ja970574c_si_001.pdf (747.34 kB)
Density Functional Study on the Electronic Structures of Model Peroxidase Compounds I and II
journal contribution
posted on 1997-11-26, 00:00 authored by Hiroshi Kuramochi, Louis Noodleman, David A. CaseThe electronic structures of
[Fe(Por)(Im)O]1+ and
[Fe(Por)(Im)O] (model compounds I and
II, respectively)
have been studied on the basis of density functional theory or DFT (Por
= porphine, Im = imidazole). The a2u
π-cation radical state (4A2u) was determined
to be the ground state of compound I with total spin equal
to 3/2, while
the a1u π-cation state (4A1u) was
found to be 0.15 eV higher in energy than the
4A2u state. Since, in both states,
the
spins were localized to the porphyrin ring (S =
1/2) and the Fe−O center (S =
1), the magnetic coupling interaction
between the two spin sites was examined by using a broken symmetry
method. The calculated J value revealed
very weak magnetic coupling for the A2u state, which
corresponded to the experimental data. The calculated J
value
revealed strong antiferromagnetic coupling for the A1u
state. The calculated Mössbauer spectrum
parameters
(quadrupole splitting and asymmetry) were similar for both the
A1u and A2u states, and both agreed well
with
experimental values. On the other hand, the calculated hyperfine
coupling constants for the nitrogen and the proton
of the porphyrin ring were different in the two states. Although
the experimental coupling constant values of the
pyrrole nitrogen atoms were intermediate between the calculated values
for the A2u and A1u states, the
experimental
values for the meso protons were closer to the values calculated for
the A2u state. These results suggest that
the
electronic structure of compound I is closer to the
A2u state than to the A1u state. However,
these results also
suggest that there is the possibility that the electronic structure of
compound I is an admixture of the A1u state
and
the A2u state. The electronic structure of compound
II was calculated and compared with the electronic structure
of
compound I. The energetics of the redox reaction
between the two compounds is discussed.