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We present a systematic study of the electronic, geometric, and magnetic properties of the actinide dioxides, UO2, PuO2, AmO2, U0.5Pu0.5O2, U0.5Am0.5O2 and Pu0.5Am0.5O2. For UO2, PuO2 and AmO2, both density functional and hybrid density functional theory (DFT and HDFT) have been used. The fractions of exact Hartree-Fock (HF) exchange chosen were 25% and 40% for the hybrid density functional. For U0.5Pu0.5O2, U0.5Am0.5O2 and Pu0.5Am0.5O2, only HDFT with 40% exact HF exchange was used. Each compound has been studied at the nonmagnetic, ferromagnetic and anti-ferromagnetic configurations, with and without spin-orbit coupling (SOC). The lattice parameters, magnetic structures, bulk moduli, band gaps and density of states have been computed and compared to available experimental data and other theoretical results. Pure DFT fails to provide a satisfactory qualitative description of the electronic and magnetic structures of the actinide dioxides. On the other hand, HDFT performs very well in the prediction and description of the properties of the actinide dioxides. Our total energy calculations clearly indicate that the ground-state structures are anti-ferromagnetic for all actinide dioxides considered here. The lattice constants and the band gaps expand with an increase of HF exchange in HDFT. The influence of SOC is found to be significant. [ABSTRACT FROM AUTHOR]

A systematic study of silicon carbide nanocones of different disclination angles and different tip geometries using the finite cluster approximation is presented. The geometries of the nanocones have been spin optimized using the hybrid functional B3LYP (Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the binding energy per atom or the cohesive energy of the nanocones depends not only on the size of the nanocones but also on the disclination angle of the nanocones. The study also shows that the electronic properties of nanocones depend on disclination angles, size of the nanocone clusters, and the structure of edge of the nanocones. A study of binding energies, natural bond orbital (NBO) charge, density of states, and the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) has been performed for all nanocones from disclination angles of 60° to 300°. [ABSTRACT FROM AUTHOR]

Quantum mechanical calculations (AM1, PM3, ab initio HF/3-21G, DFT(B3LYP/6-31G*) and MP2//(B3LYP/6-31G*) have been used to study the inverse-demand synchronous concerted Diels-Alder reactions between dimethyl-1,2,4,5-tetrazine-3,6-dicarboxylate (diene) and a variety of dienophiles (ethylene, cyclopentadiene, 1-hexene, cyclohexene). All the molecular structures (reactants, transition states, intermediates and adducts) were optimized using the semi-empirical AM1 method. The calculated energies and volumes showed that the cycloaddition reaction followed a mechanism involving the formation of an intermediate, elimination of N, and a 1,3-hydrogen shift adduct. The reaction energies of the systems were obtained by using semi-empirical AM1 calculations and showed good agreement with the experimental data. In contrast, calculations of the reaction energies using PM3, HF/3-21G, DFT (B3LYP/6-31G*) and MP2//(B3LYP/6-31G* were in poor agreement with the experimental data. Compared to the experimental data, the activation energies were overestimated using AM1, PM3 and HF/3-21G, while they were underestimated using DFT (B3LYP/6-31G*) and MP2//(B3LYP/6-31G*). [ABSTRACT FROM AUTHOR]

Mukherjee's type of multireference coupled-cluster (MkMRCC), approximate spin-projected spin-unrestricted CC (APUCC), and AP spin-unrestricted Brueckner's (APUBD) methods were applied to didehydronated ethylene, allyl cation, cis-butadiene, and naphthalene. The focus is on descriptions of magnetic properties for these diradical species such as S-T gaps and diradical characters. Several types of orbital sets were examined as reference orbitals for MkMRCC calculations, and it was found that the change of orbital sets do not give significant impacts on computational results for these species. Comparison of MkMRCC results with APUCC and APUBD results show that these two types of methods yield similar results. These results show that the quantum spin corrected UCC and UBD methods can effectively account for both nondynamical and dynamical correlation effects that are covered by the MkMRCC methods. It was also shown that appropriately parameterized hybrid density functional theory (DFT) with AP corrections (APUDFT) calculations yielded very accurate data that qualitatively agree with those of MRCC and APUBD methods. This hierarchy of methods, MRCC, APUCC, and APUDFT, is expected to constitute a series of standard ab initio approaches towards radical systems, among which we could choose one of them, depending on the size of the systems and the required accuracy. [ABSTRACT FROM AUTHOR]

