Electronic structure and properties of the ground state of copper in semiconducting cuprates

The spin-polarized discrete variational X_a method is used to calculate clusters that model the electronic structures of CuO, La₂CuO₄, and Nd₂CuO₄. It, is shown that in each of the compounds the unoccupied portion of the valence band involves mainly the O2p states, the contributions from the Cu3d orbitals being significantly smaller. The effects of the nature of holes in the valence band and of the structure of the close environment of copper on the low-energy CuK spectra and the X-ray photoelectron spectra of the above systems are discussed. Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhumal Struktumoi Khimii, Vol. 36, No. 4, pp. 636–643, July–August., 1995. Translated by I. Izvekova     

Investigation of electronic interactions in solid hydrogen fluoride

Intermolecular interactions in solid hydrogen fluoride are studied by the combined quantum chemical and X- ray diffraction method. The structure of crystalline HF is modeled by (HF)_n chains (n =2, 3,...,20, by an (HF)₄₅ cluster consisting of five (HF)₉ chains, and by an (HF)₁₀₈ cluster consisting of twelve (HF)₉ chains with nearly zero dipole moment. The quantum chemical calculations of the clusters are performed by the semiempirical AM1 method, which is most suitable for electronic structure investigations of hydrogen fluoride, as shown by comparing the X- ray experimental and theoretical spectra. The theoretical X- ray spectra are also compared with the experimental FK_α spectra of gaseous and solid hydrogen fluoride. For more detailed studies of electronic interactions in crystalline HF, fragrnent analysis of MOs of the clusters with respect to the MOs of the central molecule is carried out. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 686–695, July–August, 1997.     

The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Natural bond orbital analysis of some para-substituted N-nitrosoacetanilide biological molecules

Theoretical study of several para-substituted N-nitrosoacetanilide biological molecules has been performed using density functional B3LYP method with 6-31G(d,p) basis set. Geometries obtained from DFT calculation were used to perform natural bond orbital analysis. The p characters of two nitrogen natural hybrid orbital (NHO) σ _N3–N2 bond orbitals increase with increasing σ _p values of the para substituent group on the benzene, which results in a lengthening of the N₃–N₂ bond. The p characters of oxygen NHO σ _O1–N2 and nitrogen NHO σ _O1–N2 bond orbitals decrease with increasing σ _p values of the para substituent group on the benzene, which results in a shortening of the N₂=O₁ bond. It is also noted that decreased occupancy of the localized σ _N3–N2 orbital in the idealized Lewis structure, or increased occupancy of s_N3-N2^* \sigma_{\rm N3-N2}^{\ast} of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are also related with the resulting p character of the corresponding nitrogen NHO of σ _N3–N2 bond orbital.     

Structure of X-ray photoelectron, emission, and conversion spectra and molecular orbitals of uranium compounds

The fine structure of X-ray photoelectron spectra of uranium compounds in the range of electron binding energies from 0 to ∼50 eV is largely determined by the electrons of the outer and inner valence molecular orbitals arising from the valence atomic shells, including the U6p and Lns low-energy occupied atomic shells. This result is in agreement with the data of the electronic structure calculations of these compounds and confirmed by the nuclear electron (conversion) and X-ray emission spectroscopic investigations. It is shown that the fine structure of X-ray photoelectron spectra associated with the electrons of inner valence molecular orbitals makes it possible to judge the participation of the electrons of the occupied atomic shells in chemical bonding, the structure of the nearest environments of the atom, and the bond lengths in the compounds. The overall contribution of the electrons of these molecular orbitals to the absolute value of binding energy may prove to be comparable to the contribution of the electrons of the outer valence molecular orbitals to atomic bonding. This is a new and important fact in chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1037–1046, November–December, 1998.     

Calculation of carbon-carbon bond deformation curves for fluorinated ethylenes and the corresponding carbocations

Carbon-carbon bond deformation curves for fluorinated ethylene molecules and the corresponding carbocations were calculated by the ab initio self-consistent field method in the 5-31G basis set. The maximum force required for bond cleavage was taken as a criterion for bond strength. It has been found that for ethylene, replacement of hydrogen by fluorine insignificantly strengthens the C=C bonds in symmetric molecules. However, in molecules with an asymmetric arrangement of fluorine atoms, the bond is slightly weakened due to different charges on the carbon atoms. The configuration of the corresponding carbocations also depends on the bond polarity: an assymmetric distribution of electron density in the C=C bond region leads to the formation of σ-complexes, while a symmetric distribution of electron density (pure covalent bonding) gives π-complexes. Since the carbon-carbon bond in the σ-complexes is essentially weaker, one should expect significant weakening of the bond in high-acidity media if the bond exhibits any kind of asymmetry (chain branching, defects, etc.). For the considered molecules, an antibatic correlation has been established between the strength criterion F_max (unlike the dissociation energy) and the bond length. Institute of Physical Chemistry, Russian Academy of Sciences, Moscow. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 1, pp. 34–41, January–February, 1995. Translated by I. Izvekova     

Structure of X-ray photoelectron and X-ray emission spectra of ThO2 and ThF4 due to electrons of molecular orbitals

The fine structure of the X-ray photoelectron and O_4,5(Th) X-ray emission spectra of the low-energy (0 …) ∼50eV) electrons of thorium in ThO₂ and ThF₄ is studied. It is established that both outer (0 … ∼15 eV) and inner (15… ∼50 eV) valence molecular orbitals, which are mostly due to the Th6p and O(F)2s shells of the neighboring thorium atoms and ligands, are formed in these compounds. Translated fromZhumal Struktumoi Khimii, Vol. 39, No. 6, pp. 1052–1058, November–December, 1998.     

