扩展桥联双核铜(Ⅱ)体系的磁耦合机理

Magnetic Coupling Mechanism in Extendedly Bridged Cu(Ⅱ) Dimers

应用密度泛函理论,采用对称性破损方法研究了草酸根桥联及草酰胺桥联双核铜（Ⅱ）体系的磁耦合作用机理.结果表明:两磁中心的自旋布居大小相等,符号相反,二者之间为反铁磁耦合.而且,与磁中心相联的配体原子与磁中心具有相同符号的自旋布居,磁中心的自旋具有显著的离域效应.HOMO中磁中心未成对电子的占据轨道之间对称性和能级的匹配程度是影响磁耦合强弱的关键.当对称性匹配时,改变桥联基团占据轨道的能级,例如通过改变桥接原子的电负性,即可改变磁耦合作用的强度,电负性愈低,磁耦全作用愈强.当对称性不匹配时,体系中存在较弱的磁耦合.

The magnetic coupling mechanism of extendedly bridged dimers such as oxalato- and oxamido- bridged Cu(Ⅱ)dimers have been investigated by using the density functional theory and the broken symmetry approach. The spin densities on two Cu(Ⅱ) atoms have the same values with the opposite signs. The coupling between the two magnetic centers is of antiferromagnetic. HOMO with either a spin or β spin of these dimers is mainly composed of the singly occupied orbital of magnetic center in the local field and p-like orbitals of ligand. The degree matching with each other between the two constituents is the key factor for the coupling strength. When the symmetry and the energy levels of the two constituents match well, the coupling strength will be changed by changing the occupied orbital energy level of bridging group, for example, by varying the electronegativity of one or more atoms in the bridging ligand . The larger the electronegativity the weaker the coupling. Otherwise the smaller the electronegativity the stronger the coupling. When the symmetry does not match well the coupling is very weak.