Critical Role of the Correlation Functional in DFT Descriptions of an Agostic Niobium Complex.
Critical Role of the Correlation Functional in DFT Descriptions of an Agostic Niobium Complex.
复制标题
相关函数在不可知铌配合物的 DFT 描述中的关键作用。
DOI：
10.1021/ct700043w
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发表时间：
2007
影响因子：
5.5
通讯作者：
Pantazis DA
中科院分区：
文献类型：
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作者：
Pantazis DA
在TpMe_2NbCl（R ′ CCR ′ ′）（R）中的Agostic成键的研究中，我们采用了混合的QM/MM方法（B_3LYP：UFF），其中QM配分[Nb（Cl）（iPr）（HCCH）（NHCH_2）_3]~+]相当小，但优化的结构与实验符合得很好。在试图通过扩大QM区域的大小来改进该模型时，我们惊讶地发现，使用B3 LYP泛函对整个分子进行完全QM处理未能找到任何类型的agostic结构。密度泛函的系统评估表明，在这些系统中的B3 LYP的性能差是典型的所有DFT方法，不服从均匀电子气（UEG）的相关限制。相反，那些服从UEG极限的，当在QM水平处理完整的配体系统时，提供了对agostic结构的极好描述。我们最初的（B3 LYP：UFF）混合方法的明显良好性能可以追溯到误差的消除：B3 LYP泛函低估了竞争性Nb−Cl π键的agostic相互作用的内在强度，但这被QM区域中正电荷的存在所引入的agostic键的额外但非物理的静电分量所抵消。
In previous studies of the agostic bonding in TpMe2NbCl(R‘CCR‘ ‘)(R), we have made use of a hybrid QM/MM protocol (B3LYP:UFF) where the QM partition ([Nb(Cl)(iPr)(HCCH)(NHCH2)3]+) was rather small, but the optimized structures were nevertheless in apparently good agreement with experiment. In attempting to improve this model by expanding the size of the QM region, we were surprised to discover that a full QM treatment of the whole molecule using the B3LYP functional failed to locate an agostic structure of any kind. A systematic assessment of density functionals reveals that the poor performance of B3LYP in these systems is typical of all DFT methods that do not obey the uniform electron gas (UEG) correlation limit. Those that do obey the UEG limit, in contrast, provide an excellent description of the agostic structure when the complete ligand system is treated at the QM level. The apparently good performance of our original (B3LYP:UFF) hybrid method can be traced to a cancellation of errors: the B3LYP functional underestimates the intrinsic strength of the agostic interaction relative to competing Nb−Cl π bonding, but this is offset by an additional but unphysical electrostatic component to the agostic bond introduced by the presence of a positive charge in the QM region.