Computer simulation of low symmetry E.S.R. spectra due to vitamin B12rand model systems

Computer simulated spectra are calculated and compared with experimental E.S.R. spectra for vitamin B12r (cob(II)alamin) in the enzyme ribonucleotide reductase. The conclusions reached are that the cobalt(II) ion in the corrinoid is in a site of no higher than C 2 point group symmetry; that the cobalt g and hyperfine tensors are non-coincident, sharing the z-axis, but with their axes in the xy plane rotated 50° with respect to one another. The unpaired electron is still considered to be in an essentially d z 2 orbital, though a certain amount of mixing of states is thought to occur, accounting for the non-coincidence of the tensors. As an aid to our analysis, we present results for two model systems, firstly pentacyanocobaltate(II) ion, where close agreement with the work of Tsay, Gray and Danon is found, and secondly pentacyanocobaltate(II) ion with pyridine in the sixth coordination position.