Aromatic Ring-Forming Reactions of Metastable Diacetylene with 1,3-Butadiene

The primary reactions of the lowest energy triplet states of diacetylene (C 4 H 2 *) with 1,3-butadiene (C 4 H 6 ) in a helium buffer are characterized with a molecular beam pump−probe technique. Triplet diacetylene is prepared in the early portions of a molecular expansion by laser excitation of the 2 1 0 6 1 0 band of the 1 Δ u ← X 1 Σ + g transition in C 4 H 2 at 231.5 nm, which rapidly interconverts to high vibrational levels of the lowest energy triplet surfaces. The subsequent reactions with C 4 H 6 are allowed to proceed for 20 μs while the expansion traverses a short ceramic reaction tube or slit channel. Primary products are observed by quenching secondary processes as molecular collisions cease outside the tube. The major photochemical products C 6 H 6 and C 8 H 6 are detected in a linear time-of-flight mass spectrometer using both vacuum ultraviolet photoionization and resonant two-photon ionization (R2PI). R2PI spectra of the C 6 H 6 and C 8 H 6 products unambiguously identify them as benzene and phenylacetylene, respectively. Based on deuterium substitution experiments, a mechanism for these ring-forming reactions is proposed. The potential importance of these reactions for forming aromatics in sooting flames and planetary atmospheres is discussed.