对嘧啶的邻、间、对位双自由基体系的自旋耦合规律的研究

Study on the spin coupling rules of o,m, p-pyrimidine and its derivatives biradicals

采用量子化学abinitio法和密度泛函方法对不同取代位置的嘧啶自旋耦合规律进行研究 .两种方法比较 ,用UHF方法计算导致自旋污染严重 ,而用UB3LYP方法计算 ,自旋污染则减少了许多 .计算结果得到了双自由基之间磁性耦合的拓扑规则 :共轭体系中 ,两个自由基之间以偶数个碳 (或氮 )原子耦合 ,则有效交换积分Jij<0 ,体系具有低自旋基态 ,表现为反铁磁耦合 ;两个自由基之间以奇数个碳 (或氮 )原子耦合 ,则有效交换积分Jij>0 ,体系具有高自旋基态 ,表现为铁磁耦合 .自由基性质和铁磁耦合单元的不同位置对自旋耦合的影响较大 ,这些结论为有机磁性材料的分子设计与实验合成提供了理论依据 .

Spin coupling rules of ground state of o,m,p-pyrimidine and its different derivatives biradicals have been studied by using quantum chemical ab initio methods and DFT methods.Compared UHF with UB3LYP, the stronger spin contaminant are found within UHF calculations, the feeblish spin contaminant are found within UB3LYP calculations. It is proved that there are topological rules in magnetic coupling between radicals of plane conjugated systems: In conjugated system, the effective exchange integrals of radical sites through the even number of carbons with π-electron network are negative in sign,i.e., low spin state is stable ground state, while those through odd carbon atoms are positive in sign, i.e., high spin state is stable ground state. There are more obvious influences of properties of radical on the spin coupling, and stronger ferromagnetic coupling occurs between positive ion radicals, which provides theoretical guide for molecular design and experimental synthesis of organic magnetic materials.