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

Abstract

The primary reactions of the lowest energy triplet states of diacetylene (C4H2*) with 1,3-butadiene (C4H6) 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 2 061 0 band of the 1Δu ← X1Σ+ g transition in C4H2 at 231.5 nm, which rapidly interconverts to high vibrational levels of the lowest energy triplet surfaces. The subsequent reactions with C4H6 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 C6H6 and C8H6 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 C6H6 and C8H6 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.

Reactions of diacetylene radical cation with ethylene

“…The formation of benzene in the course of the reaction is not surprising because it is the thermodynamically favored product (Table 1) [32]. Although aromatic ring formation has been observed previously in the reaction of metastable diacetylene with 1,3-butadiene [9], the photochemical reaction of diacetylene with ethylene gives nonaromatic products identified as C 6 H 4 and C 6 H 5 [18]. Therefore, photochemical results do not mirror the radical cation chemistry.…”

“…Although no experimental evidence for the formation of aromatic molecules or PAHs in the atmosphere of Titan has been found, the formation of benzene and phenylacetylene in the reaction of metastable diacetylene (C 4 H 2 * ) with 1,3-butadiene has been reported by Arrington et al [9]. Corresponding attempts to make PAHs from the reaction of metastable diacetylene with styrene to form naphthalene and naphthalene derivatives proved unsuccessful [11].…”

“…Pathways for formation of fused aromatic rings in combustion are important to know for the modeling of flames. These PAHs form graphitic structures that are thought to be precursors to soot formation [9,12].…”

Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

“…This has a large impact on the addition rate of acetylene on the phenyl radical (k (1+2) 2 varies by more than three orders of magnitude between 150 and 300 K), which could yield a much higher production rate of PAHs at room temperature, comparable to the two other kinds of polymers. Another possibility is that we are missing some other pathways to produce benzene (or other ring molecules), such as the reaction proposed in Arrington et al (1998) between 1,3-butadiene and metastable diacetylene.…”

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

“…Subsequently, Zwier and coworkers (10,13) extensively investigated the UV photoinduced chemistry of diacetylene through reactions in a ceramic nozzle with a VUV probe of the products downstream. After excitation of the 1 ⌬ u excited state, secondary reactions were found to lead to the formation of various larger hydrocarbons (12,14,15). The laser-based studies, principally at 231 and 243 nm, also found no evidence for radical products proceeding from primary photodissociation of diacetylene (1,11,(16)(17)(18).…”