THE KINETICS OF THE FORMATION OF NITRILE COMPOUNDS IN THE ATMOSPHERESOF TITAN AND NEPTUNE

The nitrile compounds HCN and C2HCN (cyanoacetylene) were detected on Titan by Voyager 1. The ethylene-analog nitrile to cyanoacetylene C2H3 CN (acrylonitrile) remains undetected. More recently, ground-based obs ervers have detected HCN on Neptune and placed an upper limit on C2HCN . Kinetic aspects of the formation of acrylonitrile have been studied in a discharge-flow mass spectrometric system at T = 298 K. The fracti onal yield for the formation of acrylonitrile from the fast gas phase reaction CN + C2H4 --> C2H3CN + H has been determined to be (0.2 +/- 0 .1). The other possible gas phase reaction leading to the formation of acrylonitrile, HCN + C2H3 --> C2H3CN + H, has been determined to be s low under planetary conditions at k(T = 298 K) = 2-7 x 10(-14) cm3 mol ecule-1 s-1, but remains an important chemical sink for HCN. The plane tary implications of the results are discussed in terms of the ratio o f acrylonitrile to cyanoacetylene on Titan and Neptune.