The 213.8-nm Photochemistry of Gaseous 1,3-Butadiene and the Structure of Some C3H3 Radicals

Article abstract of DOI:10.1021/j100364a020

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 an 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 deg C and for /<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.