238
M. J. Toda et al.
bands of the free base corrin are primarily the result of π →π
∗ transitions, but these
alone are not sufficient to describe the full spectrum.
9.2 Cyanocobalamin
CNCbl is by far the most well-studied cobalamin to date. It has been called the
paradigm system for the evaluation of electronically excited states of B 12 derivatives [92]. There is a wealth of both experimental and theoretical data for this compound since the elucidation of its crystal structure, but the electronically excited states
of CNCbl continue to be the topic of current research as advanced spectroscopic techniques such as X-ray absorption near-edge structure (XANES) [61] progress into the
field of B 12 chemistry. Several theoretical studies have sought to understand the
electronic Abs spectrum.
Recall from Sect. 3 that CNCbl exhibits a typical Abs spectrum. Brunold and
co-workers [85] suggested the following for the assignment of the major peaks for
CNCbl. The α/β region of typical type is the result of corrin-based HOMO→LUMO
transition that is polarized along the C5· · · C15 vector. The γ band can be ascribed to
a corrin-based π →π
∗ transition. A corrin-based π →π
∗ transition is responsible for
the most intense band in the D/E region, and this is polarized along the Co· · · C10
vector. There is a notable comparison to discuss. For the Abs spectra of CNCbl
and H 2 OCbl
+ , it is important to consider that CN is a much stronger σ -donor than
H 2 O. As a result, the Co 3d z 2 for CNCbl is energetically higher than H 2 OCbl
+ .
Additionally, there is an increased contribution of the unoccupied Co 3d z 2 to the
corrin-based HOMO in CNCbl than H 2 OCbl
+ . It would seem that the α/β and γ
bands in typical cob(III)alamin Abs spectra shift to lower energy with increasing
σ -donor strength of the upper axial ligand.
The electronically excited states of CNCbl were investigated using TD-DFT with
the B3LYP functional [1]. It was found that electronic excitations were systematically overestimated, but the application of a scaling procedure can bring better
agreement with experimental results. Two models were used in this study, denoted
CN-[Co
III -corrin]-CN
+ , with CN as the lower axial base and Im-[Co
III -corrin]-CN
+
with Im as the lower axial base (their simulated absorption spectra can be seen in
Fig. 11), analogous to dicyanocobinamide (DCC) and CNCbl, respectively. In these
models, all side chains and the nucleotide loop were replaced by hydrogens. In order
to provide good agreement with experiment, the transition energies for both models were systematically shifted to the red. It would seem that the need to apply a
scaling procedure is that the B3LYP functional insufficiently treats electron correlation and that gas-phase calculations were compared with solution data. In agreement
with semi-empirical methods [14, 15, 43, 63], the α band was assigned as mainly
HOMO→LUMO. It was observed that the α band shifts to the red when the lower
axial ligand is CN instead of Im. In order to describe the β band properly the Co d
orbitals must be included. For both models the β band was ascribed to the S 0 →S 6
M. J. Toda et al.
bands of the free base corrin are primarily the result of π →π
∗ transitions, but these
alone are not sufficient to describe the full spectrum.
9.2 Cyanocobalamin
CNCbl is by far the most well-studied cobalamin to date. It has been called the
paradigm system for the evaluation of electronically excited states of B 12 derivatives [92]. There is a wealth of both experimental and theoretical data for this compound since the elucidation of its crystal structure, but the electronically excited states
of CNCbl continue to be the topic of current research as advanced spectroscopic techniques such as X-ray absorption near-edge structure (XANES) [61] progress into the
field of B 12 chemistry. Several theoretical studies have sought to understand the
electronic Abs spectrum.
Recall from Sect. 3 that CNCbl exhibits a typical Abs spectrum. Brunold and
co-workers [85] suggested the following for the assignment of the major peaks for
CNCbl. The α/β region of typical type is the result of corrin-based HOMO→LUMO
transition that is polarized along the C5· · · C15 vector. The γ band can be ascribed to
a corrin-based π →π
∗ transition. A corrin-based π →π
∗ transition is responsible for
the most intense band in the D/E region, and this is polarized along the Co· · · C10
vector. There is a notable comparison to discuss. For the Abs spectra of CNCbl
and H 2 OCbl
+ , it is important to consider that CN is a much stronger σ -donor than
H 2 O. As a result, the Co 3d z 2 for CNCbl is energetically higher than H 2 OCbl
+ .
Additionally, there is an increased contribution of the unoccupied Co 3d z 2 to the
corrin-based HOMO in CNCbl than H 2 OCbl
+ . It would seem that the α/β and γ
bands in typical cob(III)alamin Abs spectra shift to lower energy with increasing
σ -donor strength of the upper axial ligand.
The electronically excited states of CNCbl were investigated using TD-DFT with
the B3LYP functional [1]. It was found that electronic excitations were systematically overestimated, but the application of a scaling procedure can bring better
agreement with experimental results. Two models were used in this study, denoted
CN-[Co
III -corrin]-CN
+ , with CN as the lower axial base and Im-[Co
III -corrin]-CN
+
with Im as the lower axial base (their simulated absorption spectra can be seen in
Fig. 11), analogous to dicyanocobinamide (DCC) and CNCbl, respectively. In these
models, all side chains and the nucleotide loop were replaced by hydrogens. In order
to provide good agreement with experiment, the transition energies for both models were systematically shifted to the red. It would seem that the need to apply a
scaling procedure is that the B3LYP functional insufficiently treats electron correlation and that gas-phase calculations were compared with solution data. In agreement
with semi-empirical methods [14, 15, 43, 63], the α band was assigned as mainly
HOMO→LUMO. It was observed that the α band shifts to the red when the lower
axial ligand is CN instead of Im. In order to describe the β band properly the Co d
orbitals must be included. For both models the β band was ascribed to the S 0 →S 6