Equilibrium geometries, energies, and vibrational frequencies for eight N-11 isomers with low spin have been calculated at the HF/6-31G*, B3PW91/6-31G*, B3LYP/6-31G*, and MP2/6-31G* levels of theory. The results suggest that pentazole ring remains a fundamental stable structural unit in odd-numbered nitrogen cluster N-11. The thermodynamically most stable N-11 isomer is the C-2 symmetric structure I consisting of two aromatic pentazole rings. However, the investigation on decomposition exhibits that 1 has a low barrier height, 5.6 kcal/mol at the B3LYP/6-31G* level. It seems, therefore, that structure I is a kinetically unstable open shell species. (C) 2002 Elsevier Science B.V. All rights reserved.