Abstract: We calculate defect levels of dangling bonds in germanium using hybrid density functionals. To validate our approach, we first consider the dangling bond in silicon finding two well-separated defect levels, in excellent correspondence with their experimental location. Application of our scheme to the dangling bond in germanium then yields two very close defect levels lying just above the valence band, consistent with the location of the experimentally determined charge neutrality level. The small correlation energy and the proximity to the valence band edge provide an explanation for the absence of clear defect signatures in electrical and electron spin resonance experiments. [Copyright &y& Elsevier]

The mechanism for the oxidation of catechol by catechol oxidase has been studied using B3LYP hybrid density functional theory. On the basis of the X-ray structure of the enzyme, the molecular system investigated includes the first-shell protein ligands of the two metal centers as well as the second-shell ligand Cys92. The cycle starts out with the oxidized, open-shell singlet complex with oxidation states Cu2(II,II) with a μ-η2:η2 bridging peroxide, as suggested experimentally, which is obtained from the oxidation of Cu2(I,I) by dioxygen. The substrate of each half-reaction is a catechol molecule approaching the dicopper complex: the first half-reaction involves Cu(I) oxidation by peroxide and the second one Cu(II) reduction. The quantitative potential energy profile of the reaction is discussed in connection with experimental data. Since no protons leave or enter the active site during the catalytic cycle, no external base is required. Unlike the previous density functional theory study, the dicopper complex has a charge of +2. [ABSTRACT FROM AUTHOR]

Abstract: Hybrid density functional calculations are performed on a series of 21 new quinoxaline derivatives, which would likely exhibit important biological activities. Optimized geometries, harmonic vibrational frequencies, and 1H chemical shifts are reported and compared with experimental data when available. In addition, melting points of 75 derivatives are predicted resorting to the Quantitative Structure–Property Relationship (QSPR) Theory, doing the variable selection by means of the Replacement Method and using 875 theoretical descriptors obtained from Dragon 5 software. The best relationship found has seven descriptors with R =0.8818 and R l−10%−o =0.7705. [Copyright &y& Elsevier]

Oxo‐ or hydroxo‐bridged diiron centers are ubiquitous in metalloenzymes such as hemerythrin (Hr), ribonucleotide reductase, methane monooxygenase, and rubrerythrin. In each enzyme the diiron core plays a central role in the highly specific reaction. To elucidate mechanisms of these reactions, many experimental studies have been carried out, and bioinorganic model compounds have also been synthesized for the purpose. In this study electronic structures of diiron centers for Hr model compounds are investigated from the viewpoint of magnetic interactions. To this end, the Hubbard model for the three‐center four‐electron bond is analytically solved to elucidate an important role of electron correlation and the resulting superexchange interaction between localized spins. The hybrid density functional theory (DFT) calculations also are performed for Hr model compounds to provide the natural orbitals and their occupation numbers, which are crucial for computations of several chemical indices, such as effective bond order, information entropy, and unpaired electron density. These indices are useful for characterization and understanding of chemical bonds in FeOFe cores. The calculated effective exchange integrals (Jab) are wholly consistent with the available experiments. The orbital interactions in the FeOFe cores are reconsidered in relation to recent work by other groups. It is found that magnetic interactions are sensitive to the hydrogen bonds in the systems and are related to effective regulation of the activity. Implications of the calculated results are discussed in relation to the nature of chemical bonds in the FeOFe cores of several biological systems. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Hybrid density functional theory with the B3LYP functional has been used to investigate the catalytic mechanism of catechol oxidase. Catechol oxidase belongs to a class of enzymes that has a copper dimer with histidine ligands at the active site. Another member of this class is tyrosinase, which has been studied by similar methods previously. An important advantage for the present study compared to the one for tyrosinase is that X-ray crystal structures exist for catechol oxidase. The most critical step in the mechanism for catechol oxidase is where the peroxide O–O bond is cleaved. In the suggested mechanism this cleavage occurs in concert with a proton transfer from the substrate. Shortly after the transition state is passed there is another proton transfer from the substrate, which completes the formation of a water molecule. An important feature of the mechanism, like the one for tyrosinase, is that no proton transfers to or from residues outside the metal complex are needed. The calculated energetics is in reasonable agreement with experiments. Comparisons are made to other similar enzymes studied previously. [ABSTRACT FROM AUTHOR]

