University of New Brunswick, Department of Chemistry and Centre for Laser, Atomic, and Molecular Sciences, Fredericton, New Brunswick, Canada E3B 6E2.
J Phys Chem A. 2010 Feb 4;114(4):2010-21. doi: 10.1021/jp910643e.
The singlet-triplet energy differences (DeltaE(ST)) and the Heisenberg-Dirac-van-Vleck exchange parameter (J) of 6,6'-dioxo-3,3'-biverdazyl (BVD) have been studied by hybrid density functional (HDF), broken symmetry (BS), and spectroscopy oriented configuration interaction (SORCI) methods. Energy surface scans as a function of the dihedral angle between the two verdazyl rings (phi(N2C3C3'N2')) have been performed. The BS computations predict an antiferromagnetic ground state. However, the diradical index (R(BS)) ranges from 97.5 to 99.9%, implying that the interactions between the two unpaired electrons are very weak. To calculate J and DeltaE(ST), the multireference character introduced by these weak spin-spin interactions must be taken into account. Consequently, multireference difference dedicated configuration interaction (MRDDCI) methods, as implemented in the SORCI procedure, are used. The in-plane pi (IPpi), out-of-plane pi (OPpi), and sigma configurations are included in the CI expansions in a balanced fashion. The OPpi-OPpi and OPpi-IPpi overlaps are the predominant factors that influence the J and DeltaE(ST) as a function of phi(N2C3C3'N2') and cause them to peak around 40 and 140 degrees. In these regions, the antiferromagnetic interactions are minimal, and the MRDDCI methods predict a triplet ground state. At phi(N2C3C3'N2') = 0, DeltaE(ST)[MRDDCI3(14,12)] is in excellent agreement with that of 1,1',5,5'-tetramethyl-6,6'-dioxo-3,3'-biverdazyl determined experimentally from electron paramagnetic resonance (EPR) spectroscopy and differs only by 2.3%. Furthermore, DeltaE(ST)[MRDDCI3(14,12)] is consistently smaller than J(Y) as the verdazyl rings rotate with respect to each other. This corroborates the theory that the HDF-BS technique increases the singlet-triplet energy gap and favors the singlet state. Because the SORCI method is specifically designed for large molecules, the present very good results open the way for the computation of the magnetic properties of larger molecules by the SORCI method. To the best of our knowledge, this is the first time that DeltaE(ST) has been computed by the MRDDCI3 method by utilizing such a large CI reference space for a molecule of this size.
通过混合密度泛函（HDF）、破缺对称性（BS）和谱导向组态相互作用（SORCI）方法研究了 6,6'-二氧代-3,3'-联二唑（BVD）的单三重态能量差（DeltaE(ST)）和海森堡-狄拉克-范弗列克交换参数（J）。作为两个 verdazyl 环（phi（N2C3C3'N2'））之间二面角的函数进行了能量面扫描。BS 计算预测了反铁磁基态。然而，自由基指数（R（BS））范围在 97.5%到 99.9%之间，这意味着两个未配对电子之间的相互作用非常弱。为了计算 J 和 DeltaE(ST)，必须考虑这些弱自旋-自旋相互作用引入的多参考特征。因此，使用了 SORCI 程序中实现的多参考差分专用组态相互作用（MRDDCI）方法。在 CI 扩展中以平衡的方式包含面内 pi（IPpi）、面外 pi（OPpi）和 sigma 构型。OPpi-OPpi 和 OPpi-IPpi 重叠是影响 phi（N2C3C3'N2'）和 phi（N2C3C3'N2'）函数的 J 和 DeltaE(ST)的主要因素，导致它们在 40 度和 140 度左右达到峰值。在这些区域，反铁磁相互作用最小，MRDDCI 方法预测三重态基态。在 phi（N2C3C3'N2'）= 0 时，DeltaE(ST)[MRDDCI3(14,12)]与从电子顺磁共振（EPR）光谱实验确定的 1,1',5,5'-四甲基-6,6'-二氧代-3,3'-联二唑的实验值非常吻合，仅相差 2.3%。此外，DeltaE(ST)[MRDDCI3(14,12)]随着 verdazyl 环相对于彼此旋转始终小于 J(Y)。这证实了 HDF-BS 技术增加单重态-三重态能量间隙并有利于单重态的理论。由于 SORCI 方法专门针对大分子设计，因此目前非常好的结果为通过 SORCI 方法计算更大分子的磁性特性开辟了道路。据我们所知，这是第一次使用这种大小的分子如此大的 CI 参考空间通过 MRDDCI3 方法计算 DeltaE(ST)。