The Electronic Structure and Bonding of the First p-Block Paddlewheel Complex,Bi<Subscript>2</Subscript>(trifluoroacetate)<Subscript>4</Subscript>, and Comparison to d-Block Transition Metal Paddlewheel Complexes: A Photoelectron and Density Functional Theory Study

The photoelectron spectrum and a density functional computational analysis of the first p-block paddlewheel complex, Bi₂(tfa)₄, where tfa = (O₂CCF₃)⁻, are reported. The photoelectron spectrum of Bi₂(tfa)₄ contains an ionization band between the region of metal-based ionizations and the region of overlapping ligand ionizations that is not seen in the photoelectron spectra of d-block paddlewheel complexes. This additional ionization arises from an a_1g symmetry combination of the tfa ligand orbitals that is directed for σ bonding with the metals, and the unusual energy of this ionization follows from the different interaction of this orbital with the valence s and p orbitals of Bi compared to the valence d orbitals of transition metals. There is significant mixing between the Bi–Bi σ bond and this a_1g M–L σ orbital. This observation led to a re-examination of the ionization differences between Mo₂(tfa)₄ and W₂(tfa)₄, where the metal–metal σ and π ionizations are overlapping for the Mo₂ molecule but a separate and sharp σ ionization is observed for the W₂ molecule. The coalescing of the σ and π bond ionizations of Mo₂(tfa)₄ is due to greater ligand orbital character in the Mo–Mo σ bond (∼7%) versus the W–W σ bond (∼1%). In tribute to F. Albert Cotton for sharing the beauty of symmetry and the joy and excitement in the exploration of metal–metal bonds.     

X-ray spectra and electronic structure of solid ammonia

Intermolecular interactions in solid ammonia are investigated in a combined X-ray spectral and quantum chemical study. Theoretical NK_α spectra are constructed on the basis of MNDO calculations of the ammonia molecule and (NH₃)₇ and (NH₃)₁₃ clusters modeling solid ammonia; the spectra are in satisfactory agreement with the experimental X-ray spectra. Fragment analysis of the clusters with respect to the central ammonia molecule is carried out. It is shown that intermolecular electronic interactions in solid ammonia are most effective in MOs of e symmetry (σ-binding of nitrogen and hydrogen atoms). The fragment 2a₁ orbital contributes to the MO structure of the clusters to the least extent. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 721–726, July–August, 1996. Translated by L. Smolina     

Photoelectron spectrum and quantum chemical analysis of the structure of bis(1,3,6-trimethyluracilyl-5)methane

Photoelectron spectra of bis(1,3,6-trimethyluracilyl-5)methane (I) and 1,3,6-trimethyluracil (II) were studied; AM1 optimization of geometric characteristics was carried out. The total energy minimum and the best agreement between the values of IP_m and -ɛ_m were obtained for conformations with nearly orthogonal location of uracilyl fragments. In such conformations, the highest occupied orbitals are pseudodegenerate. To interpret the photoelectron spectra, we employed ab initio calculations in STO-3G and 4-31G basis sets. For uracil and its derivatives, all methods give the π, π, n⁻, n⁺, π sequence of the highest orbitals. A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 1, pp. 102–107, January–February, 1995. Translated by L. Smolina     

A study of the electronic structure of the hexafluorobenzene and pentafluorobenzene molecules by ultrasoft X-ray emission spectroscopy

The electronic structure of the hexafluorobenzene and pentafluorobenzene molecules was studied by ultrasoft X-ray emission spectroscopy. The FK _α and CK _α spectra of these compounds in the gas phase were obtained. The results of quantum-chemical calculations performed at the RHF/STO-6G//6-31G level were used to construct the theoretical spectra. The highest occupied molecular orbitals were found to consist largely of the 2p _π carbon atomic orbitals. The contribution of fluorine orbitals was small. π-Type interactions mainly involved deeper valence orbitals.     

