Home – Home – Jornals – Journal of Structural Chemistry 2017 number 6

The electronic structure of three aza-boron-dipyridomethene derivatives containing different hydrocarbon groups at the boron atom is studied by ultraviolet photoelectron spectroscopy and calculations at the density functional theory level. According to the experimental and theoretical data, the higher occupied molecular orbitals of anthracene, acridine, and the studied complexes are of the same character. For the three studied compounds, the effect of alkyl and phenyl substituents on the electronic structure is determined. The parameters of the electronic structure of aza-boron-dipyridomethene (phenyl groups at the boron atom) and its b-diketonate analogue are compared. It is shown that in an energy range up to 11 eV the calculated results correlate with the ultraviolet photoelectron spectra.

A brief review of the results of studying some classes of nitrogen-containing chelate boron complexes by ultraviolet photoelectron spectroscopy and density functional theory is reported. The quantum chemical modeling of the substitution effects of a complexing agent, heteroatoms, and functional groups in α, β, and γ positions of the chelate ring allowed us to establish the features of the electronic structure of the studied complexes. It is found that the substitution of heteroatoms in the chelate ring has no substantial influence on the structure of the highest occupied molecular orbital (HOMO). In imidoylamidinate complexes, as opposed to formazanates and b-diketonates, there is no noticeable mixing of p orbitals of the chelate and benzene rings. In condensed nitrogen heterocycles the HOMO is stabilized by 0.2-0.3 eV and p orbitals of the benzene ring are stabilized by 0.8-1.2 eV. The HOMO of substituted aza-boron-dipyridomethene correlates with anthracene and acridine p₇ orbitals, which causes the fine structure of the first band. It is shown that in an energy range below 11 eV the calculated results reproduce well the energy gaps between the ionization states of the complexes.

The infrared spectra of the complexes of boron difluoride acetylacetonate and its halogen-substituted derivatives F₂B(aaX) (X = H, Cl, Br, I) in the crystalline state are studied. The substituent effect on the geometry and force field of molecules is revealed from DFT/B3LYP quantum chemical calculations with the 6-311G( d , p ) basis set. The detailed assignment of IR absorption bands is performed based on the calculations of normal modes (NMs) and the potential energy distribution (PED). The bands most sensitive to the substituent nature belong to vibrations with prevalent involvement of ring CC and CO bonds and some low-frequency noncharacteristic NMs involving the Х atom. In support of the single crystal XRD data, intermolecular interactions have the strongest effect on the characteristic bands of the BF₂ moiety in the ranges 1280-1220 cm^-1 and 875-835 cm^-1; the sequences of IR band frequency shifts in a series of substituents corresponding to these interactions are reported.

The article examines the electronic structure and orbital nature of luminescence excitation in a series of molecular crystals with the general formula E _n AX₆, where E_n are organic and inorganic cations (diphenylguanidinium, guanidinium, and cesium); n is the number of cations; AX₆ are Te(IV) and Sb(III) anions; X are the atoms of halogens Cl or Br. The electronic structure of these molecular crystals is determined from the data of X-ray photoelectron spectroscopy of the core and valence levels and еру quantum chemical modeling фе the density functional theory level together with the previously obtained single crystal X-ray diffraction data.

The effect of an additional aromatic ligand on the electronic structure of nickel(II) and cobalt(II) bis-acetylacetonates is studied by XPS and DFT (B3LYP/def2-TZVPP). An analysis of atomic charges, the geometry of compounds, the electron density distribution, and the interaction of molecular orbitals of complex components allows the conclusion about the ionic nature of the bonding between the neutral ligand and the bis-chelate.

By UPS spectroscopy of vapor, XPS spectroscopy of the condensed phase, and quantum chemical methods the adducts of tris -b-diketonates Eu(асас)₃Phen and Eu(hfас)₃Phen are studied. The electronic structure and features of the nature of chemical bonds in the adducts are established. The geometric structure of the studied compounds in the gas phase is determined. A procedure is developed that helps to assign the bands in gas-phase HeI photoelectron spectra and also in the valence band of the XPS spectra. Quantum chemical calculations (DFT) make it possible to find the regular changes in the electronic structure of the chelate complexes depending on ligand fluorination, to study the effect of a 1,10-phenanthroline molecule on the electronic structure of the chelate rings, and also to analyze the electronic effects caused by trifluoromethyl substitution for methyl groups in the ligand.

