Spataru, T.A. Fernandez, F. 2016-06-22 Unfortunately, there are still signifi cant disagreements between experimental and theoretical data of rate constants, energy barriers for Co-C bond cleavage process and coordination numbers of vitamin B12 coenzyme species in spite of the remarkable efforts done by research community. Therefore, no grounded mechanisms for Co-C vitamin B12 coenzyme bond breaking process and subsequent reactions have been found up to now. The infl uence of the mixing orbitals e.g. Pseudo-Jahn-Teller and similar effects on the reactions paths of bond-cleavage mechanisms of vitamin B12 co-factors must be taken into account by utilizing multi-reference methods, in particular multiconfi gurational self-consistent fi eld (MCSCF) method. Then, the change in total energy along the normal coordinate Q for the stretching mode including Co-C and Co-N bonds in vitamin B12 cofactors is expected due to a “vibronic” coupling term, which couples an excited state and ground state by a second order derivative potential-energy operator. The strong state mixing effect is expected to lead to low energy barriers and to Co-C and Co-N axial bond cleavage events in agreement with experimental data. Afterward, the updated mechanisms of vitamin B12 bio-processes can be determined. 10.19261/cjm.2016.11(1).01 Chemistry Journal of Moldova 11 (1) 10-20 vitamin B12 Mechanism bio-catalysis Pseudo-Jahn-Teller effect DFT MCSCF The nature of the Co-C bond cleavage processes in methylcob(II)alamin and adenosylcob(III)alamin info:eu-repo/semantics/article