Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Katowice, Poland.
J Phys Chem B. 2011 Nov 17;115(45):13304-19. doi: 10.1021/jp200911y. Epub 2011 Oct 26.
Time-dependent density functional theory (TD-DFT) has been applied to explore electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). To explain why the Co-C bond in CNCbl does not undergo photodissociation under conditions of simple photon excitation, electronically excited states have been computed along the Co-C(CN) stretched coordinate. It was found that the repulsive (3)(σ(Co-C) → σ*(Co-C)) triplet state drops in energy as the Co-C(CN) bond lengthens, but it does not become dissociative. Low-lying excited states were also computed as function of two axial bond lengths. Two energy minima have been located on the S(1)/CNCbl, as well as T(1)/CNCbl, surfaces. The full geometry optimization was carried out for each minimum and electronic properties associated with each optimized structure were analyzed in details. One minimum was described as excitation having mixed ππ*/MLCT (metal-to-ligand charge transfer) character, while the second as ligand-to-metal charge transfer (LMCT) transition. Neither of them, however, can be viewed as pure MLCT or LMCT transitions since additional excitation to or from σ-bonds (SB) of N-Co-C unit have also noticeable contributions. Inclusion of solvent altered the character of one of the excitations from ππ*/MLCT/SBLCT to ππ*/LMCT/LSBCT-type, and therefore, both of them gained significant contribution from LMCT/LSBCT transition. Finally, the nature of S(1) electronic state has been comparatively analyzed in CNCbl and MeCbl cobalamins.
时间依赖密度泛函理论（TD-DFT）已被应用于探索维生素 B12（氰钴胺素或 CNCbl）的电子激发态。为了解释为什么在简单光子激发条件下，CNCbl 中的 Co-C 键不会发生光解，我们沿着 Co-C(CN)拉伸坐标计算了电子激发态。结果发现，排斥的（3）（σ（Co-C）→σ*（Co-C））三重态随着 Co-C(CN)键的延长而降低能量，但不会发生解离。还计算了与两个轴向键长相关的低能激发态。在 S(1)/CNCbl 和 T(1)/CNCbl 表面上已经定位了两个能量最小值。对每个最小值进行了完整的几何优化，并详细分析了与每个优化结构相关的电子性质。一个最小值被描述为具有混合ππ*/MLCT（金属到配体电荷转移）特征的激发，而另一个则为配体到金属电荷转移（LMCT）跃迁。然而，它们都不能被视为纯 MLCT 或 LMCT 跃迁，因为来自 N-Co-C 单元的σ-键（SB）的额外激发也有显著贡献。溶剂的包含改变了其中一个激发的性质，从ππ*/MLCT/SBLCT 到ππ*/LMCT/LSBCT 型，因此，它们都从 LMCT/LSBCT 跃迁获得了显著贡献。最后，比较分析了 CNCbl 和 MeCbl 钴胺素中 S(1)电子态的性质。
