The 213.8-nm photochemistry of gaseous 1,3-butadiene and the structure of some C3H3 radicals

Abstract

A systematic study of the 213.8-nm (zinc line) photochemistry of 1,3-butadiene has been made either in the absence or in the presence of various additives-such as radical scavengers (02, NO, DI) and collisional quenchers-in the gas phase (pressure between 1 and 500 Torr). The major fate of the photoexcited 1,3-butadiene molecule is isomerization to the 1,2-butadiene structure which may then decompose to methyl and C3H3 radicals ( = 0.64 ± 0.04 at 1 Torr of 1,3-butadiene). Minor processes include decomposition to the acetylene + ethylene couple ( = 0.22 ± 0.02) or to vinylacetylene ( = 0.038 ± 0.003) and molecular hydrogen. These two minor processes occur from different excited states. Some 2-butyne ( < 0.015) is formed by a unimolecular isomerization process. The photolysis of 1,3-butadiene-l,l,4,4-d¡, indicates that at least three different intermediates are involved in the formation of molecular ethylene and acetylene. The C3H3 radicals are not easily intercepted by DI: k(C3H3 + 1,3-butadiene)/A:(C3H3 + DI) = 0.09 ± 0.03. Also at 21 °C and for [DI]/ [ 1,3-butadiene] = 10, the highest ratio used, (3 1 ß + propyne)/^(CH3D) = 0.72 and a fraction of the C3H3 radicals are still not accounted for (reaction with 1,3-butadiene and/or recombination?). The relative energies obtained by ab initio RHF-SCF geometry optimizations for the doublet electronic state of the C3H3 radical structures are £(propargyl) < £(propyn-l-yI) < £(cyclopropen-l-yl) < £(allenyl). General valence bond geometry optimizations and a multiconfigurational self-consistent-field surface scan also show that the propargyl species (1 2B, state) is the lowest energy one. There are probably at least two distinct C3H3 radical structures (different states) present in the far-UV photolysis of 1,3-butadiene.

Kinetics and products of propargyl (C3H3) radical self-reactions and propargyl-methyl cross-combination reactions

“…However, two of the products detected in the present experiments, CH 2 " C " CHCl and CH 2 " C " C 9 CH 2 -C # CH, suggest that the free-electron density on the photolytically generated C 3 H 3 radicals is not exclusively localized on the 9 CH 2 end of the radical, and C 3 H 3 with allenic characteristics may also be present under these experimental conditions. Similarly, Collin et al [17], in their investigations of the 213-nm photolysis of 1,3-butadiene, have reported production of both propargyl-like and allenic-like structures of C 3 H 3 radicals.…”

Formation of fulvene in the reaction of C2H with 1,3-butadiene

“…However, numerous studies explored the excitation and photolysis of 1,3-butadiene in the 193-220 nm range, from which it is found that the primary dissociative channel leads to formation of C 3 H 3 + CH 3 [14,[57][58][59][60]. In addition to C 3 H 3 + CH 3 , the following minor photolysis channels have also been reported in the 193-220 nm energy range: C 4 H 5 + H, C 4 H 4 + H 2 , C 2 H 3 + C 2 H 3 and C 2 H 2 + C 2 H 4 with varying relative ratios [14,59,60]. The 1,3-butadiene cross-section for absorption at 248 nm is nearly two orders of magnitude smaller than at 193 nm and certain channels are energetically inhibited, such as formation of two vinyl radicals (C 2 H 3 ) by cleavage of the central CÀ ÀC bond.…”

Development and testing of a comprehensive chemical mechanism for the oxidation of methane

“…Even recently, the struc-H · C " C " O, tures of H 2 CCCH and H 2 CCCCH have been debated; these were identified as the propargyl radical and butenyn-2-yl ( · CH CCH 4 CH " C " CH · ) 2 2 respectively [24,25].…”