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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. February 2005, Volume 4, Number 2, 94-105 |
Density Functional Study on the Highest and Lowest Spin States of
[Mn2O2(H2O)8]q+ (q = 0, 2, 4)
Masaki Mitani, Takeharu Katsurada, Yohei Wakamatsu, and Yasunori Yoshioka
Internet Electron. J. Mol. Des. 2005, 4, 94-105
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Abstract:
Water oxidation mechanism of oxygen-evolving complex (OEC)
in photosystem II (PSII) has not been established yet. Recently,
the X-ray structure of PSII has been reported and it has been
suggested that OEC contains cubane-like Mn3CaO4 cluster linked
to Mn by mono-μ-oxo bridge. The Mn-Mn and Mn-Ca in
cubane-like cluster are connected by di-μ-oxo bridge to form a
face of cubane-like structure. As the first step to examine the
mechanism of dioxygen generation by OEC, the geometrical and
electronic structures of model complexes
[Mn2O2(H2O)8]q+
(q = 0, 2, 4)
are investigated for the highest and lowest spin
configurations from theoretical viewpoint in this work. Model
complexes were constructed by placing eight H2O molecules as
ligands around Mn-Mn core with di-μ-oxo bridges to form
six-coordinated Mn sites. Full geometry optimizations were carried
out by the hybrid-type DFT method with B3LYP functional. For
Mn, the LanL2DZ basis set with double-zeta quality was used
and the effective core potential was applied. The unrestricted and
broken-symmetry wave functions were obtained for the highest
and lowest spin states, respectively. The coordinated positions of
H2O molecules in Mn(II)-Mn(II) complex are different from
those in Mn(III)-Mn(III) or Mn(IV)-Mn(IV) complex, while
Mn(III)-Mn(III) and Mn(IV)-Mn(IV) complexes have similar
coordination pattern. In Mn-Mn core connected by di-μ-oxo
bridges, the spin densities are well localized on Mn, and the
antiferromagnetic coupling is more stable than the ferromagnetic
coupling by 0.46, 5.90, and 2.58 kcal/mol for Mn(II)-Mn(II),
Mn(III)-Mn(III), and Mn(IV)-Mn(IV) oxidation states,
respectively. The two-electron or four-electron oxidations of
di-μ-oxo-bridged Mn-Mn complex occur from octahedral "eg"
orbitals of Mn.
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