Formation of the Interstellar Molecules HNC 3 and HC 3 N from a C 3 Carbon Cluster and Ammonia

Abstract

The reaction of the carbon cluster, C 3 , with ammonia (NH 3 ) 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 -NH 3 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 HNC 3 molecule. Upon further photolysis, HNC 3 is destroyed, and HC 3 N (cyanoacetylene) is generated. The calculated potential energy surface for this reaction reveals that HNC 3 is produced from the C 3 -NH 3 complex by loss of H 2 , while HC 3 N is formed from HNC 3 by H migration along the NC 3 backbone. © 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.