Quantum chemical and x-ray spectral studies of the structures of superstoichiometric fluorocarbons

The possibility of hole formation in the structures of superstoichiometric fluorocarbons is studied. Different geometries are modeled by removing one, two, or six CF groups from the stoichiometric fluorocarbon lattice. The positions of fluorine atoms in the internal CF₂ groups are optimized using the semiempirical MNDO method. The quantum chemical calculations of fluorocarbon clusters containing holes of different geometries suggest the preferential formation of six-center hole structures in fluorocarbon lattices. The X-ray emission CK_α-spectra of the superstoichiometric CF_x (x=1.20 and 1.33) samples are obtained. Based on the cluster calculations, theoretical CK_α-spectra of CF_x are constructed. A comparison of the theoretical and experimental results shows that the spectra of the superstoichiometric fluorocarbons are characterized by a short-wave maximum, whose intensity increases with x. Deceased Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1072–1080, November–December, 1996. Translated by I. Izvekova     

X-ray and quantum chemical study of the electronic structure of glycine and its metal complexes

OK_α spectra of glycine and some transition metal complexes with glycine ligands were obtained. The electronic structure of the glycine zwitterion is calculated by a quantum chemical method, and a theoretical X-ray spectrum of the glycine molecule is constructed. The nature of the metal-ligand bond in the compounds is discussed on the basis of experimental spectra and theoretical calculations. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Moscow State Academy of Light Industry, Novosibirsk Branch. Moscow State Academy of Light Industry. Novosibirsk State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 112–116, July–August, 1994. Translated by L. Smolina     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of fluorobenzene was investigated by X-ray emission spectroscopy (using the F−Kα- and C−Kα-spectra) and quantum-chemical MNDO calculations. Molecular orbitals of fluorobenzene were compared with those of benzene and hydrogen fluoride. The P_π−p_π-interaction between the phenyl ring and the fluorine atom in the fluorobenzene molecule is weak for both the outer and inner π levels. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1454–1460, August, 1997.     

X-ray spectral study of the nature of electronic interactions in transition metal dithiocarbamates

SK_β spectra are studied for a series of nickel(II), zinc(II), and cadmium(II) dithiocarbamate complexes. Model quantum chemical calculations of the electronic structure of planar chelate rings are reported. It is shown that the metal-ligand interaction forming a coordination bond between the dithiocarbamate ligand and the metal mainly involves the nonbonding n-electrons and the weakly bonding π-electrons localized on the sulfur atoms. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1121–1126, November–December, 1998.     

Isomerism in OBe3F3 + cation: anab initio study

Ab initio MP2/6-31G^*//HF/6-31G^*+ZPE(HF/6-31G^*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe₃F₃ ⁺ cation detected in the mass spectra of μ₄-Be₄O(CF₃COO)₆ were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol⁻¹ and those of the remaining four structures lie in the range 20–40 kcal mol⁻¹. Two most favorable isomers, aC _2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC _3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol⁻¹, except for the pyramidalC _3v isomer (∼16 kcal mol⁻¹). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol⁻¹). A planarD _3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe₃H₃ ⁺ cation, a hydride analog of the OBe₃F₃ ⁺ ion; the energies of the remaining five isomers are more than 25 kcal mol⁻¹ higher. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999.     

Synthesis and structures of ethoxymethylene derivatives of 1,3-diethoxycyclohexenylium perchlorate

1,3-Diethoxy-4,6-di(ethoxymethylene)-5,5-dimethylcyclohexenylium perchlorate (6a) was synthesized from 1,3-diethoxy-5,5-dimethylcyclohexenylium perchlorate by successive introduction of ethoxymethylene groups. Perchlorate6a was also obtained from dimedone in one preparative step. Compound6a was studied by X-ray diffraction analysis. The bond lengths in the fragment containing the C(1)–C(4) and C(6) atoms of the six-membered ring of the cation are substantially equalized and close to the C_arom-C_arom bond length. Apparently, this fact indicates that the positive charge is primarily delocalized over the aromatic system. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2300–2304, November, 1998.     

Investigation of azomethine group vibrations in aromatic schiff bases and their N-oxides by vibrational spectroscopy methods

The effects of fluorine substitution in benzene rings and quaternization of the nitrogen atom in Schiff bases of the general formula R−CH=N−R′ on the vibration characteristics of the azomethine group are analyzed. Normal vibrations and intensities of IR bands are calculated. It is shown that the spectroscopic behavior of the C=N bond is almost independent of the electronic effects of substituents in the benzene rings of the molecules studied, and the changes in the intensities of ν_C=N bands are caused by interactions between vibrations. This points to the stability of this bond in the series of molecules under consideration. For diphenylnitrones, N→O vibrations are identified. L. M. Litvinenko Institute of Physical Organic Chemistry and Carbon Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 310–315, March–April, 1995. Translated by L. Smolina     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of the phenol molecule in the gas phase was studied by X-ray emission spectroscopy (using the O-Kα and C-Kα spectra). MNDO calculations were performed, which made it possible to construct theoretical spectra and interpret experimental spectra. The structure of the molecular orbitals of phenol was compared with those of benzene and water. The π-interaction of the phenyl fragment with the oxygen-containing substituent was investigated. The contribution of the 2p atomic orbital of the oxygen atom to the π-HOMO of phenol is considerably less than that to lower-lying orbitals. For Part 3, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2187–2193, December, 1997.