NiTi过渡金属团簇的结构和芳香性的理论研究

Theoretical Studies on Structures and Aromaticity of NiTi Clusters

在B3PW91/6-311+G(d)计算水平上,计算并讨论了Ni4Ti2,[Ni4Ti2]2+,[Ni4Ti2]2-与Ni4Ti4,[Ni4Ti4]2+,[Ni4Ti4]2-团簇的几何结构和芳香性.在构型优化过程中得到了Ni4Ti2(D4h),[Ni4Ti2]2+(D4h),[Ni4 Ti2]2-(D4h)和Ni4 Ti4(D2h)4个稳定构型,发现当引入上下2个Ti原子后,Ni4环成为了平面正方形构型.核无关化学位移(NICS)计算结果表明,Ni4Ti2(D4h)与Ni4Ti4(D2h)的NICS值为正,而[Ni4Ti2]2+(D4h)和[Ni4Ti2]2-(D4h)的NICS值为负,且[Ni4Ti2]2-(D4h)的NICS值更负.同时还发现,由s与d轨道参与形成的反磁性环流是引起[Ni4Ti2]2+(D4h)和[Ni4Ti2]2-(D4h)具有较大芳香性的主要原因;其中Ti原子主要提供dz2与s轨道,而Ni原子主要利用其dz2与dx2-y2轨道形成正方形环,它们之间构成了球状的d轨道环流,且[Ni4Ti2]2+(D4h)和[Ni4Ti2]2-(D4h)中还有非常明显的π轨道环流.

The structures and aromaticity of Ni4,2＋ and 2-,Ni4Ti2,[Ni4Ti2]2＋ and [Ni4Ti2]2-,Ni4Ti4,[Ni4Ti4]2＋ and [Ni4Ti4]2-were studied at the B3PW91/6-311＋G（d） basis level with density functional theory（DFT）.Introduction of Ti atoms to both sides of Ni4 ring brings about stable conformations Ni4Ti2（D4h）,[Ni4Ti2]2＋（D4h）,[Ni4Ti2]2-（D4h） and Ni4Ti4（D2h） with square-planar ring of Ni4.Nucleus-independent chemical shifts（NICS） calculations show positive NICS value in Ni4Ti2（D4h） and Ni4Ti4（D2h）,negative NICS value in [Ni4Ti2]2＋（D4h）,and more negative NICS value in [Ni4Ti2]2-（D4h）.Analysis results show that the formation of diamagnetic circulation involving s and d orbitals is the main cause for the relatively strong aromaticity of [Ni4Ti2]2＋（D4h） and [Ni4Ti2]2-（D4h）.In such case,Ti atoms provide dz2 and s orbitals,and Ni atoms form square with their dz2 and dx2-y2 orbitals,with which a spherical electron current forms in [Ni4Ti2]2＋（D4h） and [Ni4Ti2]2-（D4h） with conspicuous π electron current.