Ultraviolet Photochemistry of Diacetylene: Reactions with Benzene and Toluene

Abstract

The reactions of metastable diacetylene with benzene and toluene are explored using a molecular beam pump−probe time-of-flight mass spectrometer. Diacetylene is laser-excited to the 210610 band of the 1Δu ← X1Σ+g transition, whereupon rapid intersystem crossing occurs to the lowest triplet states. The triplet state diacetylene then reacts with either benzene or toluene as the gas mixture traverses a short reaction tube (∼20 μs). The reactions are quenched as the gas mixture expands into the ion source region of a time-of-flight mass spectrometer where the primary photoproducts are detected using vacuum ultraviolet (VUV) photoionization or resonant two-photon ionization (R2PI). The major products from the reaction of diacetylene and benzene have molecular formulas C8H6 and C10H6, and are identified as phenylacetylene and phenyldiacetylene using R2PI spectroscopy. The major products from metastable diacetylene's reaction with toluene are C9H8 and C11H8. The C9H8 product is confirmed as a mixture of o-, m-, and p-ethynyltoluene, with the ortho product dominating. Mechanisms for the formation of the above products are proposed based on deuterium substitution studies of the reactions. The potential importance of these reactions is discussed as they relate to hydrocarbon growth in sooting flames.