Combined Spectroscopic and Computational Analysis
of the Vibrational Properties of Vitamin B<sub>12</sub> in its Co<sup>3+</sup>, Co<sup>2+</sup>, and Co<sup>1+</sup> Oxidation States
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Abstract
While the geometric and electronic
structures of vitamin B<sub>12</sub> (cyanocobalamin, CNCbl) and its
reduced derivatives Co<sup>2+</sup>cobalamin (Co<sup>2+</sup>Cbl)
and Co<sup>1+</sup>cobalamin (Co<sup>1+</sup>Cbl<sup>–</sup>) are now reasonably well established, their vibrational properties,
in particular their resonance Raman (rR) spectra, have remained quite
poorly understood. The goal of this study was to establish definitive
assignments of the corrin-based vibrational modes that dominate the
rR spectra of vitamin B<sub>12</sub> in its Co<sup>3+</sup>, Co<sup>2+</sup>, and Co<sup>1+</sup> oxidation states. rR spectra were collected
for all three species with laser excitation in resonance with the
most intense corrin-based π → π* transitions. These
experimental data were used to validate the computed vibrational frequencies,
eigenvector compositions, and relative rR intensities of the normal
modes of interest as obtained by density functional theory (DFT) calculations.
Importantly, the computational methodology employed in this study
successfully reproduces the experimental observation that the frequencies
and rR excitation profiles of the corrin-based vibrational modes vary
significantly as a function of the cobalt oxidation state. Our DFT
results suggest that this variation reflects large differences in
the degree of mixing between the occupied Co 3d orbitals and empty
corrin π* orbitals in CNCbl, Co<sup>2+</sup>Cbl, and Co<sup>1+</sup>Cbl<sup>–</sup>. As a result, vibrations mainly involving
stretching of conjugated C–C and C–N bonds oriented
along one axis of the corrin ring may, in fact, couple to a perpendicularly
polarized electronic transition. This unusual coupling between electronic
transitions and vibrational motions of corrinoids greatly complicates
an assignment of the corrin-based normal modes of vibrations on the
basis of their rR excitation profiles