X-ray photoelectron spectroscopy and quantum chemistry methods are used to study Nd(tol)₃ and Nd(сor)₃ carboxylate complexes and their adducts with 1,10-phenanthroline (Nd(tol)₃Phen and Nd(сor)₃Phen₂). The electronic structure and specific features of the nature of chemical bonds are studied, as well as the effect of 1,10-phenanthroline on the electronic structure of the adduct. We propose the band assignment of the valence band of the XPS spectra of all compounds.

A series of Ni- and/or Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium are examined by X-ray photoelectron spectroscopy. Binding energies of core electrons, elemental composition, chemical state of elements, and features of the structural organization of the surface and near-surface layers of the coatings are determined. A combination of the data collected indicates similar regularities of the composition and significant distinctions in the structure of the coatings formed. It is shown that the coatings formed on titanium are characterized by a considerably higher phosphorus concentration, and correspondingly, phosphates, unlike the coatings formed on aluminum, in which base metal and 3 d element (Ni or Cu) oxides are dominant. In both cases, Cu is mainly concentrated in the surface layers of the coatings whereas Ni is mainly concentrated in the near-surface layers.

Mineral valleriite of the Talnakh deposit, which consists of alternating copper-iron sulfide layers and brucite-like layers of magnesium-aluminium hydroxide is studied for the first time by XPS at photon excitation energies ranging from 1.253 keV to 6 keV and CuL FeL, SL, AlL, MgK, and OK edge TEY XANES using synchrotron radiation. The comparison of the XPS and XANES spectra of valleriite and chalcopyrite, in particular, demonstrates that in the sulfide layers of valleriite, Cu⁺ and Fe³⁺ are in a tetrahedral coordination, however, a local positive charge on both cations is slightly lower than that in chalcopyrite, apparently, due to a structure disorder. The concentration of oxygen-bound iron decreases with an increase in the depth of the analyzed layer even after ion etching; probably, Fe does not enter into the brucite-like layer, but mainly forms its own surface structures.

XPS, Pb L ₃ and Cu K EXAFS, solid state NMR, and EPR techniques are used to study insoluble products formed in the interaction of aqueous solutions of lead(II) nitrate and copper (II) sulfate with potassium n -butylxanthogenate (PX). The XPS spectra of lead xanthogenates with the composition PbX₂ are similar to those of PX, and interatomic distances of 0.279 nm suggest a nearly ionic character of Pb-S bonds. In copper xanthogenate precipitating together with dixanthogene (approximately 15 wt.%), the Cu(I)-S bond length is smaller (0.229 nm), and copper coordination number of 2.9 in a composite with dixanthogene increases to 3.3 after its removal by washing with acetone. The XPS spectra indicate the covalent character of the bond and non-equivalence of xanthogenate radicals. Solid state ¹H and ¹³C NMR spectra as well as the actual absence of metal lines under the measurement conditions demonstrate strong disordering of the structure of xanthogenates, which is stronger for PbX₂ and weakest in CuX after the removal of dixanthogene. EPR reveals sulfur-containing radicals and Cu²⁺ in CuX, however, their amounts are insignificant and decrease after the washing with acetone. The results of the work are significant for the understanding of the reactivity of xanthogenates, in particular, under the conditions of flotation beneficiation of base metal ores.

