A density functional study of the structures and energies of CnN5− (n = 1–13) clusters

We designed numerous models of CnN5− (n = 1–13) using molecular graphics software. Geometry optimization and calculation of vibration frequency were carried out by means of the B3LYP density functional approach. After comparison of structure stability, we found that the ground-state structure of CN5− shows a N4CN chain configuration whereas those of CnN5− (n = 2–13) contain a planar pentagonal ring compose of nitrogen and carbon atoms when n ≤ 8, and of carbon atoms only when 9 ≤ n. For odd-n ground-state isomers with 1 < n ≤ 9, one carbon atom of the ring is bonded to a nitrogen atom and the other carbon atom(s) are bonded to a cyano (–CN) group. When n is even and with 2 ≤ n ≤ 10, all the carbon atom(s) within the ring are bonded to a cyano group. When n = 11–13, one of the carbon atoms of the ring is bonded to a straight NC2–, NC3– and NC4– chain, respectively, while the other carbon atoms of the ring are each bonded to a cyano group. The NC2– and NC4– chains show cumulenic-like structures whereas the NC3– chain shows polyacetylene-like structure. According to total energies, the CnN5− with even n are more stable than those with odd n, matching the peak patterns observed in the mass spectra of CnN5−. The trend of such odd/even alternation can be explained based on concepts of bonding characters, energy differences, electron affinities, and incremental binding energies.