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 C 2 H+HCN→products and C 2 H+CH 3 CN→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, C 2 H, relevant to planetary atmospheres such as those of Jupiter and Saturn and its satellite Titan. Laser photolysis of C 2 H 2 is used to produce C 2 H, and transient infrared laser absorption is employed to measure the decay of C 2 H to obtain the subsequent reaction rates in a transverse flow cell. Rate constants for the reaction C 2 H+HCN→products are found to increase significantly with increasing temperature and are measured to be (3.9–6.2)×10 −13 cm 3 molecules −1 s −1 over the temperature range of 297–360 K. The rate constants for the reaction C 2 H+CH 3 CN→products are also found to increase substantially with increasing temperature and are measured to be (1.0–2.1)×10 −12 cm 3 molecules −1 s −1 over the temperature range of 262–360 K. For the reaction C 2 H+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 C 2 H+HCN→products are significantly smaller than values currently employed in photochemical models of Titan, which will affect the HC 3 N distribution.