THE 213.8-NM PHOTOCHEMISTRY OF GASEOUS 1,3-BUTADIENE AND THE STRUCTURE OF SOME C3H3 RADICALS

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 (O2, 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 (Φ = 0.64 ± 0.04 at 1 Torr of 1,3-butadiene). Minor processes include decomposition to the acetylene + ethylene couple (Φ = 0.22 ± 0.02) or to vinylacetylene (Φ = 0.038 ± 0.003) and molecular hydrogen. These two minor processes occur from different excited states. Some 2-butyne (Φ < 0.015) is formed by a unimolecular isomerization process. The photolysis of 1,3-butadiene-1,1,4,4-d4 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)/k(C3H3 + DI) = 0.09 ± 0.03. Also at 21°C and for [DI]/[1,3-butadiene] = 10, the highest ratio used, Φ(allene + propyne)/Φ(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 E(propargyl) < E(propyn-1-yl) < E(cyclopropen-1-yl) < E(allenyl). General valence bond geometry optimizations and a multiconfigurational self-consistent-field surface scan also show that the propargyl species (2B1 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. © 1990 American Chemical Society.