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

Understanding the diverse reactivitiesof vitamin B-12 and its derivatives, collectively calledcobalamins, requires detailedknowledge of their geometric and electronic structures. Electronicabsorption (Abs) and resonance Raman (rR) spectroscopies have proveninvaluable in this area, particularly when used in concert with computationaltechniques such as density functional theory (DFT). There remain,however, lingering uncertainties in the computational descriptionof electronic excited states of cobalamins, particularly surroundingthe vibronic coupling that impacts the Abs bandshapes and gives riseto rR enhancement of vibrational modes. Past computational analysesof the vibrational spectra of cobalamins have either neglected rRenhancement or calculated rR enhancement for only a small number ofmodes. In the present study, we used the recently developed ORCA_ASAcomputational tool in conjunction with the popular B3LYP and BP86functionals to predict Abs bandshapes and rR spectra for vitamin B-12. The ORCA_ASA/B3LYP-computed Abs envelope in the visiblespectral region and rR spectra of vitamin B-12 agree remarkablywell with our experimental data, while BP86 fails to reproduce both.This finding represents a significant advance in our understandingof how these two commonly used density functionals differently modelthe electronic properties of cobalamins. Guided by the computed frequenciesfor the Co-C stretching and Co-C-N bending modes,we identified, for the first time, isotope-sensitive features in ourrR spectra of (12)CNCbl and (13)CNCbl that can beassigned to these modes. A normal coordinate analysis of the experimentallydetermined Co-C stretching and Co-C-N bendingfrequencies indicates that the Co-C force constant for vitaminB(12) is 2.67 mdyn/& ANGS;, considerably larger than the Co-Cforce constants reported for alkylcobalamins. Resonance Raman (rR) spectroscopy wasused in combinationwith electronic absorption spectroscopy and density functional theorycalculations to probe vibronic coupling in vitamin B-12.The computed absorption envelope in the visible spectral region andrR spectra of vitamin B-12 agree remarkably well with theexperimental data, which enabled the computationally guided identificationof the elusive cobalt-carbon stretching mode in our rR spectra.

Automated summary

Understanding the reactivities of cobalamins requires knowledge of their geometric and electronic structures. In this study, the ORCA_ASA computational tool was used with B3LYP and BP86 functionals to predict Abs bandshapes and rR spectra for vitamin B-12. The ORCA_ASA/B3LYP-computed Abs envelope and rR spectra were in good agreement with experimental data, while BP86 failed to reproduce them. This finding is important for understanding the different modeling of electronic properties by these density functionals.