Are CnN− clusters really bent?

The CnN− (n=1–7) anions have been investigated using the B3LYP (Becke 3-parameter-Lee–Yang–Parr) density functional method and energetics compared with ab initio calculations at several levels of theory (namely MP2, MP4, QCISD(T) and CCSD(T)). Equilibrium structures, vibrational frequencies and relative energies have been determined for the lowest-lying linear and cyclic isomers. All the methods employed (except Møller–Plesset perturbation theory) reveal that the CnN− (n=2–7) clusters are linear in their ground state (1Σ+ for odd n and 3Σ− for even n).