Formation of the interstellar molecules HNC3 and HC3N from a C3 carbon cluster and ammonia

The reaction of the carbon cluster, C-3, with ammonia (NH3) in cryogenic argon matrices (at 12 K), conditions that mimic, at least in part, interstellar ice, has been monitored by infrared spectroscopy. We present evidence that, in the first reaction step, a C-3-NH3 complex is formed without an entrance barrier. The calculated [MP2/6-311++G(d, p)] zero-point-corrected binding energy of the complex is 14.39 kJ mol(-1) at its equilibrium geometry. After UV-visible photolysis, the complex dissociates and forms the HNC3 molecule. Upon further photolysis, HNC3 is destroyed, and HC3N (cyanoacetylene) is generated. The calculated potential energy surface for this reaction reveals that HNC3 is produced from the C-3-NH3 complex by loss of H-2, while HC3N is formed from HNC3 by H migration along the NC3 backbone.