An experimental study of the reactivity of CN− and C3N− anions with cyanoacetylene (HC3N)

The reactions of the CN− and C3N− anions with cyanoacetylene HC3N, of special interest for the chemistry of Titan's upper atmosphere, have been investigated by means of FTICR mass-spectrometry. Primary ions, CN− and C3N−, have been produced by dissociative electron attachment (DEA) from BrCN and BrC3N, and prepared in a clean way before reaction. Total rate constants have been measured for both reactions at 300 K and are found to be as follows: (3.9 ± 0.5) × 10−9 and (1.0 ± 0.2) × 10−10 cm3 s−1 for the reaction of HC3N with CN− and C3N−, respectively. For the CN− + HC3N reaction, proton transfer is found to be the only reactive channel within our detection limits. Proton transfer is also dominant for the C3N− + HC3N reaction but the resulting ionic product being identical to the primary ion C3N−, this process is transparent for the kinetics of the C3N− + HC3N reaction and the kinetic rate retrieved corresponds to a slow and competitive detachment pathway. Yet the nature and energetics of the neutral product(s) formed through this process remain unknown. Additional experiments using isotopic products have allowed to retrieve specific rate constants associated with the proton transfer channel in the C315N− + HC3N and C3N− + HC315N reactions and the measured rates are found to be significantly lower than for the CN− + HC3N system. This decrease and the evolution of reactivity when going from CN− to C3N− and the opening of a new detachment pathway are finally discussed.