240
M. J. Toda et al.
singlet and triplet excited states have been further investigated via TD-DFT [52, 53].
This investigation included thorough analysis of potential energy curves (PECs) that
were constructed as a function of axial bond lengths for S 0 , singlet, and triplet states as
well as, potential energy surfaces (PESs) that were constructed as a function of both
axial bond lengths for the S 0 , S 1 , and T 1 . There are some important conclusions to
highlight. The excited states computed along the Co–C bond length show a repulsive
triplet state with
3 (σ Co–C →σ
∗
Co–C ) character that does not become dissociative. Two
energy minima were located on the S 1 PES. The first has ππ
∗ /MLCT/σ -bond–
ligand CT (SBLCT) character and the second can be characterized as ligand-to-metal
CT/ligand-to-σ -bond CT (LMCT/LSBCT).
The photophysics of CNCbl, based on experiment and simulations, can be summarized as follows. The initial excitation of CNCbl leads to a population of higher
π →π
∗ states. From this Franck–Condon point, there is a decay to a π →d intermediate LMCT state that corresponds to only modest elongation of the axial bonds.
This short-lived intermediate further relaxes to a LMCT minimum characterized as
π →σ
∗ (d z 2 ). As the axial bond lengths continue to elongate, IC occurs as the S 0
crosses the S 1 . As a result of this crossing, photodissociation does not occur as axial
bonds elongate and thus CNCbl is designated as photostable. From an experimental
perspective, the decay to the S 0 is dependent on both temperature and solvent. Less
polar solvents increase the excited-state lifetimes.
Recently, a combined theoretical and experimental study involved the analysis
of CNCbl with XANES to further characterize the photochemistry of CNCbl [61].
Evidence of very good agreement between experiment and theory in terms of nature
of geometrical changes in electronically excited states was exhibited. This experiment
confirmed that the formation of a Franck–Condon excited state is the initial photoinduced event. There is an IC to the π
1 (σ
∗ d z 2 )
1 intermediate. This intermediate is
marked by elongated axial bonds. The major conclusion from this work is that, upon
photoexcitation of CNCbl, the largest structural changes that occur are related to
axial bond lengths. As a result, PESs constructed as a function of axial bond lengths
are an important tool to use in order to understand the mechanisms associated with
photodissociation of other cobalamins.
9.3 Methylcobalamin
Like CNCbl, the electronically excited states of MeCbl have been thoroughly investigated using both experimental and computational methods. MeCbl exhibits a unique
Abs spectrum. Brunold and co-workers [85] proposed that two factors are primarily responsible for the features in unique spectra, (1) a net increase in the number
of donor MOs available for electronic transitions and (2) extensive mixing of the
Co 3d and corrin π orbitals. The α/β band in MeCbl and AdoCbl Abs spectra is
a HOMO→LUMO transition polarized along the C5· · · C15 vector. Specifically,
for MeCbl, the corrin-based HOMO has significant σ -bonding character. The more
intense β band, in comparison to the typical spectra, is the result of excited-state dis-
M. J. Toda et al.
singlet and triplet excited states have been further investigated via TD-DFT [52, 53].
This investigation included thorough analysis of potential energy curves (PECs) that
were constructed as a function of axial bond lengths for S 0 , singlet, and triplet states as
well as, potential energy surfaces (PESs) that were constructed as a function of both
axial bond lengths for the S 0 , S 1 , and T 1 . There are some important conclusions to
highlight. The excited states computed along the Co–C bond length show a repulsive
triplet state with
3 (σ Co–C →σ
∗
Co–C ) character that does not become dissociative. Two
energy minima were located on the S 1 PES. The first has ππ
∗ /MLCT/σ -bond–
ligand CT (SBLCT) character and the second can be characterized as ligand-to-metal
CT/ligand-to-σ -bond CT (LMCT/LSBCT).
The photophysics of CNCbl, based on experiment and simulations, can be summarized as follows. The initial excitation of CNCbl leads to a population of higher
π →π
∗ states. From this Franck–Condon point, there is a decay to a π →d intermediate LMCT state that corresponds to only modest elongation of the axial bonds.
This short-lived intermediate further relaxes to a LMCT minimum characterized as
π →σ
∗ (d z 2 ). As the axial bond lengths continue to elongate, IC occurs as the S 0
crosses the S 1 . As a result of this crossing, photodissociation does not occur as axial
bonds elongate and thus CNCbl is designated as photostable. From an experimental
perspective, the decay to the S 0 is dependent on both temperature and solvent. Less
polar solvents increase the excited-state lifetimes.
Recently, a combined theoretical and experimental study involved the analysis
of CNCbl with XANES to further characterize the photochemistry of CNCbl [61].
Evidence of very good agreement between experiment and theory in terms of nature
of geometrical changes in electronically excited states was exhibited. This experiment
confirmed that the formation of a Franck–Condon excited state is the initial photoinduced event. There is an IC to the π
1 (σ
∗ d z 2 )
1 intermediate. This intermediate is
marked by elongated axial bonds. The major conclusion from this work is that, upon
photoexcitation of CNCbl, the largest structural changes that occur are related to
axial bond lengths. As a result, PESs constructed as a function of axial bond lengths
are an important tool to use in order to understand the mechanisms associated with
photodissociation of other cobalamins.
9.3 Methylcobalamin
Like CNCbl, the electronically excited states of MeCbl have been thoroughly investigated using both experimental and computational methods. MeCbl exhibits a unique
Abs spectrum. Brunold and co-workers [85] proposed that two factors are primarily responsible for the features in unique spectra, (1) a net increase in the number
of donor MOs available for electronic transitions and (2) extensive mixing of the
Co 3d and corrin π orbitals. The α/β band in MeCbl and AdoCbl Abs spectra is
a HOMO→LUMO transition polarized along the C5· · · C15 vector. Specifically,
for MeCbl, the corrin-based HOMO has significant σ -bonding character. The more
intense β band, in comparison to the typical spectra, is the result of excited-state dis-