Orbitals Of Anions Research Articles

Low-energy electron collisions with O2: Test of the molecularR-matrix method without diagonalization

Electron collisions with O${}_{2}$ at scattering energies below 1 eV are studied in the fixed-nuclei approximation for a range of internuclear separations using the ab initio molecular R-matrix method. The ${}^{2}{\ensuremath{\Pi}}_{\mathrm{g}}$ scattering eigenphases and quantum defects are calculated. The parameters of the resonance and the energy of the bound negative ion are then extracted. Different models of the target that employ molecular orbitals calculated for the neutral target are compared with models based on anionic orbitals. A model using a basis of anionic molecular orbitals yields physically correct results in good agreement with experiment. An alternative method of calculation of the R matrix is tested, where instead of performing a single complete diagonalization of the Hamiltonian matrix in the inner region, the system of linear equations is solved individually for every scattering energy. This approach is designed to handle problems where diagonalization of an extremely large Hamiltonian is numerically too demanding.

Manifestation of dynamic Jahn–Teller distortions and surface interactions in scanning tunnelling microscopy images of the fullerene anion C−60

Using scanning tunnelling microscopy (STM), it is possible to observe detailed structure of the molecular orbitals (MOs) of fullerene anions C−60. However, understanding the experimental observations is not straightforward because of the inherent presence of Jahn–Teller (JT) interactions, which (in general) split the MOs in one of a number of equivalent ways. Tunnelling between equivalent distortions means that any observed STM image will be a superposition of images arising from the individual configurations. Interactions with the surface substrate must also be taken into account. We will show how simple ideas involving a symmetry analysis and Hückel molecular orbital theory can be used to understand observed STM images without need for the more usual but more complicated density functional calculations. In particular, we will show that when the fullerene ion is adsorbed with a pentagon, hexagon or double-bond facing the surface, STM images involving the lowest unoccupied molecular orbital (LUMO) can be reproduced by adding together just two images of squares of components of the LUMO, in ratios that depend on the strength of the JT effect and the surface interaction. It should always be possible to find qualitative matches to observed images involving any of these orientations by simply looking at images of the components, without doing any detailed calculations. A comparison with published images indicates that the JT effect in the C−60 ion favours D3d distortions.

X-ray absorption spectra of the spinel LiCu 0.5Mn 1.5O 4

X-ray absorption spectra of the spinel LiCu 0.5Mn 1.5O 4 were recorded at the Cu K and L 3, Mn K and L 3, and O K edge. The spectra are aligned on a common energy scale with the aim to establish an experimental picture of the conduction band structure. The fine structures observed a few electron volts around the absorption thresholds are discussed in terms of hybridisation of cation and anion orbitals. Emphasis is put on the identification of spectral features correlated with the presence of lithium on the tetrahedral sites of the spinel structure. The consequences of lithium insertion/extraction on the intensities of these spectral structures are discussed. Previous studies by X-ray absorption spectroscopy of lithium insertion/extraction in various spinels are found to agree with our expectations.

Magnetism of CaAs, CaP, and CaN half-metals

First principles calculations indicate that CaAs, CaP, and CaN compounds are ferromagnets. Analysis of these and other II–V compounds that do not contain transition metal atoms shows that their magnetism derives from the spin polarization of the atomic p(anion) orbitals, which persists in the compounds of Ca, Sr, and Ba due to their large atomic radii.

Structural, electrical, and magnetic properties of a series of molecular conductors based on BDT-TTP and lanthanoid nitrate complex anions (BDT-TTP = 2,5-Bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene).

