Assessing Electronically Excited States of Cobalamins …
243
designed under the premise that the B3LYP functional requires a uniform shift to
the red, while BP86 requires a uniform shift to the blue to simulate Abs spectra with
good alignment with experiment [85]. However, this method is problematic because
it operated under the assumption that the energy separation between Abs maximum
bands is correlated between theory and experiment and this is not the case. The technique suggested by Kozlowski and co-workers [2] resolves this issue and involves
two parameters, one for the low-energy excitations and another for the high-energy
excitations. The scaling procedure is marked by the equation:
E
i
scaled = ζ E
i
T D−DFT + E shift
(1)
where E
i
T D−DFT is the TD-DFT computed ith electronic excitation. ζ and E shift are
empirical parameters that will adjust the simulated spectrum. The values of ζ and
E shift are dependent upon the electronic transitions that are selected. The empirical
parameters are further denoted as:
ζ = (E
γ
exp − E
α
exp )/(E
γ
T D−DFT − E
α
T D−DFT )
(2)
and
E shift = (E
γ
exp + E
α
exp )/2 − ζ(E
γ
T D−DFT + E
α
T D−DFT )/2
( 3 )
This scaling procedure can be applied to other cobalamin systems. In fact, it was
applied in a TD-DFT analysis of AdoCbl which will be discussed in Sect. 9.4.
In addition to these Abs studies, MeCbl has been investigated using TAS. An
important conclusion to note is that the photolysis of MeCbl is wavelength dependent [82]. Upon excitation at 400 nm, a partitioning between prompt bond homolysis
and a metastable cob(II)alamin photoproduct is observed. With excitation at 520 nm,
no prompt bond homolysis is observed. The reason for the wavelength dependence
for the photolysis of MeCbl is still not well understood from a theoretical point of
view.
The electronically excited states of ethylcobalamin (EtCbl) were also investigated
in the comparison with MeCbl [51]. The EtCbl model was derived by replacing the
methyl group in MeCbl with ethyl. Overall, the simulated Abs spectrum for EtCbl and
MeCbl are quite similar (Fig. 12). The difference in these species is in the mechanism
of photolysis associated with TAS. For both EtCbl and MeCbl, after excitation to
the α/β band, internal conversion (IC) will result in the population to the S 1 state
where the photolytic fate is either photolysis or IC to the ground state. The barrier
for photolysis is lower for EtCbl, so photolysis is more prevalent. The reverse is true
for MeCbl where only partial photolysis is observed.
243
designed under the premise that the B3LYP functional requires a uniform shift to
the red, while BP86 requires a uniform shift to the blue to simulate Abs spectra with
good alignment with experiment [85]. However, this method is problematic because
it operated under the assumption that the energy separation between Abs maximum
bands is correlated between theory and experiment and this is not the case. The technique suggested by Kozlowski and co-workers [2] resolves this issue and involves
two parameters, one for the low-energy excitations and another for the high-energy
excitations. The scaling procedure is marked by the equation:
E
i
scaled = ζ E
i
T D−DFT + E shift
(1)
where E
i
T D−DFT is the TD-DFT computed ith electronic excitation. ζ and E shift are
empirical parameters that will adjust the simulated spectrum. The values of ζ and
E shift are dependent upon the electronic transitions that are selected. The empirical
parameters are further denoted as:
ζ = (E
γ
exp − E
α
exp )/(E
γ
T D−DFT − E
α
T D−DFT )
(2)
and
E shift = (E
γ
exp + E
α
exp )/2 − ζ(E
γ
T D−DFT + E
α
T D−DFT )/2
( 3 )
This scaling procedure can be applied to other cobalamin systems. In fact, it was
applied in a TD-DFT analysis of AdoCbl which will be discussed in Sect. 9.4.
In addition to these Abs studies, MeCbl has been investigated using TAS. An
important conclusion to note is that the photolysis of MeCbl is wavelength dependent [82]. Upon excitation at 400 nm, a partitioning between prompt bond homolysis
and a metastable cob(II)alamin photoproduct is observed. With excitation at 520 nm,
no prompt bond homolysis is observed. The reason for the wavelength dependence
for the photolysis of MeCbl is still not well understood from a theoretical point of
view.
The electronically excited states of ethylcobalamin (EtCbl) were also investigated
in the comparison with MeCbl [51]. The EtCbl model was derived by replacing the
methyl group in MeCbl with ethyl. Overall, the simulated Abs spectrum for EtCbl and
MeCbl are quite similar (Fig. 12). The difference in these species is in the mechanism
of photolysis associated with TAS. For both EtCbl and MeCbl, after excitation to
the α/β band, internal conversion (IC) will result in the population to the S 1 state
where the photolytic fate is either photolysis or IC to the ground state. The barrier
for photolysis is lower for EtCbl, so photolysis is more prevalent. The reverse is true
for MeCbl where only partial photolysis is observed.