二氮双环阳离子游离基中N,N'-三电子键

用从头算HF/3-21+G^*优化了二氮双环m,n,l]游离基阳离子(m,n,l≥2～5)。分子4,4,4]和2,2,2]^+.,3,3,3]^+.,4,4,4]^+.游离基阳离子的优势构型有D3对称性，而其它游离基阳离子的优势构型为非对称性。通过比较这些阳离子几何构型，HOMO和NHOMO(即NextHOMO和HOMO-1)，和由MNDO计算确定的原子对作用能，表明当二氮双环游离基阳离子的桥链(CH2)n的n≥3时，桥头氮原子通过空间相互用用形成了一个弱的N,N'-三电子σ键。形成的三电子键强度不随环的扩大而增强。而三电子键强度被两个因素影响：一个是桥头氮原子间的p轨道重叠的取向；另一个是它们相应p轨道成分。

N,N'-three-electron bond in the radical cation of medium ring bicyclic diamine

ab initio HF/3-21+G^* calculations have been performed to optimize the structure of a series of bicyclic diaminem, 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.