilization channel at P ϭ 2.1 Torr He, which increased to ϳ78% at P ϭ 200 Torr He.…”

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…CH + C 3 H 4 → H + CH 2 CHC 2 H 4.2e-10 0 0 1.4 14 , Butler et al 1981, Daugey et al 2005, Goulay et al 2009, Loison & Bergeat 2009 12.! At low pressure (Fahr et al 1999, Stoliarov et al 2000, Hébrard et al 2013, Thorn et al 2000 → Rate constant in the 10-50K deduced from (Sabbah et al 2007), branching ratio mainly from (Savee et al 2012) (see also (Knyazev et al 1992, Cavallotti et al 2014, Leonori et al 2015 35.! 86.7 % de C 3 H x (supposed to be C 3 H 4 **) and 13.3% of CH x + C 2 H y (Angelova et al 2004a, Angelova et al 2004b.…”