P. V. Avramov

Ab initio calculations of endo-and exohedral C60 fullerene complexes with Li+ ion and the endohedral C60 fullerene complex with Li2 dimer

The results of ab initio Hartree-Fock calculations of endo-and exohedral C60 fullerene complexes with the Li+ ion and Li2 dimer are presented. The coordination of the Li+ ion and the Li2 dimer in the endohedral complexes and the coordination of Li+ ion in the exohedral complex of C60 fullerene are determined by the geometry optimization using the 3–21G basis set. In the endohedral Li+C60 complex, the Li+ ion is displaced from the center of the C60 cage to the centers of carbon hexa-and pentagons by 0.12 nm. In the Li2 dimer encapsulated inside the C60 cage, the distance between the lithium atoms is 0.02 nm longer than that in the free molecule. The calculated total and partial one-electron densities of states of C60 fullerene are in good agreement with the experimental photoelectron and X-ray emission spectra. Analysis of one-electron density of states of the endohedral Li+@C60 complex indicates an ionic bonding between the Li atoms and the C60 fullerene. In the Li+C60 and Li+@C60 complexes, there is a strong electrostatic interaction between the Li+ ion and the fullerene.

Magnetic, optical, and electrical properties of solid solutions VxFe1−xBO3

Complex studies of magnetic, electrical, and optical properties of Vx Fe1−xBO3 solid solutions are carried out in the entire range of concentrations between the extreme compounds VBO3 and FeBO3. A concentration semiconductor-insulator transition accompanied by a change in the magnetic structure is observed. It is found that the physical properties of the solid solution under investigation differ from those predicted in the model of a virtual crystal in the form of an aggregate of V and Fe centers taken with the weight of x and 1−x, respectively. The systems of electron energy levels of the VB6O6 and FeB6O6 clusters are calculated from first principles using the Hartree-Fock method. The calculated electron structure forms the basis for simulating the optical absorption spectra, which are in good agreement with experimental results. A qualitative explanation is given for the entire body of data on electrical conductivity and magnetization.

Possible scheme of synthesis-assembling of fullerenes

A new scheme of fullerene formation is proposed on the basis of the similarity between the experimentally detected carbon structures. According to experimental data, the microclusters of C2 and C10 are synthesized first and then either an intermediate nucleus cluster or an obtainable lower fullerene is assembled from them. A high-symmetry fullerene can be assembled with a high probability from a nucleus cluster with a “good” symmetry. The atomic and electronic structures of molecules such as C36, C60, C70, and C76 are analyzed. For C36, the NMR spectra are calculated and compared with the experimental data.

Correlation of the chemical properties of carbon nanotubes with their atomic and electronic structures

The nature of chemical bonding in carbon nanoclusters is investigated by the PM3 semiempirical quantum-chemical method. The influence of the atomic structure on the electronic characteristics and chemical properties of nanoclusters is analyzed. A σ-π model is proposed for the chemical bonding in nanotubes. It is shown that, in the framework of the proposed model, nanotubes are objects characterized by a small contribution of π states to the valence band top.

Theory of X-ray absorption spectra of strongly correlated copper oxides

Abstract It has been shown that theoretical X-ray absorption spectra of highly correlated systems can be presented as a product of a single-electron part obtained by the self-consistent field Xα-scattered wave (SCFXα-SW) method, and of a multi-electron part obtained by exact diagonalization of the Hamiltonian of the multi-band multi-electron p-d model. Using that model, the influence of strong correlation effects on the Cu K- and CuL2,3-spectra of La2−xSrxCuO4 (x = 0, 0.2, 1) has been studied. In terms of that model, the main peak of the Cu K-spectrum for x = 0 was assigned to the Cu d 10 L - configuration and only one satellite was assigned to the Cud9-configuration. Comparison of the theoretical with the experimental data shows that the ground state of the two-holes in the CuO4 cell is triplet. In that case additional satellites of Cu d 9 L - and Cud8-configurations are observed. The same conclusions have been made concerning polarized CuL2,3-spectra for the fully doped LaSrCuO4 excluding the peak with the energy 2.8 eV above the threshold, which was assigned to the transitions into quasi-stationary states due to existence of a high barrier in the Cud-state Hartree-Fock potential.

