Assessing Electronically Excited States of Cobalamins …
241
tortion that lengthens the Co–C bond. The second intense feature in the α/β region of
MeCbl arises from a corrin-based π →π
∗ transition and a Co 3d xz → corrin π
∗ transition. The γ region in unique Abs spectra results from significant mixing between
occupied Co 3d and corrin π orbitals. As a result of this mixing, there is distribution
of the relatively intense transitions across a larger range of the UV region and a single
intense γ band is not observed. To summarize, there is a high degree of σ -donation
from the alkyl ligands (Me or Ado, for example) to the Co and this destabilizes the
Co 3d orbitals. The contribution of the Co 3d z 2 orbital to the HOMO increases with
increasing σ -donor strength of the upper axial ligand [85].
Later, another TD-DFT analysis shed further light on the assignment of the bands
in MeCbl Abs, CD, and MCD spectra [84]. The BP86 functional was used in this
study, and it is now well established that for cobalamins this functional provides
much better agreement with experiment than others such as hybrids [38, 45]. For
MeCbl, the α band is less intense than the β band. Like in CNCbl, the α/β band
of MeCbl does not appear to be based on a vibrational progression. Rather, BP86
calculations indicate that the α/β band arises from several electronic transitions with
the first two characterized as π /d→π
∗ and d/π →π
∗ for the S 1 and S 2 , respectively.
This is consistent with results from TAS that indicate that the S 1 state is MLCT [82].
Based on the BP86 simulated Abs spectrum, it would also seem that the S 2 state
gives rise to the α band. The β band assignment can be determined upon inspection
of both Abs and CD spectra. The S 3 electronic excitation appears to be responsible
for the β band, and the transition has been characterized as π /d→π
∗ . The γ band
in MeCbl, which is much broader than in the typical Abs spectra, was ascribed to
a manifold of transitions in the BP86 simulated Abs spectrum where the occupied
molecular orbitals are a mixture of d and π character. This is in stark contrast to
Brunold’s work [85] where this region was described as solely π →π
∗ transitions.
Returning to the nature of the lowest energy part of the spectrum of MeCbl, the α/β
band, there have been two divergent interpretations for the assignment of this region.
Historically, this region was considered to be dominated by π →π
∗ excitations [24,
68]. Another more recent study indicated that there was a second electronic transition
involved [85]. TAS measurements indicate that these low-energy excitations are
MLCT. Consequently, a study based on several theoretical methods was conducted
to resolve this discrepancy [39]. The technical details were described in Sect. 8. The
major conclusion from this work was that, a pure GGA like BP86, a meta-GGA,
or a LC-BLYP functional should be employed in order to properly describe the S 1
state in MeCbl as having MLCT character. Hybrid functionals interpret the S 1 state as
having π →π
∗ character, which is now widely understood as a misinterpretation. The
reasons for these varying interpretations between BP86 and B3LYP are mainly due
to the differences associated with how these functionals describe bonding. B3LYP
tends to describe corrin π →π
∗ excitations well, while BP86 describes d/π →π
∗ /d
excitations (MLCT) properly.
In addition to work related to the S 1 state and its comparison with TAS data, [39]
the Abs spectrum for MeCbl as well as its base-off derivative (MeCbi-H 2 O) was
simulated [2]. Two functionals, BP86 and B3LYP, were used in this study, and as has
been seen in other cases, these functionals offer different insights for the electronic
241
tortion that lengthens the Co–C bond. The second intense feature in the α/β region of
MeCbl arises from a corrin-based π →π
∗ transition and a Co 3d xz → corrin π
∗ transition. The γ region in unique Abs spectra results from significant mixing between
occupied Co 3d and corrin π orbitals. As a result of this mixing, there is distribution
of the relatively intense transitions across a larger range of the UV region and a single
intense γ band is not observed. To summarize, there is a high degree of σ -donation
from the alkyl ligands (Me or Ado, for example) to the Co and this destabilizes the
Co 3d orbitals. The contribution of the Co 3d z 2 orbital to the HOMO increases with
increasing σ -donor strength of the upper axial ligand [85].
Later, another TD-DFT analysis shed further light on the assignment of the bands
in MeCbl Abs, CD, and MCD spectra [84]. The BP86 functional was used in this
study, and it is now well established that for cobalamins this functional provides
much better agreement with experiment than others such as hybrids [38, 45]. For
MeCbl, the α band is less intense than the β band. Like in CNCbl, the α/β band
of MeCbl does not appear to be based on a vibrational progression. Rather, BP86
calculations indicate that the α/β band arises from several electronic transitions with
the first two characterized as π /d→π
∗ and d/π →π
∗ for the S 1 and S 2 , respectively.
This is consistent with results from TAS that indicate that the S 1 state is MLCT [82].
Based on the BP86 simulated Abs spectrum, it would also seem that the S 2 state
gives rise to the α band. The β band assignment can be determined upon inspection
of both Abs and CD spectra. The S 3 electronic excitation appears to be responsible
for the β band, and the transition has been characterized as π /d→π
∗ . The γ band
in MeCbl, which is much broader than in the typical Abs spectra, was ascribed to
a manifold of transitions in the BP86 simulated Abs spectrum where the occupied
molecular orbitals are a mixture of d and π character. This is in stark contrast to
Brunold’s work [85] where this region was described as solely π →π
∗ transitions.
Returning to the nature of the lowest energy part of the spectrum of MeCbl, the α/β
band, there have been two divergent interpretations for the assignment of this region.
Historically, this region was considered to be dominated by π →π
∗ excitations [24,
68]. Another more recent study indicated that there was a second electronic transition
involved [85]. TAS measurements indicate that these low-energy excitations are
MLCT. Consequently, a study based on several theoretical methods was conducted
to resolve this discrepancy [39]. The technical details were described in Sect. 8. The
major conclusion from this work was that, a pure GGA like BP86, a meta-GGA,
or a LC-BLYP functional should be employed in order to properly describe the S 1
state in MeCbl as having MLCT character. Hybrid functionals interpret the S 1 state as
having π →π
∗ character, which is now widely understood as a misinterpretation. The
reasons for these varying interpretations between BP86 and B3LYP are mainly due
to the differences associated with how these functionals describe bonding. B3LYP
tends to describe corrin π →π
∗ excitations well, while BP86 describes d/π →π
∗ /d
excitations (MLCT) properly.
In addition to work related to the S 1 state and its comparison with TAS data, [39]
the Abs spectrum for MeCbl as well as its base-off derivative (MeCbi-H 2 O) was
simulated [2]. Two functionals, BP86 and B3LYP, were used in this study, and as has
been seen in other cases, these functionals offer different insights for the electronic