New Isomers of N8 without Double Bonds

Quantum mechanical methods have been used to examine isomers of N8. In addition to the previously studied octaazacubane structure (Oh symmetry, 1), we have investigated two new structures, of D 2h (2) and C 2v (3) symmetry, without NN double bonds. The C 2v structure may be designated octaazacuneane. The following basis sets and methods have been employed to optimize geometries: (1) (SCF, CISD, CCSD)/DZP; (2) (SCF, MP2, B3LYP, B3P86, BHLYP)/6-31G*; (3) B3LYP/6-311+G*. The single-point energies have also been estimated at the CISD+Q/DZP level of theory with CISD/DZP geometries and at the CCSD(T)/DZP level with CCSD/DZP geometries. The harmonic vibrational frequencies and their infrared intensities have also been predicted with the SCF, MP2, and DFT methods. The results show that the two new isomers are both minima on the N8 potential energy hypersurface and of higher energy than isomers containing NN double bonds. Among the three isomers with only single bonds, the ones with the higher symmetry have higher energies.