构象对同位乙烯双自由基体系基态自旋多重度及其稳定性影响的理论研究

用DFT,CASSCF和QCISD(T)方法6-31G^*基组计算了构象对同位二取代乙烯双自由基体系基态自旋多重度及其稳定性的影响.结果表明,用DFT或CASSCF方法计算的单、三重态的能量差随自由基与乙烯间的二面角增加成不规则变化;用QCISD(T)方法计算的单、三重态的能量差随二面角的增加而逐渐降低,并呈规律性变化,说明QCISD(T)方法用于计算分子的磁性是可信的.对于同位二取代乙烯双自由基体系,无论双自由基旋转,还是单自由基旋转,高自旋基态稳定性随自由基与乙烯间二面角的增加而降低,只是降低的幅度不同,当二面角接近90°时,同位乙烯由具有平面或近似平面构象时强的铁磁耦合单元变成接近垂直平面构象时弱的反铁磁耦合单元或弱的铁磁耦合单元.

Theoretical Studies on Effect of Conformation on the Spin Multiplicity Ground State in Parity Ethylene Type Biradical System and Stability

The effect of conformation on the spin multiplicity of ground state and it′s stability were investigated for parity ethylene type biradical system by means of comparative study with DFT, CASSCF and QCISD(T) methods. It is found that based on the continuous twisting of radical centers out of conjugation with the ethylene, both the absolute values and the tendency for the stability of triplet ground state (ΔES-T) of the investigated system change dramatically with variation the calculation methods. DFT and CASSCF methods cannot give a smooth change for ΔES-T, when the radical centers in the ethylene are twisted. However, the ΔES-T result obtained by QCISD(T) method offers a smooth change in the continuous twisting of radical centers. It is suggested that QCISD(T) method is reliable for dealing with the conformational problem of high-spin molecules. Furthermore, the ΔES-T values for ethylene type biradicals with one and two twisted radical centers were explored, respectively. When two or one radical centers in the ethylene are twisted sufficiently out of conjugation, and dihedral angles are close to 90°, ethylene turns into a weak antiferromagnet and weak ferromagnetic coupling units, respectively. Though the ethylene is a robust ferromagnetic coupling unit under planar conformation.