242
M. J. Toda et al.
excitations. Starting with MeCbl, the α/β band has been ascribed to two transitions
located at 520 and 496 nm based on BP86 calculations. The calculated transition
at 520 nm has mixed character (d→π
∗ and π →π
∗ ) and is the HOMO→LUMO
excitation. The calculated transition at 496 nm also has mixed character that includes
d orbitals. Based on results using the BP86 functional, the γ band for MeCbl arises
from three electronic transitions that have d/π →π
∗ /σ
∗ character. The α/β region
appears to arise from transitions that are predominately π →π
∗ when B3LYP is
employed. Using B3LYP, the γ band experimentally observed at 316 nm arises
from the transitions that were calculated at 293 and 279 nm with d→π
∗ and d→σ
∗
character. For MeCbi-H 2 O, which represents the base-off configuration that persists
in low pH environments, differences can also be noted in the calculated electronic
transitions with the two different functionals. Like the base-on case, transitions with
d character have very low oscillator strengths when B3LYP is employed. However,
B3LYP is reliable in reproducing the blue-shift of the lowest-intensity band that
is observed experimentally for MeCbi-H 2 O in relation to MeCbl. BP86 predicts
the transitions with d orbital character with high oscillator strengths for calculated
transitions at 457, 461, and 405 nm. The calculated transition at 315 nm corresponds
to the experimentally observed band at 315 nm and is of mixed character including
π /d→π
∗ /σ
∗ . For the experimental band at 264 nm, the BP86 results suggest this
arises from primarily σ /π →σ
∗ /π
∗ excitations as well as n→π
∗ which involves
corrin nitrogen lone pairs. Clearly, regardless of the model (base-on/base-off MeCbl),
B3LYP and BP86 provide unique insights into the nature of the electronic transitions
responsible for the Abs spectra.
There are some additional conclusions to note based on the comparison between
the TD-DFT/B3LYP and TD-DFT/BP86 results [2]. First, both functionals can reproduce Abs spectra in good accordance with experiment. B3LYP reproduced the intensity of the bands in a better fashion than BP86, although it suggested these transitions
were basically π →π
∗ . While BP86 predicts Co–C BDE much better than B3LYP,
it is less effective than B3LYP for describing the corrin-based electronically excited
states. These differences are related to the bonding descriptions provided by the two
functionals. BP86 describes the Co–C bond with proper energy for its covalent nature,
while the energy produced by B3LYP is far too low. All of this indicates that BP86
is the better functional choice for studies involving the cleavage of the Co–C bond.
Also, BP86 is better for describing MLCT electronic transitions that involve the Co
d orbitals while B3LYP does a reliable job for corrin π →π
∗ transitions. Taking all
of this into account, this study suggests that two different functionals are required
to provide a proper description of electronically excited states. Practically speaking,
determining a reliable functional for studies of complex systems like cobalamins
comes down to the question that is being asked. When photolytic cleavage of the
Co–C bond is the target, BP86 is certainly the more reliable choice. However, B3LYP
is a good option for reproducing peak intensities well.
Even with the use of the proper functional, it still may be necessary to apply a
scaling procedure to obtain agreement between experiment and theory. An important
implication that arose from TD-DFT analysis of MeCbl Abs spectra was the introduction of a dependable scaling procedure [2]. A previous scaling technique was
M. J. Toda et al.
excitations. Starting with MeCbl, the α/β band has been ascribed to two transitions
located at 520 and 496 nm based on BP86 calculations. The calculated transition
at 520 nm has mixed character (d→π
∗ and π →π
∗ ) and is the HOMO→LUMO
excitation. The calculated transition at 496 nm also has mixed character that includes
d orbitals. Based on results using the BP86 functional, the γ band for MeCbl arises
from three electronic transitions that have d/π →π
∗ /σ
∗ character. The α/β region
appears to arise from transitions that are predominately π →π
∗ when B3LYP is
employed. Using B3LYP, the γ band experimentally observed at 316 nm arises
from the transitions that were calculated at 293 and 279 nm with d→π
∗ and d→σ
∗
character. For MeCbi-H 2 O, which represents the base-off configuration that persists
in low pH environments, differences can also be noted in the calculated electronic
transitions with the two different functionals. Like the base-on case, transitions with
d character have very low oscillator strengths when B3LYP is employed. However,
B3LYP is reliable in reproducing the blue-shift of the lowest-intensity band that
is observed experimentally for MeCbi-H 2 O in relation to MeCbl. BP86 predicts
the transitions with d orbital character with high oscillator strengths for calculated
transitions at 457, 461, and 405 nm. The calculated transition at 315 nm corresponds
to the experimentally observed band at 315 nm and is of mixed character including
π /d→π
∗ /σ
∗ . For the experimental band at 264 nm, the BP86 results suggest this
arises from primarily σ /π →σ
∗ /π
∗ excitations as well as n→π
∗ which involves
corrin nitrogen lone pairs. Clearly, regardless of the model (base-on/base-off MeCbl),
B3LYP and BP86 provide unique insights into the nature of the electronic transitions
responsible for the Abs spectra.
There are some additional conclusions to note based on the comparison between
the TD-DFT/B3LYP and TD-DFT/BP86 results [2]. First, both functionals can reproduce Abs spectra in good accordance with experiment. B3LYP reproduced the intensity of the bands in a better fashion than BP86, although it suggested these transitions
were basically π →π
∗ . While BP86 predicts Co–C BDE much better than B3LYP,
it is less effective than B3LYP for describing the corrin-based electronically excited
states. These differences are related to the bonding descriptions provided by the two
functionals. BP86 describes the Co–C bond with proper energy for its covalent nature,
while the energy produced by B3LYP is far too low. All of this indicates that BP86
is the better functional choice for studies involving the cleavage of the Co–C bond.
Also, BP86 is better for describing MLCT electronic transitions that involve the Co
d orbitals while B3LYP does a reliable job for corrin π →π
∗ transitions. Taking all
of this into account, this study suggests that two different functionals are required
to provide a proper description of electronically excited states. Practically speaking,
determining a reliable functional for studies of complex systems like cobalamins
comes down to the question that is being asked. When photolytic cleavage of the
Co–C bond is the target, BP86 is certainly the more reliable choice. However, B3LYP
is a good option for reproducing peak intensities well.
Even with the use of the proper functional, it still may be necessary to apply a
scaling procedure to obtain agreement between experiment and theory. An important
implication that arose from TD-DFT analysis of MeCbl Abs spectra was the introduction of a dependable scaling procedure [2]. A previous scaling technique was