Hybrid density functional theory (B3-LYP and B3-PW91) and counterpoise-corrected ab initio (MP2) calculations have been performed for the adducts of the main-group metal ions Na+, Mg+, Al+, K+, and Ca+ with the phenyl radical C6H5. While the calculated bond dissociation energies (BDEs) and complexation geometries of the alkali metal ion adducts do not differ substantially from those determined for the analogous M+/benzene π-complexes, the alkaline earth ions exhibit a strong preference for σ-bond formation at the bare C atom of the phenyl ring, with calculated BDEs exceeding those determined for benzene complexation by approximately 80 and 150 kJ mol−1 for Mg+ and Ca+, respectively. This σ-coordination is only feasible when the unpaired spins on the reactant Mg+ (or Ca+) and C6H5 radicals are opposed, thereby leading to a closed-shell (singlet state) adduct ion. When the unpaired spins are aligned, the triplet-state adduct thus produced adopts π-complex minimum energy geometry with a BDE significantly below that of the corresponding M+/benzene complex. Both π- and σ-coordination are also found to be local minima on the AlC6H5+ potential energy surface, with the BDE of the Al+/C6H5 σ-complex being approximately 50 kJ mol−1 larger than that of the corresponding π-complex, and about 20 kJ mol−1 above the BDE for Al+/benzene. The overall results of our study show that the phenyl radical exhibits a much greater degree of selectivity between the main-group metal ions represented here than is evident for benzene. The phenyl radical BDEs range from 59.7 (K+) to 264.7 kJ mol−1 (Ca+) according to calculations at the B3-PW91/6-311+G(2df,p) level of theory, which contrasts with the 65.5 (K+) to 147.9 kJ mol−1 (Al+) BDE values determined for benzene at the same level of theory. [Copyright &y& Elsevier]

The HNO isomerization into HON was examined with a variety of density functional theory methods, including the B1LYP, B3LYP, and the MPW1PW91 method coupled to a relatively large 6-311G(2d,2p) basis set. In addition, the CBS-Q, and G2 methods were used to further evaluate the barrier to isomerization, which is believed to occur through a three-center cyclic transition state. The difference in energy between the two isomers is predicted to being around 40–41 kcal mol−1 when any of the methods are applied. [Copyright &y& Elsevier]

In this study, the B3LYP hybrid density functional theory was used to investigate the electromechanical characteristics of C70 fullerene with and without point charges to model the effect of the surface of the gate electrode in a C70 single-electron transistor (SET). To understand electron tunneling through C70 fullerene species in a single-C70 transistor, descriptors of geometrical atomic structures and frontier molecular orbitals were analyzed. The findings regarding the node planes of the lowest unoccupied molecular orbitals (LUMOs) of C70 and both the highest occupied molecular orbitals (HOMOs) and the LUMO of the C70 anion suggest that electron tunneling of pristine C70 prolate spheroidal fullerene could be better in the major axis orientation when facing the gate electrode than in the major (longer) axis orientation when facing the Au source and drain electrodes. In addition, we explored the effect on the geometrical atomic structure of C70 by a single-electron addition, in which the maximum change for the distance between two carbon sites of C70 is 0.02 Å. [ABSTRACT FROM AUTHOR]