Ultraviolet Photochemistry of Diacetylene: Metastable C4H2*+C2H2 Reaction in Helium and Nitrogen

Abstract

The collisional photochemistry of diacetylene triplet metastable state (C4H2*) with acetylene in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. The photolysis laser counterpropagates the molecular expansion through the reaction tube exciting the C4H2 'Au -I&+ 2'06'0 transition at 231.4 nm. Efficient intersystem crossing forms the triplet state from which reaction occurs. The finite 8 mm length of the tube serves to quench the reaction after 10-30 ps so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuumultraviolet light and mass analyzed in a linear time-of-flight mass spectrometer. The primary photochemical processes are C4H2* + C2Hz -C6H2 + 2H (H2) and C4H2* + C2H2 -C6H3 -I-H. Under all conditions examined, C6H2 dominates the observed products, with C6H3 present above the I3CCsH2 background only at high nitrogen concentrations which deactivate diacetylene within its metastable vibrational manifold more efficiently than does helium. C6DH is the major product in the C4H2* + C2D2 reaction, indicating that both C4H2* and C2H2 contribute one hydrogen to the C6H2 product. CS& is observed as a new minor product of the reaction of excited diacetylene with C a 2 , but only at long reaction times. Formation of c 8 H 4 is significantly enhanced by the presence of C2H2. A simple mechanism for the reaction of diacetylene and acetylene with metastable excited diacetylene is formulated. Assuming equal photoionization cross sections for the products, concentration data are used to determine that k(C4H~)/k(C2H2) = 11 f 2 in helium buffer and 7 f 1 in nitrogen buffer, where k(i) is the total rate constant for the reaction of C4H2* with species i.

IR Spectrum of C8H2: Integrated Band Intensities and Some Observational Implications

“…Indeed, the lightest polyyne, C 4 H 2 , has been observed on Titan by the infrared interferometer spectrometer (IRIS) of the mission Voyager (3) and experimental simulations of Titan's atmosphere also revealed the presence of gaseous C 6 H 2 (4,5). Moreover, ultraviolet photochemical experiments on diacetylene showed the formation of C 8 H 2 (6,7). All these results strongly support the involvement of polyynes in Titan's organic chemistry.…”

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

“…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). Although triplet diacetylene reactions invoked to account for the observed chemistry are now often incorporated in models of Titan's atmosphere, with an assumed metastable lifetime of 1 ms or more, to clarify their role, the triplet lifetime must be measured directly and as a function of excitation energy (19).…”

Reactions of diacetylene radical cation with ethylene

“…The reactions of metastable diacetylene with small organic molecules such as ethylene, acetylene, and propylene have also been investigated in detail [17,18]. It has been demonstrated that metastable diacetylene has a rich chemistry with unsaturated hydrocarbons.…”

“…It has been demonstrated that metastable diacetylene has a rich chemistry with unsaturated hydrocarbons. For example, the dominant product formed from the reaction of metastable diacetylene with ethylene has been characterized as 1-hexene-3,5-diyne [18], whereas reactions with acetylene and propylene result in the formation of triacetylene [17] for the reaction with acetylene, and C 7 H 6 (either 5-heptene-1,3-diyne or 2-methyl-1-hexene-3,5-diyne), and 1-hexene-3,5-diyne [18] for the reaction with propylene. Although the photochemical reactivity of diacetylene has been explored, little is known about the reactivity of diacetylene radical cation [19], which is also likely present in planetary nebulae [7] and combustion systems [12].…”