A density functional study of the structures and energies of C_nN₅^- (n = 1-13) clusters

We designed numerous models of C_nN₅^- (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 C_nN₅^- (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 C_nN₅^- with even n are more stable than those with odd n, matching the peak patterns observed in the mass spectra of C_nN₅^-. The trend of such odd/even alternation can be explained based on concepts of bonding characters, energy differences, electron affinities, and incremental binding energies.