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In this article, we performed spin-unrestricted Hartree Fock (UHF) based coupled-cluster (CC) and Bruckner CC double calculations of methylene, fluorine, H&bond;He&bond;H diradical, and ozone. Mukherjee-type (Mk) Multireference coupled-cluster (MRCC) calculations of these species were also conducted by the use of UHF natural orbital (UNO) and UHF localized natural orbitals (ULO) as reference orbitals to elucidate size-consistent error. Comparing calculated results of spin-unrestricted Hartree-Fock based coupled-cluster (UCC) and spin-unrestricted Bruckner-Double (UBD) with those of MkMRCC, it was found that spin-contamination errors of UCC and UBD solutions still remained, though this type of errors for these solutions was considered to be negligible in general. The comparison also revealed scope and applicability of approximate spin-projection (AP) scheme for UCC and UBD, indicating that the AP procedure corrected spin-contamination errors to yield good agreement with MRCC in energy. We also performed pure- and hybrid- density functional theory (DFT) calculations of the same systems, and those calculations provided reasonable singlet-triplet energy gaps, which were qualitatively good agreement with those of MRCC, AP-UCC, and AP-UBD calculations. Implications of the computational results are discussed in relation to applicability of hybrid DFT to large superexchange coupled systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

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We have investigated the reaction pathways for the primary hydroxylation reaction of trimethylmethane by a high-valent Fe(IV)&dbond;O porphyrin π-cation radical species known as compound I at the B3LYP/CEP-31G level. The isoelectronic analogy of the Fe(IV)&dbond;O core of compound I to a molecular oxygen (O2) has been successfully used to clarify the important roles of the singlet excited state of the Fe(IV)&dbond;O core in the alkane hydroxylation, which has hitherto been neglected. The reaction is initiated by the rate-determining hydrogen-atom abstraction from the substrate to give a discrete radical intermediate complex, in accordance with the conventional radical rebound mechanism. Similar to the chemistry of O2, however, one of the singlet excited states, i.e., the diradical component of the 1Δ state of the Fe(IV)&dbond;O core intercepts the triplet ground state (the 3Σ state) in the region of the transition state for the hydrogen abstraction. Our findings strongly indicate that the exchange polarization or intersystem crossing for the nonradiative transition to the locally singlet state is highly important to enhance the reactivity of compound I. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)&dbond;O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)&dbond;O and a doublet ligand radical. This excited state lies 20.8 kcal mol-1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ΔE# = 26.6 kcal mol-1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: 3O(3p) + CH4. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (3O-model) and insertion (1O-model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

The radical harvesting state of a ribonucleotide reductase R2 subunit (RNR-R2) is theoretically investigated at the broken-symmetry DFT level. Recently, a high-resolution X-ray crystallography revealed a precise location of the active site structure of RNR-R2 (Class 1b) from C. ammoniagenes, and found a linking water near the tyrosine (Tyr) radical site. In this study, the magnetic interaction between the Tyr radical and the diiron core JTyr&bond;Fe is investigated for the first time. Used theoretical model for the active site is composed of diiron core, the Tyr residue, and surrounding side-chain groups. After the geometrical structure optimization of the core structures, the magnetic interaction between the iron centers was calculated to be JFe&bond;Fe = -77.82 cm-1, which was comparable to the experimental result of JFe&bond;Fe = -84 cm-1. The natural orbital analysis clearly showed that the magnetic interaction between the two iron centers mainly interact through the p orbital of the bridging μ-oxo. It was also calculated that the JTyr&bond;Fe interaction is negligibly weak, and the Tyr radical is almost in an isolated or a free radical state. These results suggest that the linking water may be used for (1) a confinement of the reactive Tyr radical in the hydrogen bonds or (2) a hydrogen-transfer coupled with electron transfer at the radical formation step of the Tyr. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

A quantum spin-correction scheme based on spin-correlation functions is proposed for multiple bond-breaking processes described by the unrestricted single determinant (USD) approaches, such as unrestricted Hartree–Fock (UHF) and unrestricted Kohn–Sham (UKS) density functional theory (DFT). It is known that natural orbitals (NOs) and occupation numbers (ONs) contain full information of USD solutions. We introduce a bond-operator representation of UHF and UKS wave functions to exploit information about those NOs, enabling us not only to facilitate the calculus, but also to understand the essential feature of the USD and those quantum spin-corrected solutions. We found that usual spin-projected solutions do not work for multiple bonds in the Heisenberg model mapping scheme. We propose quantum spin-correction schemes to obtain the spin-symmetry allowed solutions from the spin-unrestricted KS-DFT solutions. We examine these schemes for a chromium dimer, which is a typical example exhibiting multiple bonds. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Stabilization effects of nonsynchronous transition structures (TSs) of the Cope rearrangements of phenyl-substituted 1,5-hexadienes were examined by the hybrid density functional theory (DFT) and UHF methods. Natural orbital analyses of the hybrid UDFT and UHF solutions were performed to analyze the instability of molecular orbitals at nonsynchronous TSs. It was found that the chemical characteristics at the TSs are made explicit in the behavior of the information entropy and related chemical indices, which are defined by the occupation numbers of natural orbitals. The computational results are discussed in connection with the utility of chemical indices and the applicability of the hybrid DFT methods for pericyclic reactions of large systems. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the scope and applicability of Mukherjee’s state-specific multireference coupled cluster singles and doubles (Mk-MRCCSD) method on the magnetic interactions in dichloro-, oxo-, and peroxo-bridged binuclear Cu(II) complexes. Several reference orbitals including broken-symmetry natural orbitals (BSNOs) are examined for the Mk-MRCCSD computations. We compare the effective exchange integral (J) values calculated by Mk-MRCCSD with those calculated by conventional complete active space self-consistent field (CASSCF) and MRMP2. The BS methods with approximate spin-projection (AP) method are also performed for comparison. It is found that the Mk-MRCCSD method can be applicable to the prediction of exchange couplings in binuclear copper(II) containing systems. [Copyright &y& Elsevier]