Laboratory investigation of ion-molecule reactions of HC3N in dense interstellar clouds

Abstract

Rate coefficients from laboratory measurements at 300 K are reported for reactions of cyanoacetylene, HC₃N, with 18 ions of interstellar significance. Reactions between HC₃N and ions of the type YH⁺ were found to be exclusively proton transfer except in the case where Y = NH₃ where no reaction occurred. These observations limit the proton affinity of HC₃N to the range between 824 and 866 kJ/mol. Reactions between HC₃N and ions with non-transferable protons, Y⁺, were exclusively charge transfer with rate coefficients in most cases exceeding the predictions of the simple polarization and orientated dipole models. A further consequence of the high proton affinity of HC₃N is that HC₅N will not be formed from HC₃N via the reaction$$\text{C}_{2}\text{H}_{3}^{+}+\text{HC}_{3}\text{N}\rightarrow \text{H}_{2}\text{C}_{5}\text{N}^{+}+\text{H}_{2}$$.