The platelike crystals of a series of novel molecular conductors, which are based on the pi-donor molecules BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) with a tetrathiapentalene skeleton and lanthanide nitrate complex anions [Ln(NO3)x](3-x)(Ln = La, Ce, (Pr), Tb, Dy, Ho, Er, Tm, Yb, Lu) with localized 4f magnetic moments, were synthesized. Except for the Ce complex, the salts were composed of (BDT-TTP)(5)[Ln(NO(3))(5)] and were isostructural. Even though the Ce crystal had a different composition, (BDT-TTP)(6)[Ce(NO(3))(6)](C(2)H(5)OH)(x)() (x approximately 3), the crystals all had the space group P(-)1. Although the X-ray examination of the Pr salts was insufficient, the existence of two modifications was suggested in these systems by preliminary X-ray examination. Previously, we reported the crystal structures and unique magnetic properties of (BDT-TTP)(5)[Ln(NO(3))(5)] (Ln = Sm, Eu, Nd, Gd). Thus, by combining the results of this work with previous one, we for the first time succeeded in obtaining a complete set of organic conductors composed of the identical pi-donors (BDT-TTP in this case) and all the lanthanide nitrate complex anions (except the complex with Pm(3+)). The crystals were all metallic down to 2 K. Electronic band structure calculations resulted in two-dimensional Fermi surfaces, which was consistent with their stable metallic states. Except for the Lu complex, which lacked paramagnetic moments, the magnetic susceptibilities were measured on the six heavy lanthanide ion complex salts by a SQUID magnetometer (Ln = Tb, Dy, Ho, Er, Tm, Yb). The large paramagnetic susceptibilities, which were caused by the paramagnetic moments of the rare-earth ions, were obtained. The Curie-Weiss law fairly accurately reproduced the temperature dependence of the magnetic susceptibilities of (BDT-TTP)(5)[Ho(NO(3))(5)] in the experimental temperature range (2-300 K) and a comparatively large Weiss temperature (|THETAV;|) was obtained (THETAV;(Ho) = -15 K). A Weiss temperature (THETAV;(Tm) = -8 K) was also obtained for Tm. The |THETAV;| values of other (BDT-TTP)(5)[Ln(NO(3))(5)] salts and (BDT-TTP)(6)[Ce(NO(3))(6)](C(2)H(5)OH)x(x approximately 3) were as follows: |THETAV;|/K = 4 (Er), < or =2 (Ce, Tb, Dy, Yb). The comparatively strong intermolecular magnetic interaction between Ho(3+) ions, which was suggested by the |THETAV;| value, is inconsistent with the traditional image of strongly localized 4f orbitals shielded by the electrons in the outer 5s and 5p orbitals. The dipole interactions between Ln(3+) ions causing the Curie-Weiss behavior and the comparatively large THETAV; value of (BDT-TTP)(5)[Ho(NO(3))(5)] is inconsistent with the data, since the complexes exhibit isostructural properties and there is not a clear relationship between the magnitudes of THETAV; values and those of magnetic moments. Therefore, it is possible that the 4f orbitals of Ho atom are sensitive to the ligand field, which will have an effect on the orbital moment of the Ho(3+) ion and/or produce a small amount of mixing between 4f and ligand orbitals to give rise to "real" intermolecular antiferromagnetic interaction through intermolecular overlapping between pi (BDT-TTP) and ligand orbitals of lanthanide nitrate complex anions.

Distance Dependence of the Mn-Mn Exchange Interaction in IV-VI Semimagnetic Semiconductors

In the present paper we calculate the exchange interaction between two manganese ions in IV-VI semiconductors with the rocksalt structure. The method of calculations is based on the fourth order perturbation theory with respect to hybridization between band states and localized d orbitals of Mn ions. This hybridization is described by three Harrison integrals: V p d σ , V p d π , and V s d σ . The band states of IV-VI semiconductor are obtained from the semiempirical tight binding model built from s and p orbitals of cations and anions. The resulting exchange term in the Hamiltonian is of the form - Σ i , j = x y z J i j S 1 i S 2 j , however nondiagonal terms of the exchange integral tenso J i > not = j are very small. The dependence of J i j on the Mn-Mn distance is non-monotonic. We also discuss the influence of the local crystal deformations on the exchange integral.

