Mechanism of cyanoacetylene photochemistry at 185 and 254 nm

Abstract

The role of cyanoacetylene (HC3N) in the atmospheric photochemistry of Titan and its relevance to polymer formation are discussed. Investigation of the relative light absorption of HC3N, acetylene (C2H2), and diacetylene (C4H2) revealed that HC3N is an important absorber of UV light in the 205- to 225-nanometer wavelength region in Titan's polar regions. Laboratory studies established that photolysis of C2H2 initiates the polymerization of HC3N even though the HC3N is not absorbing the UV light. Quantum yield measurements establish that HC3N is 2-5 times as reactive as C2H2 for polymer formation. Photolysis of HC3N with 185-nanometer light in the presence of N2, H2, Ar, or CF4 results in a decrease in the yield of 1,3,5-tricyanobenzene (1,3,5-tcb), while photolysis in the presence of CH4, C2H6, or n-C4H10 results in an increase in 1,3,5-tcb. The rate of loss of HC3N is increased by all gases except H2, where it is unchanged. It was not possible to detect 1,3,5-tcb as a photoproduct when the partial pressure of HC3N was decreased to 1 torr. Photolysis of HC3N with 254-nanometer light in the presence of H2 or N2 results in the formation of 1,2,4-tcb, while photolysis in the presence of CH4, C2H6, or n-C4H10 results in the formation of increasing amounts of 1,3,5-tcb. Mechanisms for the formation of polymers are presented.

The Design and Use of a Photochemical Flow Reactor: A Laboratory Study of the Atmospheric Chemistry of Cyanoacetylene on Titan

“…The initial steps in the photolysis of HC 3 N are given in (R2)-(R4) in Scheme 1. The quantum yields determined for the loss of HC 3 N (Table IV) are consistent with the quantum yields obtained in the static system (Clarke and Ferris 1996). The SCHEME 1 quantum yields in the static system decrease with the decreasing of HC 3 N pressure from 25 to 2.5 Torr and then increase in the 2.5 to 1.0 Torr range.…”

“…4). The spectrum of the polymer formed at a 10 −3 mixing ratio of HC 3 N was comparable to that of the polymers prepared by the static irradiation of HC 3 N (Clarke and Ferris 1996). Peaks could be assigned to N-H (3330 cm −1 ), C-H in CH 3 (2960 and 2870 cm −1 ), C-H in CH 2 (2920 and 2854 cm −1 ), and C≡N (2210 cm −1 ).…”

“…Comparison of peak areas at 3320 and 3351 cm −1 to standard curves was used for the determination of HC 3 N loss and HCN formation, respectively. The gases were then transferred to a quartz cell and extracted into a weighed amount of CDCl 3 (Clarke and Ferris 1996). Acrylonitrile (CH 2 CHCN) and acetonitrile (CH 3 CN) were identified by comparison of their NMR spectra with authentic samples.…”

“…HC 3 N is the third most prevalent nitrogen-containing compound in the atmosphere, after N 2 and HCN. Its light absorbing properties and photochemical reactivity also indicate that it could have an important role in the atmospheric chemistry on Titan (Clarke and Ferris 1996). HC 3 N absorbs light at wavelengths as long as 260 nm King 1966a, 1966b) and is photoreactive at wavelengths out to at least 254 nm (Ferris and Guillemin 1990).…”

Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan's atmosphere

“…This radiation is not absorbed by methane or nitrogen but is absorbed by acetylene, ethylene and cyanoacetylene. Some of the photoproducts of these gases absorb the 254 nm radiation (Clarke and Ferris, 1996). The structures of the photoproducts were determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC/MS).…”