Vibronic Coupling in Vitamin B12: A Combined Spectroscopic and Computational Study.

ABSTRACT

Understanding the diverse reactivities of vitamin B12 and its derivatives, collectively called cobalamins, requires detailed knowledge of their geometric and electronic structures. Electronic absorption (Abs) and resonance Raman (rR) spectroscopies have proven invaluable in this area, particularly when used in concert with computational techniques such as density functional theory (DFT). There remain, however, lingering uncertainties in the computational description of electronic excited states of cobalamins, particularly surrounding the vibronic coupling that impacts the Abs bandshapes and gives rise to rR enhancement of vibrational modes. Past computational analyses of the vibrational spectra of cobalamins have either neglected rR enhancement or calculated rR enhancement for only a small number of modes. In the present study, we used the recently developed ORCA_ASA computational tool in conjunction with the popular B3LYP and BP86 functionals to predict Abs bandshapes and rR spectra for vitamin B12. The ORCA_ASA/B3LYP-computed Abs envelope in the visible spectral region and rR spectra of vitamin B12 agree remarkably well with our experimental data, while BP86 fails to reproduce both. This finding represents a significant advance in our understanding of how these two commonly used density functionals differently model the electronic properties of cobalamins. Guided by the computed frequencies for the Co-C stretching and Co-C-N bending modes, we identified, for the first time, isotope-sensitive features in our rR spectra of 12CNCbl and 13CNCbl that can be assigned to these modes. A normal coordinate analysis of the experimentally determined Co-C stretching and Co-C-N bending frequencies indicates that the Co-C force constant for vitamin B12 is 2.67 mdyn/Å, considerably larger than the Co-C force constants reported for alkylcobalamins.