On the role of exchange interaction in magnetic ordering and conduction of manganates

A model of chemical bonding between ions in manganites involving covalent one-electron $\sigma$ bonding is suggested. The covalent one-electron $\sigma$ bonding gives rise to a strongly correlated state of electrons resulting from the exchange interaction between electrons when they are simultaneously at cation and anion orbitals. The manifestation of the correlatred state is the spin and spatial ordering of the electrons resulting in the formation of a spin- ordered electron lattice. The conductivity of manganites in this model is the consequence of displacement of the electron lattice (or its part) from one localization site to another and depends on the type of spin ordering of the electrons in the electron lattice and the localization energy determined by the energy of the one-electron $\sigma$ bond. The model also assumes a strong polarization of an anion by cations, which facilitates the 3s2p hybridization of the anion and transition of one of the pairs of 2p electrons from the singlet state into the triplet state. This transition leads to formation of the spin-polarized electron lattice (electron spins are parallel) and ferromagnetic ordering of manganese ions. In the model, the effect of colossal magnetoresistance is explained by a change of the conductivity mechanism on application of an external magnetic field, i.e., transition from the conductivity mechnism typical of an ionic crystal to the conductivity provided by the spin-polarized electron lattice.

Synthesis and Electronic Structures and Linear Optics of Solid State Compound SrB2O4

AbstractThe cluster (SrB2O4)2 existing in crystalline states is employed to model the electronic structure and linear optical properties of solid state compound SrB2O4. This compound is synthesized by high temperature solution reaction, and it crystallizes in the orthorhombic space group Pbcn with cell dimensions a = 1.1995(3), b = 0.4337(1), c = 0.6575(1) nm, V = 0.34202 nm3, and Z = 4, μ = 15.14 cm−1, Dcaled = 3.36 g/cm3. The dynamic refractive indices are obtained in terms of INDO/SCI following combination with the Sum‐Over‐States method. A width of the calculated gap is 4.424 eV between the valence band and conduction band, and the calculated average refractive index is 1.980 at a wavelength of 1.065 μm. The charge transfers from O2‐ anion orbitals to Sr2+cation orbitals make the significant contributions to linear polarizability in terms of analyses of atomic state density contributing to the valence and conduction bands.

Syntheses, Crystal and Electronic Structures, and Linear Optics of LiMBO3 (M = Sr, Ba) Orthoborates

The syntheses, crystal and electronic structures, and linear optical properties of the new orthoborates LiMBO3 (M = Sr, Ba) are reported here. These compounds, which crystallize in the monoclinic space group P21/n with cell dimensions a = 6.476(2), b = 6.684(3), c = 6.843(3) Å, β = 109.41(3)°, and Z = 4 for M = Sr, a = 6.372(1), b = 7.022(3), c = 7.058(1) Å, β = 113.89(1)°, and Z = 4 for M = Ba, are modeled in terms of the cluster units (LiMBO3)2. The calculated electronic structures show that the top of the valence band consists of mostly the O 2p orbitals and the bottom of the conduction band consists of cationic orbitals. The dynamic refractive indices of these orthoborates are obtained in the framework of the INDO/SCI approximation together with the “sum-over-states” method. It is found that the refractive index is larger for LiSrBO3 than for LiBaBO3 and the charge transfer from O2- anionic orbitals to cationic orbitals appears to provide significant contribution to the linear polarizability of these compounds.

Electronic Structures and Linear Optics of A2B2O5 (A = Mg, Ca, Sr) Pyroborates

The electronic structure and linear optical properties of the pyroborate A2B2O5 (A = Mg, Ca, Sr) compounds are reported here. These compounds, which crystallize with four formula units in the monoclinic space group P21/c, are modeled in terms of the cluster units (A2B2O5)2. The calculated electronic structures show that the top of the valence band consists of mostly the O-2p orbitals and the bottom of the conduction band consists of cationic orbitals. The dynamic refractive indices of these pyroborates are obtained in the framework of the INDO/SCI approximation together with the sum-over-states method. It is found that the refractive index increases with an increase of alkaline-earth cationic radius for pyroborates, and the charge-transfer for O2- anion orbitals to A2+ cation orbitals appears to provide a significant contribution to the linear polarizability of these compounds.

Crystal and Electronic Structures and Linear Optics of Strontium Pyroborate

The compound Sr2B2O5has been synthesized by high temperature solution reaction. It crystallizes in the monoclinic space groupP21/cwith cell dimensionsa=7.719(4),b=5.341(1),c=11.873(2)Å,β=92.71(2)°,V=488.9(3) Å3, andZ=4,Dcalc=3.76 g/cm3. The structure comprises SrO7and SrO6polyhedra and B2O5groups. The electronic structure has been calculated by the INDO/S method for this compound, and the dynamic refractive indices have been obtained in terms of INDO/SCI following combination with the sum-over-states method. The calculated energy gap is 4.26 eV between the upper valence band and lower conduction band, and the calculated average refractive index is 1.791 at a wavelength of 1.065 μm. The charge transfers from O2−anion orbitals to Sr2+cation orbitals make the significant contributions to linear polarizability in terms of analyses of atomic state density contributing to the valence and conduction bands.

