Electronic and Structural Properties of Low-lying Excited States of Vitamin B12

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-CcN stretched coordinate. It was found that the repulsive (3)(sigma(Co-C) -> sigma*(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 pi pi*/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 a-bonds (SB) of N-Co-C unit have also noticeable contributions. Inclusion of solvent altered the character of one of the excitations from pi pi*/MLCT/SBLCT to pi pi*/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.