ab initio HF/3-21+G^* calculations have been performed to optimize the structure of a series of bicyclic diamine[m, n, l] (m,n,l≥2～5) radical cations. The calculated geometries show that neutral [4,4,4] and the radical cation [2,2,2]^+., [3,3,3]^+. and [4,4,4]^+. have D3 symmetry, while the other radical cations are asymmetrlic. Through comparison of the geometries, HOMO and NHOMO(i.e.Next HOMO or HOMO-1) for the radical cations calculated by the ab initio 3-21+G^* method, and the atom-pair interaction energy determined by NMDO calculation, it is shown that when n is equal to or largey than 3 in the bridge chain (CH2)n for bicyclo diamine radical cations, the bridgehead nitrogen atoms form a weak N, N'-three-electron σ-bond through space. The strength of the three-electron bond formed does not increase with enlargement of the ring. However, the strength was found to be influenced by two factors: One is the orientation of the overlapping p orbitals of the bridgehead nitrogen atoms the other is the component of their corresponding p orbitals.

Key words: NITROGEN HETEROCYCLICS, CATION, FREE RADICAL, BRIDGE BOND, ATOM PAIRS, AB INITIO CALCULATION, BICYCLIC COMPOUND