Characterization of molecular shape resonances using different decouplings of the dilated electron propagator with application to 2Π CO− and 2B2g C2H4− shape resonances

The zeroth (Σ0), second order (Σ2), quasiparticle second order (Σ2q), diagonal two-particle one-hole Tamm Dancoff approximation (Σ2ph-TDA) and the quasiparticle diagonal 2ph-TDA (Σq2ph-TDA) decouplings have been applied to investigate the 2Π CO− and 2B2g C2H4− shape resonances. An examination of the resonant roots and the corresponding Feynman Dyson amplitudes (FDAs) reveals that the most economic and effective description is offered by the second order decoupling. The more demanding diagonal two-particle one-hole Tamm Dancoff approximation (2ph-TDA) is shown to be less effective and the quasiparticle decouplings are shown to be no better than the zeroth order (bivariational self-consistent field) approximation in the description of molecular shape resonances. The correlation and relaxation effects incorporated by the Σ2 and Σ2ph-TDA decouplings are shown to assist resonance formation by lowering the antibonding nature of the lowest unoccupied molecular orbitals (LUMOs) on the real line and by turning these into anionic diffuse orbitals suitable for metastable electron attachment for the optimal value of the complex scaling parameter. The use of complex resonance energies calculated here to construct a nonempirical optical potential for the investigation of vibrational dynamics of these resonances is suggested.

Electronic structure of Li? and F? calculated directly in momentum space

A self-consistent field method is applied to compute directly in momentum space the electronic structures of the bound anions Li− and F− at the Hartree-Fock level. The convergence towards the Hartree-Fock limit, starting from STO-3G, 3–21G, 3–21+G and 6–311+G AO's, is stable and monotonous. Substantial improvement in the quality of the anion orbitals is noted already after one iteration. Particularly interesting is the efficiency with which the method modifies and improves the shape of the trial functions.

Theoretical estimation of the maximal value of the first, second and higher electron affinity of chemical compounds

A method for an estimation of the maximal values of the first, second, third, and higher electron affinities (EAs) of chemcial compounds is suggested. The estimation is based on the known data of energies of the highest occupied nonbonding molecular orbitals of anions. The maximal EA values for the MF k compounds are estimated for every k from the results of quantum chemical calculations. It is found that hexafluorides may possess a second positive EA; i.e., their doubly charged anions may, in principle, be stable to the loss of an electron in anisolated state

An Oxygen Exchange of Arsenate and Alkylarsonate Ions with Water

Abstract The oxygen exchange rates, R′s, between the alkylarsonates (methyl- and butyl-) and water were measured over the pH range greater than 9 and at 30 °C. The rates were analyzed in terms of the rate law, R=k1[R′-AsO3H−]+k2[R′-AsO32−] (R′-; alkyl-) to obtain k1 to be 6.64×10−4 s−1 for methyl- and 4.38×10−4 s−1 for butylarsonate ions, respectively. The first-order oxygen exchange rate constants, k1’s, on the alkylarsonate univalent anions (methyl and butyl) are almost the same values, which indicates that the steric effect on the rate is absent. The rate constants, k1’s, correlate well with energy levels of the lowest unoccupied molecular orbitals (ELUMO’s) of univalent anions of arsenic, arsenious, and alkylarsonic acids.

