Photolysis of Methylcobalamin:  Identification of the Relevant Excited States Involved in Co−C Bond Scission

The relevant excited states involved in the photolysis of methylcobalamin (MeCbl) have been examined by means of time-dependent density functional theory (TD-DFT). The low-lying singlet and triplet excited states have been calculated along the Co−C bond at the TD-DFT/BP86/6-31g(d) level of theory in order to investigate the dissociation process of MeCbl. These calculations have shown that the photodissociation is mediated by the repulsive ³(σ_Co_-_C → σ*_Co_-_C) triplet state. The key metastable photoproduct involved in Co−C bond photolysis was identified as an S₁ state having predominantly d_Co → π*_corrin metal−ligand charge transfer (MLCT) character.