Rate coefficients for reactions of ethynyl radical (C2H) with HCN and CH3CN:: Implications for the formation of complex nitriles on Titan

Rate coefficients for the reactions of C2H + HCN --> products and C2H + CH3CN --> products have been measured over the temperature range 262-360 K. These experiments represent an ongoing effort to accurately measure reaction rate coefficients of the ethynyl radical, C2H, relevant to planetary atmospheres such as those of Jupiter and Saturn and its satellite Titan. Laser photolysis of C2H2 is used to produce C2H, and transient infrared laser absorption is employed to measure the decay of C2H to obtain the subsequent reaction rates in a transverse flow cell. Rate constants for the reaction C2H + HCN --> products are found to increase significantly with increasing temperature and are measured to be (3.9-6.2) x 10(-13) cm(3) molecules(-1) s(-1) over the temperature range of 297-360 K. The rate constants for the reaction C2H + CH3CN --> products are also found to increase substantially with increasing temperature and are measured to be (1.0-2.1) x 10(-12) cm(3) molecules(-1) s(-1) over the temperature range of 262-360 K. For the reaction C2H + HCN --> products, ab initio calculations of transition state structures are used to infer that the major products form via an addition/elimination pathway. The measured rate constants for the reaction of C2H + HCN --> products are significantly smaller than values currently employed in photochemical models of Titan, which will affect the HC3N distribution.