The states of components of highly efficient Pt/CeO₂ catalysts for low-temperature oxidation of carbon monoxide are studied in detail by X-ray photoelectron spectroscopy (XPS). Using the precise calibration of the spectra relative to the internal standard and the fitting of Ce3d and Pt4f spectra by elementary doublets, we found the features of the platinum interaction with the ceria lattice. It is shown that when the co-deposition technique is used, depending on the quality of stock solutions, it is possible to obtain both homogeneous solid solutions of platinum in the ceria lattice and solutions containing polyatomic platinum associates of the (PtO)_m type. It is found that when homogeneous PtCeO_x solid solutions are stored in air at room temperature, the homogeneous solutions slowly pass into the state of solutions with platinum associates. Mechanical mixtures of metallic platinum and ceria nanoparticles, synthesized by laser ablation, were also investigated in the course of their annealing in the air. The results obtained from the Pt4f spectra completely confirm the specific features of the interaction of platinum with ceria.

The regularities of the electron energy dissipation found in the subsurface atomic layer are valid in the bulk of a solid, too. On the example of model graphite-based materials it is shown that energy losses in X-ray photoelectron spectra agree with the calculated valence electron excitation spectra in analogous unit cells. The control of conjugate electron transitions opens the way to gain new data on the geometry, character, and order of bonding between atoms in the sample by the conventional electron spectroscopy and quantum chemistry methods.

The electronic structure of hexanuclear Mn(II,III) pivalate complexes with tetrahydrofuran and isonicotinamide are studied by X-ray photoelectron spectroscopy and X-ray emission spectroscopy. It is shown that when isonicotinamide substitutes for tetrahydrofuran the spin state of manganese ions is retained, and the electron density increases on the manganese and oxygen atoms of the [Mn₆(O)₂Piv₁₀] core.

X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are used for in situ studies of the electronic structure of lithiated natural graphite produced by thermal deposition of lithium upon graphite in a vacuum. By XPS and NEXAFS spectroscopy it is found that lithium vapor thermal deposition results in the formation of a lithiated graphite surface layer and a change in its electronic structure. Based on the quantum chemical simulation of the experimental СK_α XES spectrum of lithiated graphite, it is found that lithium atoms are located mostly on the edges of graphite crystallites. Atomic force microscopy reveals that the size of natural graphite flakes varies from 50 nm to 200 nm.

The electronic structure and functional composition of the products of graphite oxide (GO) interaction with concentrated sulfuric acid (H₂SO₄) depending on the oxygen content in the precursor and the treatment temperature are studied. X-ray photoelectron spectroscopy measuring the near-edge Xray absorption fine structure at the CK edge, and infrared spectroscopy are used to show the reduction of the π-electron system of graphene planes at a temperature of 200° C. Aggregation of oxygen groups in GO containing more than 40 % of oxygen causes the formation of vacancy defects in the planes with oxygen groups at the edges. When the GO treatment temperature rises to 280°C, the oxidation of the basal plane π-regions occurs. The results can be the basis to control the functional composition, vacancy number and size in graphite materials.

The functional composition and electrochemical behavior of the samples of N121 oxidized nanodisperse technical carbon in aqueous electrolytes are studied. For oxidation 30% aqueous hydrogen peroxide (H₂O₂) solution and 2% H₂O₂ with the addition of singlet oxygen or ozone were used. By means of X-ray photoelectron spectroscopy data and the analysis of the near edge X-ray absorption fine structure the features of the chemical structure of the samples are found. The oxygen concentration did not exceed 5 at.% in the samples. The analysis of cyclic voltammograms reveals that at low potential sweep rates the specific capacity of the material is determined by the functional composition of the surface. The sample oxidized by 30% H₂O₂ solution and containing the largest number of -OH and -COOH groups demonstrated the highest capacity in 6M KOH and in 1М H₂SO₄ it was the sample with the highest concentration of C=O groups formed during the oxidation with singlet oxygen. The stability of carbon electrodes is studied in supercondensor models.

The work reports the study of the structure of carbon nanoparticles prepared by pyrolysis of helium-diluted acetylene under adiabatic compression in a piston reactor. At a pushing gas pressure of 0.5 MPa, 0.7 MPa, and 0.9 MPa the reaction gas heated to a temperature of 400° C, 600° C, and 750° C. By transmission electron microscopy it is found that carbon nanoparticles have a spherical form and their size varies from 20 nm to 60 nm. The structural features of carbon nanoparticles are determined from the Xray photoelectron spectroscopy data and the analysis of near-edge Xray absorption fine structure. Carbon nanoparticles prepared at a pushing gas pressure of 0.5 MPa have an amorphous structure and consist of hydrogenated carbon with an impurity of polycyclic aromatic fragments. At a stronger compression ratio, carbon nanoparticles with a layered structure consisting of mainly sp ² hybridized carbon atoms form. The capacitance properties and electrochemical impedance of electrodes based on carbon nanoparticles in supercapacitors are compared.

