A density functional study on nitrogen-doped carbon clusters C _nN₃^-(n=1-8)

Using molecular graphics software, we designed numerous models of C _nN₃ (n= 1-8). Geometry optimization and calculation on vibration frequency were carried out by the B3LYP density functional method. After comparison of structure stability, we found that the structures of ground-state CN₃^- and C₂N₃ ^- are bent chains with a nitrogen atom at either end, whereas when n = 3-8, the ground-state clusters show three branches, each with a nitrogen atom located at the end. When n= 5-8, the longest branch of C_nN ₃^- is polyacetylenelike. When n = 5 or 7, the longest branch is connected to the central sp² carbon in a nonlinear manner. The C_nN₃^-(n= 1 -8) with an even number of carbon atoms are more stable than those with odd numbers, matching the peak pattern observed in laser-induced mass spectra of C_nN ₃^-. The trend of such odd/even alternation is explained based on concepts of bonding characteristics, electron affinities, and incremental binding energies.