MECHANISM OF CYANOACETYLENE PHOTOCHEMISTRY AT 185 AND 254 NM

The role of cyanoacetylene (HC3N) in the atmospheric photochemistry of Titan and its relevance to polymer formation are discussed. Investiga tion of the relative light absorption of HC3N, acetylene (C2H2), and d iacetylene (C4H2) revealed that HC3N is an important absorber of UV li ght in the 205- to 225-nm wavelength region in Titan's polar regions. Laboratory studies established that photolysis of C2H2 initiates the p olymerization of HC3N even though the HC3N is not absorbing the UV lig ht. Quantum yield measurements establish that HC3N is 2-5 times as rea ctive as C2H2 for polymer formation. Photolysis of HC3N with 185-nm li ght in the presence of N-2, H-2, Ar, or CF4 results in a decrease in t he 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 H-2, where it is unchanged. It was not possible to detect 1,3,5-tcb as a photopr oduct when the partial pressure of HC3N was decreased to 1 torr. Photo lysis of HC3N with 254-nm light in the presence of H-2 or N-2 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.