Ultraviolet Photochemistry of Diacetylene with Alkynes and Alkenes: Spectroscopic Characterization of the Products

The primary reaction products formed when metastable diacetylene (C 4 H 2 *) reacts with ground-state diacetylene (C 4 H 2 ), ethylene (C 2 H 4 ), and propene (CH 3 −CH CH 2 ) have been spectroscopically characterized using a laser pump−probe scheme. Reaction is initiated in a constrained expansion which limits reaction times to about 20 μs. The molecular structures of the C 6 H 2 product formed in the C 4 H 2 * + C 4 H 2 reaction is identified from its R2PI spectrum and two-photon ionization threshold (9.49 ± 0.01 eV) as triacetylene (H−C⋮C−C⋮C−C⋮C−H). The analogous scans of the C 8 H 2 product show a clear two-photon ionization onset at 9.09 ± 0.02 eV, but little resolved vibronic structure above this threshold, consistent with its assignment as tetraacetylene. The R2PI spectra of the C 6 H 4 product formed in the reactions of C 4 H 2 * with C 2 H 4 and CH 3 −CH CH 2 lead to an identification of this product as 1-hexene-3,5-diyne (CH 2 CH−C⋮C−C⋮CH). The C 7 H 6 product formed in the reaction of C 4 H 2 * with CH 3 −CH CH 2 possesses an R2PI spectrum close to that of 1-hexene-3,5-diyne, supporting its identification as a methylated derivative with the same ene−diyne structure. Two C 7 H 6 isomers meet this criterion, 5-heptene-1,3-diyne (CH 3 −CH CH 2 −C⋮C−C⋮C−H) and 2-methyl-1-hexene-3,5-diyne (H 2 C C(CH 3 )-C⋮C−C⋮C−H), between which we cannot distinguish. Implications for the mechanisms for these C 4 H 2 * reactions are discussed.