246
M. J. Toda et al.
D/E band is π /d→π
∗ type. The broad γ band clearly results from multiple transitions,
and those with significant oscillator strengths are characterized as mostly π /d→π
∗ ,
π /d→d and π /d→σ
∗ . The δ band arises from π →π
∗ transitions. For BP86, the α/β
region arises from d/π →π
∗ and d→π
∗ transitions. The D/E band is a mixture of
several transitions including π /d→d, π /d→d/π
∗ , and π /d→σ
∗ type. The γ band
results from several transition types and the ones with the most significant oscillator
strengths of the simulated spectrum being π /d→σ
∗ and d→π
∗ /d at 385 nm, π /d→π
∗
at 339 and 327 nm, σ →π
∗ at 310 nm, and finally d→π
∗ at 307 nm. The δ band
arises from transitions that are σ →π
∗ /d character.
The electronic spectrum for a model of the base-off form of AdoCbl (AdoCbiH 2 O) was also simulated in this study (Fig. 13) [3]. This model denoted H 2 O-[Co
III -
corrin]-Ado
+ is representative of the low pH situation where the lower axial DBI
group is replaced by water. The α/β region is shifted to higher energies in the AdoCbiH 2 O spectrum. B3LYP properly predicted this shifting of bands to shorter wavelengths in accordance with experiment. The B3LYP assignments for AdoCbi-H 2 O
are as follows. The α/β band arises from three transitions that are primarily π /d→π
∗ .
The D/E band results from two d/π →π
∗ -type transitions. As in the AdoCbl base-on
case, the γ region is the result of several transitions which are mainly π /d→π
∗ ,
but contributions from π /d→σ
∗ and σ →π
∗ persist as well. The δ band arises from
several transitions of varying oscillator strengths. Those with the largest oscillator
strengths are π →π
∗ transitions of the adenosine moiety and of π /d→π
∗ /d character.
The electronic Abs spectrum was also computed with BP86; the following details are
of note. As was the case for the Im-[Co
III -corrin]-Ado
+ model, the first transition is
LRCT when employing BP86. Again there are some differences in the assignments
of key regions when comparing the assignments of the key features of AdoCbi-H 2 O
simulated spectra in terms of B3LYP and BP86 methods. The α/β region arises from
d/π →π
∗ and π /d→π
∗ -type transitions. The D/E band is a mixture of two transitions that can be characterized as d/π →π
∗ and d→σ
∗ . These transitions are more
complex than at first glance. The D/E transitions involve excitation from free electron pairs from the adenine group to the σ
∗ (Co-Ado) orbital or the π
∗ orbital of the
corrin. The γ band results from several transition types and the ones with the most
significant oscillator strengths being π /d→π
∗ and π /d→σ
∗ character. There are
also contributions from Ado(n)→σ
∗ and Ado(n)→d excitations. The δ band arises
from transitions that are σ →π
∗ /d character. The δ part arises from transitions of
several types including σ →σ
∗ , σ →π
∗ , d/π →Ado(π
∗ ), π →π
∗ of the adenosine,
Ado(n)→π
∗ , and finally σ →π
∗ types. Clearly, the interpretation of Abs spectrum
of AdoCbl and AdoCbi-H 2 O is quite involved and requires a thorough analysis of
the electronic structure and relevant molecular orbitals.
A major conclusion of this work [3] is that B3LYP and BP86 can be used in
a complimentary manner to provide a more complete picture of the electronically
excited states. B3LYP is reliable in simulating the spectral pattern of the experimental
Abs spectra for AdoCbl even though it performs poorly in replicating accurate axial
bond lengths. Because the two functionals indicate a different orbital ordering and
HOMO/LUMO character, the lowest energy transitions are predicted to be of different
types. For B3LYP, the lowest energy transitions are of π /d→π
∗ , π /d→σ
∗ , and
M. J. Toda et al.
D/E band is π /d→π
∗ type. The broad γ band clearly results from multiple transitions,
and those with significant oscillator strengths are characterized as mostly π /d→π
∗ ,
π /d→d and π /d→σ
∗ . The δ band arises from π →π
∗ transitions. For BP86, the α/β
region arises from d/π →π
∗ and d→π
∗ transitions. The D/E band is a mixture of
several transitions including π /d→d, π /d→d/π
∗ , and π /d→σ
∗ type. The γ band
results from several transition types and the ones with the most significant oscillator
strengths of the simulated spectrum being π /d→σ
∗ and d→π
∗ /d at 385 nm, π /d→π
∗
at 339 and 327 nm, σ →π
∗ at 310 nm, and finally d→π
∗ at 307 nm. The δ band
arises from transitions that are σ →π
∗ /d character.
The electronic spectrum for a model of the base-off form of AdoCbl (AdoCbiH 2 O) was also simulated in this study (Fig. 13) [3]. This model denoted H 2 O-[Co
III -
corrin]-Ado
+ is representative of the low pH situation where the lower axial DBI
group is replaced by water. The α/β region is shifted to higher energies in the AdoCbiH 2 O spectrum. B3LYP properly predicted this shifting of bands to shorter wavelengths in accordance with experiment. The B3LYP assignments for AdoCbi-H 2 O
are as follows. The α/β band arises from three transitions that are primarily π /d→π
∗ .
The D/E band results from two d/π →π
∗ -type transitions. As in the AdoCbl base-on
case, the γ region is the result of several transitions which are mainly π /d→π
∗ ,
but contributions from π /d→σ
∗ and σ →π
∗ persist as well. The δ band arises from
several transitions of varying oscillator strengths. Those with the largest oscillator
strengths are π →π
∗ transitions of the adenosine moiety and of π /d→π
∗ /d character.
The electronic Abs spectrum was also computed with BP86; the following details are
of note. As was the case for the Im-[Co
III -corrin]-Ado
+ model, the first transition is
LRCT when employing BP86. Again there are some differences in the assignments
of key regions when comparing the assignments of the key features of AdoCbi-H 2 O
simulated spectra in terms of B3LYP and BP86 methods. The α/β region arises from
d/π →π
∗ and π /d→π
∗ -type transitions. The D/E band is a mixture of two transitions that can be characterized as d/π →π
∗ and d→σ
∗ . These transitions are more
complex than at first glance. The D/E transitions involve excitation from free electron pairs from the adenine group to the σ
∗ (Co-Ado) orbital or the π
∗ orbital of the
corrin. The γ band results from several transition types and the ones with the most
significant oscillator strengths being π /d→π
∗ and π /d→σ
∗ character. There are
also contributions from Ado(n)→σ
∗ and Ado(n)→d excitations. The δ band arises
from transitions that are σ →π
∗ /d character. The δ part arises from transitions of
several types including σ →σ
∗ , σ →π
∗ , d/π →Ado(π
∗ ), π →π
∗ of the adenosine,
Ado(n)→π
∗ , and finally σ →π
∗ types. Clearly, the interpretation of Abs spectrum
of AdoCbl and AdoCbi-H 2 O is quite involved and requires a thorough analysis of
the electronic structure and relevant molecular orbitals.
A major conclusion of this work [3] is that B3LYP and BP86 can be used in
a complimentary manner to provide a more complete picture of the electronically
excited states. B3LYP is reliable in simulating the spectral pattern of the experimental
Abs spectra for AdoCbl even though it performs poorly in replicating accurate axial
bond lengths. Because the two functionals indicate a different orbital ordering and
HOMO/LUMO character, the lowest energy transitions are predicted to be of different
types. For B3LYP, the lowest energy transitions are of π /d→π
∗ , π /d→σ
∗ , and