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[45.05] Laboratory investigation on the formation of unsaturated nitriles in Titan's atmosphere via reactions of CN radicals with unsaturated hydrocarbons as studied in crossed molecular beams experiments

R.I. Kaiser, N. Balucani, O. Asvany, Y.T. Lee (Academia Sinica, Taipei, Taiwan, ROC)

Reactions of CN radicals with unsaturated hydrocarbons are thought to be of fundamental relevance to form unsaturated nitriles in Titan's atmosphere. Because of that, numerous studies of CN reactions with unsaturated molecules have been performed by exploiting the most modern kinetic techniques. Nevertheless, only the CN decay rates were recorded and the reaction mechanisms speculated. A deeper understanding of CN reaction mechanisms requires investigation at the microscopic level. The reaction mechanism has been elucidated, so far, only for the simplest CN radical reaction, that with molecular hydrogen. Even if this system may be regarded as a prototype for CN reactions because of its simplicity, the reaction mechanism with unsaturated hydrocarbons is expected to be different with the addition of CN to the \pi bond being dominant. In this contribution we report on the first systematic investigation of reactions of CN radicals with unsaturated hydrocarbons by using the crossed molecular beam technique with mass spectrometric detection. The first reactions we have looked at are those involving simple unsaturated hydrocarbons with respect to the CN/H exchange channels:

(1) CN + C₂H₂ -> HCCCN + H

(2) CN + C₂H₄ -> C₂H₃CN + H

(3) CN + CH₃CCH -> CH₃CCCN + H \ H₂CCCH(CN) + H

(4) CN + CH₃CCCH₃ -> CH₂CC(CN)CH₃ + H

(5) CN + H₂CCCH₂ -> H₂CCCH(CN) + H

Our results show that reactions (1) - (5) proceed without entrance barrier through the formation of a complex following the addition of the CN radical to the \pi system of the unsaturated hydrocarbon. Each complex decomposes to form the nitrile and a H atom. The explicit identification of this CN - H exchange channel opens a versatile route to form unsaturated nitriles and predict their formation in Titan's atmosphere. Further, our studies provide a solid data base on reaction products and shall guide chemical investigation of the NASA-ESA Cassini-Huygens mission to identify these nitriles in Titan.

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The author(s) of this abstract have provided an email address for comments about the abstract: kaiser@po.iams.sinica.edu.tw

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