A CAS-DFT study of fundamental degenerate and nearly degenerate systems

ABSTRACT:The effect of hydrogen bonds attaching to sulfur atoms of cysteine ligands on 4Fe-4S cluster is examined by using UB3LYP method. Calculated results indicate that an existence of the hydrogen bonds scarcely changes SOMO-SUMO gap, shapes of Kohn-Sham orbitals, and magnetic interactions between Fe ions, although it stabilizes Kohn-Sham orbital energies of SOMO and SUMO about 1.0 eV. And they also make a reduced state stable in comparison with an oxidized state. In addition, the point charges of ϩ0.1e (e: elementary electric charge) at the position of the hydrogen atoms give almost same results to the hydrogen bonds quantitatively. The results suggest that a positive environment from the hydrogen bonds around the clusters is important for a redox potential of 4Fe-4S clusters.

Section: Discussionmentioning

confidence: 99%

Theoretical studies on effects of hydrogen bonds attaching to cysteine ligands on 4Fe‐4S clusters

Kitagawa

Saitô

Nakanishi

et al. 2008

“…where W D means the weight of doubly excited configuration involved in the diradical pair [81][82][83]. The effective bond order B HOMO after spin projection, normalized information entropy I n and residual information entropy are given, respectively, by…”

Section: Chemical Indices For Labile Mn-o Bondsmentioning

confidence: 99%

Possible mechanisms of water splitting reaction based on proton and electron release pathways revealed for CaMn₄O₅ cluster of PSII refined to 1.9 Å X‐ray resolution

Saitô

Yamanaka

Kanda

et al. 2011

Recently, Umena et al. have revealed the X-ray diffraction structure of the CaMn 4 O 5 cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) refined to 1.9 Å resolution. Their X-ray structure has first elucidated hydrogen-bonding networks and proton release pathways at OEC of PSII. Here, several working hypotheses Additional Supporting Information may be found in the online version of this article Correspondence to: S. Yamanaka; e-mail: syama@chem.sci.osaka-u.ac.jp Contract grant sponsor: Grants-in-Aid for Scientific Research; Contract grant numbers: 19750046, 19350070, 18350008, 23550016 (heuristic principles) for water splitting reaction are derived from their X-ray structure for theoretical modeling. These hypotheses suggest how water can be oxidized at OEC of PSII: namely possible reaction mechanisms for the reaction. To confirm them, we have also performed broken-symmetry (BS) UB3LYP calculations for active site models based on their XRD structure. The bond lengths of formal Mn(V)AO with labile dp-pp bonds are optimized to clarify possible roles of the species that are often introduced as a key intermediate in the catalytic (Kok) cycle for water splitting reaction at OEC of PSII. Location of the transition structure for the oxygen-oxygen (OAO) bond formation is also performed by the energy optimization technique. The natural orbital (NO) analysis of the UB3LYP solutions has been performed to obtain the natural molecular orbitals and their occupation numbers that have been useful for classification of localized d-electrons, labile chemical bonds and closed-shell (valence) orbitals. The localized d-electrons characterized by the NO analysis are the origins for the magnetism revealed by ENDOR and other magnetic experiments. On the other hand, the nature of labile (soft) dp-pp bonds responsible for the OAO bond formation has been investigated on the basis of chemical indices such as effective bond order (b), diradical character (y), and spin density (Q) indices that are calculated using the orbital overlap between broken-symmetry orbitals. These chemical indices have been calculated for the transition structure of the OAO bond formation at OEC of PSII. Implications of present computational results are discussed in relation to the derived hypotheses and available accumulated experimental results.

“…On the other hand, several groups have developed the hybrid method combining multireference (MR) wavefunction and DFT to cover multireference effects 34–45. Some of the recently developed MR‐DFTs are obviously designed to satisfy the Euler equation, i.e., to be an exact DFT 37–39, 42, 43. To our knowledge, there is no attempt to apply the MR‐DFT method for the ion‐radical systems systematically, although the difficulty in describing these systems is related to the MR character of the electronic structure.…”

Section: Introductionmentioning

confidence: 99%

Resonating broken symmetry CI approach for ion‐radical systems: Comparison with UHF, hybrid‐DFT, and CASSCF‐DFT

Nishihara

Yamanaka

Ukai

et al. 2008

ABSTRACT:The resonating broken-symmetry configuration interaction (RBS-CI) approach, in which we employ two spin-unrestricted Hartree-Fock (UHF) solutions as the basis, is applied for the ion-radical systems. The RBS-CI results are compared with those of UHF, various hybrid density functional theory (DFT), and CASSCF-DFT. The hybrid DFT leads to the spurious delocalized state as is well known, whereas the UHF to the localized state. The CASSCF-DFT results show that the excess electron localized, missing the spatial symmetry of the systems. On the other hand, the Res-HF CI results satisfy the spatial symmetry as well as the Perdew, Parr, Levy, and Balduz's relation, implying that this method is so-called many-electron self-interaction free (ME-SIF), whereas all other methods are not ME-SIF. These results suggest that the RBS-CI approach will become not only a powerful ab initio approach, but also a guideline to construct a new exchange-correlation functional of hybrid-DFT, for the ion-radical systems.