Handai Rigaku Techno Research

Toyonaka, Japan

Handai Rigaku Techno Research

Toyonaka, Japan
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Isobe H.,Okayama University | Shoji M.,University of Tsukuba | Yamanaka S.,Osaka University | Mino H.,Nagoya University | And 6 more authors.
Physical Chemistry Chemical Physics | Year: 2014

Full geometry optimizations followed by the vibrational analysis were performed for eight spin configurations of the CaMn4O 4X(H2O)3Y (X = O, OH; Y = H2O, OH) cluster in the S1 and S3 states of the oxygen evolution complex (OEC) of photosystem II (PSII). The energy gaps among these configurations obtained by vertical, adiabatic and adiabatic plus zero-point-energy (ZPE) correction procedures have been used for computation of the effective exchange integrals (J) in the spin Hamiltonian model. The J values are calculated by the (1) analytical method and the (2) generalized approximate spin projection (AP) method that eliminates the spin contamination errors of UB3LYP solutions. Using J values derived from these methods, exact diagonalization of the spin Hamiltonian matrix was carried out, yielding excitation energies and spin densities of the ground and lower-excited states of the cluster. The obtained results for the right (R)- and left (L)-opened structures in the S1 and S3 states are found to be consistent with available optical and magnetic experimental results. Implications of the computational results are discussed in relation to (a) the necessity of the exact diagonalization for computations of reliable energy levels, (b) magneto-structural correlations in the CaMn4O5 cluster of the OEC of PSII, (c) structural symmetry breaking in the S 1 and S3 states, and (d) the right- and left-handed scenarios for the O-O bond formation for water oxidation. © 2014 the Partner Organisations.


Yamaguchi K.,Osaka University | Yamaguchi K.,Handai Rigaku Techno Research | Shoji M.,University of Tsukuba | Isobe H.,Handai Rigaku Techno Research | And 5 more authors.
Polyhedron | Year: 2013

Activation mechanisms of oxygen dianion and hydroxide by high-valent transition metal ions for the oxygen-oxygen (O-O) bond formation of water splitting reaction have been investigated on the theoretical grounds, together with experimental results. First of all, broken-symmetry MO formulations are revisited to elucidate the instability of the dπ-pπ bond in high-valent metal-oxo M(X)O (M = Mn, Fe, Ru, etc.; X = IV, V) systems that react with hydroxide anion (or radical) or water to afford hydroperoxide anion (or peroxide). The triplet instability of these bonds entails strong or intermediate diradical characters: •M(X-1)O• and ••M(X-2)O••; the broken-symmetry (BS) molecular orbitals (MO) resulted from strong electron correlation, leading to the concept of electron localizations and local spins. As a continuation of these theoretical results, the BS MO interaction diagrams, namely orbital and spin correlation diagrams, for one-electron and electron-pair transfer mechanisms for the O-O bond formation have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of mechanisms of oxygenation reactions by p450, methane mono- oxygenase (MMO) and homolytic radical coupling mechanisms by oxygen evolving complex (OEC) of PSII. The spin alignments in high-valent M(X)O systems are found directly corresponding to possible mechanisms of the O-O bond formation between Mn(X)O and hydroxide (OH) anion via one-electron, electron-pair transfer and their superposed (chameleon) processes. The broken-symmetry (BS) UB3LYP calculations of the model systems have been performed to confirm these mechanisms for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms and orbital and spin correlation diagrams. Implications of the theoretical results are discussed in relation to three scenarios of the O-O bond formation for water oxidation: (a) HO-OH, (b) OOH and (c) O-O, and water oxidation of OEC of PSII. © 2013 Elsevier Ltd. All rights reserved.


Yamaguchi K.,Osaka University | Yamaguchi K.,Handai rigaku Techno Research | Isobe H.,Okayama University | Shoji M.,University of Tsukuba | And 2 more authors.
Molecular Physics | Year: 2016

Magneto-structural correlations in oxygen-evolving complex (OEC) of photosystem II (PSII) have been elucidated on the basis of theoretical and computational results in combination with available electron paramagnetic resonance (EPR) experimental results, and extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) results. To this end, the computational methods based on broken-symmetry (BS) UB3LYP solutions have been developed to elucidate magnetic interactions in the active manganese catalyst for water oxidation by sunlight. The effective exchange interactions J for the CaMn(III)Mn(IV)3O5(H2O)3Y(Y = H2O or OH-) cluster (1) model of OEC of PSII have been calculated by the generalised approximate spin projection (GAP) method that eliminates the spin contamination errors of the BS UB3LYP solution. Full geometry optimisations followed by the zero-point energy (ZPE) correction have been performed for all the spin configurations of 1 to improve the J values that are compared with accumulated EPR in the S2 state of Kok cycle and magnetic susceptibility results of Christou model complex Ca2Mn(IV) 3O4 (2). Using the calculated J values, exact diagonalisation of the spin Hamiltonian matrix has been carried out to obtain excitation energies and spin densities of the ground and lower excited states of 1. The calculated excitation energies are consistent with the available experimental results. The calculated spin densities (projection factors) are also compatible with those of the EPR results. The calculated spin densities have been used to calculate the isotropic hyperfine (Aiso) constants of 55Mn ions revealed by the EPR experiments. Implications of the computational results are discussed in relation to the structural symmetry breaking (SSB) in the S1, S2 and S3 states, spin crossover phenomenon induced by the near-infrared excitation and the right- and left-handed scenarios for the O-O bond formation for water oxidation. © 2015 Taylor & Francis.


Yamaguchi K.,Handai Rigaku Techno research | Yamaguchi K.,Osaka University | Yamanaka S.,Osaka University | Isobe H.,Handai Rigaku Techno research | And 2 more authors.
International Journal of Quantum Chemistry | Year: 2012

Electronic and spin states of putative ruthenium (Ru)-quinine(Q) complex(1) mononuclear[Ru(trpy)(3,5-t-Bu 2Q)(OH 2)(trpy = 2,2':6',2′-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone)(2) and binuclear [Ru 2(btpyan)(3,6-di-Bu 2Q) 2(OH 2)](SbF 6) 2 (btpyan = 1,8-bis(2,2':6',2′- terpyrid-4'-yl)anthracene,3,6-t-Bu 2Q = 3,6-di-tert-butyl-1,2- benzoquinone) (3) were investigated by broken-symmetry (BS) hybrid density functional (DFT) methods. BS DFT computations for 3 have elucidated that the closed-shell structure (3b): Ru(II)-quinone (Q) complex is less stable than the open-shell structure (3bb) consisted of Ru(III) and semiquinone fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model has been derived to elucidate common theoretical pictures for these systems. Effective exchange interactions for four spin systems have been determined using total energies of the 15 different DFT functionals. The natural orbital (NO) analysis of these BS DFT solutions have also been performed to elucidate the NOs and their occupation numbers that are useful for lucid understanding of the nature of chemical bonds of these Ru complexes (1, 2, and 3). Implications of the computational results are discussed in relation to proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems, similarity between artificial and native water splitting systems, and elements science of water splitting reaction. © 2012 Wiley Periodicals, Inc.

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