Monolayer carbon framework nanoparticles (nanohorns) (CNHs) are synthesized by two methods: electric arc and electron evaporation of graphite in the inert atmosphere. Based on the data of electron microscopy and Raman light spectroscopy distinctions in the structures of materials obtained by different methods are revealed. By X-ray photoelectron and NEXAFS spectroscopy a change in the chemical structure of the CNH surface during their oxidation is considered. It is found that oxidation causes the destruction of CNH agglomerates and weakly affects the structure of graphene nets. However, these changes are sufficient for an increase in the infrared radiation absorption by the dispersion of nanohorns in water. It is shown that the efficiency of laser heating with a wavelength of 808 nm of CNH dispersion depends on the synthesis method and chemical modification of nanoparticles, which enables their potential use for local hyperthermia of cells of living organisms in cancer therapy.

The Cu(II)GHK (Cu(II)-Gly-His-Lys (glycyl-hystidyl-lysine) complex is of interest as a model peptide for the elaboration of a procedure to study the structure of metal centers in proteins, in particular, the copper binding center in β-amyloid. X-ray absorption spectra are measured for an aqueous solution of Cu(II)GHK. The stability of the complex under X-ray radiation is controlled by optical spectroscopy. Structural models with different coordinations of the copper center, which were constructed based on the crystallographic structure, are considered. Based on the analysis of theoretical X-ray absorption spectra of the derived structures, two optimal models are chosen. For the selected models the structural parameters are optimized. It is found that the best agreement with the experiment is observed for the spectrum of a five-coordinated model with water molecules in the equatorial and lower axial positions with Cu-O distances of 1.97 Å and 2.31 Å respectively.

The electronic energy structures of three phosphorus-containing sulfides are studied from the experimental K and L_2,3 X-ray emission spectra, K absorption spectra of sulfur and phosphorus, X-ray photoelectron spectra, and also quantum chemical calculations based on density functional theory (DFT). The full-potential and all-electron quantum chemical calculations are carried out using the LAPW+lo basis set implemented in the WIEN2k software package [1]. The following exchange-correlation potentials are used for the calculations: PBE, PBE+U, and mBJ [2]. The spin-orbit coupling of Tl 5d_3/2 and 5d_5/2 electronic states are taken into account in Tl₃PS₄. All the specific features of the electronic energy structures of the compounds under study are determined in the valence and conduction bands near the Fermi level. The obtained band gaps Eg are in good agreement with the literature experimental data.

A series of Cu, Ni, and Mn complexes based on formylpyrone and formylcoumarine azomethynes with 1,3-diaminepropanol-2. The analysis of XANES and EXAFS spectra of the Cu, Ni, and Mn complexes, which were processed by both Fourier and wavelet transforms, enables the determination of local atomic structural parameters and the unambiguous evidence of the formation of both dimeric and monomeric structures for these coordination compounds. The possibility of forming binuclear compounds is shown to depend mainly on the nature of the bridging ligand and be independent of the type of the polydentate azomethyne ligand: pyrone or coumarine. The obtained structural results are well consistent with the magnetochemical data for the complexes.

Nanocomposite systems based on ternary semiconductor compounds ZnS _x Se_1-x with different compositions (x = 0, 0.3, 0.5, 0.7, 1) in dielectric matrices of nanoporous anodic alumina (AA) are synthesized by ultrahigh vacuum thermal sputtering of a mixture of lead sulfide and selenide powders. The effect of the atomic concentration of solid solutions and structural parameters of the AA template matrix on the crystal structure of synthesized nanocomposites and the local atomic environment of Zn and Se atoms is investigated.