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.
Chem. 61, 1970Chem. 61, (1983. La photolyse du cyclohcx&ne gazeux a CtC systdmatiqucment dtudide h 184.9 nrn en I'absence ou cn prCsence d'intcrcepteur radicalairc tel que Oz, NO, H2S ou H1 entre 1 ct 70 Torr. En outre. le propane ou le soufre hcxafluord a CtC utilisC cornme agent stabilisant. L'Cthylkne et Ic butadiknc-1,3 ont Ctd les seuls produits majeurs obscrvCs avec des rendcrnents quantiqucs supdrieurs i 0.5 quelles quc soient les conditions expCrimentales. Lcs analyscs isotopiques de I'Cthylknc formd dans la photolyse du cycIohexknc-3,3,6,6-(1, montrent la prksence preponddrantc dc la varidtd perhydrogknCe. Ces rdsultats, avec d'autres observations tiries dc la IittCrature, favorisent un mdcanisme dc fragmentation de la moldcule cxcitde en unc Ctape ou en deux dtapcs, mais dans ce cas via un intermediaire dont Ic temps dc vie cst plus court que cclui de la molCcule photoexcitCc.GUY J. COL-LIN and HELENE DESLAURIERS. Can. J . Chem. 61, 1970Chem. 61, (1983. We have studied the 184.9 nrn photolysis of gaseous cyclohcxcne either in the absence or in the presence of radical scavengers such as 02. NO, HZS, or HI at pressures between I and 70 Torr. Propane or sulfur hexatluoride has also been used as stabilizing agent. In all cases, ethylene and I ,3-butadicne have rather high quantum yields (@ 2 0.5). The isotopic analysis of the ethylene formed in the photolysis of cyclohexene-3,3,6.6-rl, shows the high importance of the perhydrogenated species. These results, togcthcr with others taken from the literature, favor a one-step fragmentation mechanism or a double-step mechanism involving an intermediate which has a lifetime shorter than that of the photocxcited molecule.
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