Effect of carbon network defects on the electronic structure of semiconductor single-wall carbon nanotubes

For a single-wall (14, 0) carbon nanotube, the total density of electronic states of the ideal structure and of some possible defect structures is calculated in the framework of the band theory approach using Gaussian-type orbitals and the approximation of the generalized density gradient. It is shown that allowance for defects of the atomic structure of a nanotube makes it possible to adequately describe the existing experimental data on nanotube electronic structure. In the framework of the same approach, the total density of electronic states is calculated for an intermolecular contact of (5, 5) and (10, 0) single-wall carbon nanotubes formed due to the creation of a 5–7 defect. It is shown that the electronic states related to the contact region and the 5–7 defect lie in vicinity of the Fermi level.

Hypothesis of Hemoprotein Sensor Confirmed by Ab initio Quantum-Chemical Method

The nature of the chemical bond of complexes of iron and cobalt porphyrinates with ligands is studied by the quantum-chemical method in the Hartree–Fock self-consistent field approximation using the 3-21G basis set. The addition of oxygen molecule to the MP and MPIm complexes (M = Fe, Co; Im is imidazole) is established to be more favorable than water addition. However, imidazole, which is the second ligand in the MPImO2 and MPImH2O complexes (M = Fe, Co), increases the M–O2 and M–H2O binding energies for iron, but decreases them for cobalt. The Co atom is bound with the porphyrin ring more strongly than the iron atom due to the larger total overlap of the atomic orbitals. The calculations of the binding energy in the complexes demonstrate similar changes in the structures of the spatial conformation of the deoxy form (FeP + H2O) of iron porphyrinate and the oxy form (CoP + O2) of cobalt porphyrinate. This is an argument in favor of the hypothesis of hemoprotein sensor of partial oxygen stress in tissues.

Theoretical Study of the Toroidal Forms of Carbon and Related Endohedral Complexes with Lithium

The atomic and electron structures of toroidal carbon molecules (C240 and two C120 isomers) and related endohedral complexes with lithium (Li2@Cn and Li4@Cn) were theoretically studied using both nonempirical (3–21G basis set) and semiempirical (MNDO) calculation schemes. For the metal-containing compounds, the behavior of lithium atoms embedded into internal cavities of the carbon framework was studied using methods of molecular dynamics. It is demonstrated that the structure of electron levels of metal-containing carbon complexes exhibits an embedded state in the forbidden band, which appears due to the presence of electrons accepted from metal atoms. The position of this embedded state and the bandgap width depend both on the initial carbon structure and on the amount of metal atoms incorporated.

The Role of Histidine in the Ligand-Bonding Capacity of the Hemoglobin Gene

The atomic and electronic structures of heme complexes with His, Gly, and Cys residues (Heme–His, Heme–Gly, and Heme–Cys) in the fifth coordination position of the Fe atom and with oxygen and nitrogen oxide molecules in the sixth Fe position were studied by the semiempirical quantum-chemical method PM3. A comparative analysis of internuclear distances showed that the strength of chemical bonding between the ligand molecules (oxygen and nitrogen oxide) is greater for Heme–Cys than for Heme–His and Heme–Gly complexes. Consequently, the strengthening of the chemical bond of the oxygen (or nitrogen oxide) molecule with Heme–Cys substantially weakens the chemical bond in the ligand molecule. The Mulliken population analysis showed that the electronic density of ligand (oxygen or nitrogen oxide) p-orbitals is transferred to the d-orbitals of the Fe atom, whose charge, calculated according to the Mulliken analysis, formally becomes negative. In the Heme–His complex with oxygen, this charge is substantially greater than in the complex with NO, and the oxygen molecule becomes polarized. No oxygen polarization is observed in the Heme–Cys complex, and the electron density (judging from the change in the Fe charge) is transferred to the coordinated sulfur atom. This is also characteristic of Heme–Cys complexes with nitrogen oxide. An analysis of charges on the atoms indicates that the character of chemical bonding of the oxygen molecule in Heme–Cys and Heme–Gly complexes is similar and basically differs from that in the case of the Heme–His complex.

Electronic and atomic structures of the isomers of endohedral and exohedral fullerene complexes with two lithium atoms

The electronic structures of all the possible isomers of endohedral and exohedral C60 fullerene complexes with two lithium atoms are theoretically investigated. It is found that the electronic structures of these compounds are characterized by an impurity filled-level state determining the band gap. The location of the impurity state and, correspondingly, the band gap of the exohedral fullerene complexes depend on the coordination mode and the distance between the alkali metal ions. A similar dependence is observed for the total energy of the exohedral fullerene complex under investigation.