Structure and stability of new N-7 isomers

Ab initio quantum mechanics methods have been used to examine isomers with low spin of N-7 cluster. In additional to the previously studied five N-7 i somers, five new structures were investigated in this paper. Their structur es, energies, zero point energies, net charges were calculated at the UHF/6 -31G*, UMP2/6-31G*, B3LYP/6-31G*, B3LYP/6-311+G* levels. The harmonic vibra tional frequencies and their infrared intensities have been predicted with SCF, MP2 and DFT methods. Two of them [structure 2 (Cs) and 5 (Cs)] are rea sonable local minima on the N-7 energy hypersurface at the all of the level s we used. The twisted open-chain N-7 (C-2v) is still the most stable in al l of the twelve N-7 isomers. Based on the NBO analysis, a new non-classical Lewis structure was suggested for the twisted open-chain N-7 (C-2v), which is helpful for understanding its special bond nature. The similar and diff erent properties between the N-7 clusters and even-number nitrogen clusters were discussed. (C) 2001 Elsevier Science B.V. All rights reserved.