Electron paramagnetic resonance studies of some ternary oxides of copper (II)

The electron paramagnetic resonance (EPR) of ternary oxides of Cu(II) has been studied between 4.2 and 300 K. The systems include those with 180 degrees Cu-O-Cu interactions (such as Ln2CuO4, Sr2CuO2Cl2, Sr2CuO3 and Ca2CuO3) or 90 degrees Cu-O-Cu interactions (such as Y2Cu2O5 or BaCuO2) as well as those in which the Cu2+ ions are isolated (such as Y2BaCuO5, La1.8Ba1.2Cu0.9O4.8 and Bi2CuO4). The change in the EPR susceptibility as a function of temperature is compared with that of the DC magnetic susceptibility. Compounds with extended 180 degrees Cu-O-Cu interactions which have a low susceptibility also do not give EPR signals below room temperature. For compounds such as Ca2CuO3 with one-dimensional 180 degrees Cu-O-Cu interactions a weak EPR signal is found the temperature dependence of which is very different from that of the DC susceptibility. For Y2BaCuO5 as well as for La1.8Ba1.2Cu0.9O4.8 the EPR susceptibility as well as its temperature variation are comparable with those of the static susceptibility near room temperature but very different at low temperatures. Bi2CuO4 also shows a similar behaviour. In contrast, for Y2Cu2O5, in which the copper ions have a very distorted nonsquare-planar configuration, the EPR and the static susceptibility show very similar temperature dependences. In general, compounds in which the copper ions have a square-planar geometry give no EPR signal in the ground state (0 K) while those with a distortion from square-planar geometry do give a signal. The results are analysed in the light of recent MS Xalpha calculations on CuO46- square-planar clusters with various Cu-O distances as well as distortions. It is suggested that in square-planar geometry the ground state has an unpaired electron in anionic orbitals which is EPR inactive. Competing interactions from other cations, an increase in Cu-O distance or distortions from square-planar geometry stabilise another state which has considerably more Cu 3d character. These states are EPR active. Both these states, however, are magnetic. For isolated CuO46- clusters the magnetic interactions seem to involve only the states which have mainly anionic character.

Covalency parameters in Co 0.55Cu 0.45Cr 2S 4−ySe y

From the Anderson's-Kanamori theory of superexchange the transfer integrals, after taking into account the observed exchange constants, have been estimated. With the use of the conventional molecular orbital model the admixture, the covalent-mixing parameters, the electron spin transfer coefficients as well as the total charge transfer have been determined. The anomalous features of the e g(t 2g)-s bonding with s orbitals of anions could explain the influence of the ligand field and the overlap of the orbitals on the magnetic properties of the spinels under study.

Anomalous temperature dependence of fundamental gap of AgGaS 2 and AgGaSe 2 chalcopyrite compounds

We have studied the temperature variation of the fundamental gap energy of the chalcopyrite-like ternary semiconductors AgGaS 2 and AgGaSe 2 from reflectivity measurements performed between 5 K and room temperature. The temperature evolution of the gap energy is found to be clearly different from that observed for the II-VI and III-V zincblende-like binary semiconductors. The temperature coeffecients are positive under 80 K and negative above 100 K with values clearly smaller than those encountered for the II-VI and III-V binaries. Results are discussed taking into account the hybridization between the d orbitals of Ag and the p orbitals of anion (S or Se).

The electronic density of states of disordered compounds

The electronic properties of liquid alkali metal (Li, Na, K, Rb, Cs)-group IV (Si, Ge, Sn, Pb) alloys have been discussed by the authors using a tight-binding model. Only anion orbitals (=group IV) are taken into account. Disorder is described by a pseudo-lattice, which takes into account local coordination in one of the sublattices (cation or anion) only. It is shown that this approximation is consistent with the usual valence rules used by structural chemists for crystalline structures. The solutions for the density of states of the tight-binding Hamiltonian are studied for a number of pseudo-lattices. The approach outlined is capable of describing the electronic density of states due to various degrees of local order in a sublattice. Some of the peculiarities occurring in the solution of the density of states of certain pseudo-lattices, such as poles outside the band, are discussed.

Semiempirical model of the exchange polarization mechanism of transferred hyperfine interactions in ionic radicals

This paper presents a semiempirical method for calculating metal hyperfine interaction constants in ionic radicals whose zero-order wave function places the unpaired electron completely on the anion with zero overlap between it and the relevant cation orbitals. The mechanistic basis of the calculation is spin polarization of closed-shell anion orbitals by their exchange interaction with the unpaired electron, followed by partial transfer of the resulting unpaired electron density in these anion orbitals to the cation orbitals via overlap. The exchange polarization is described by a perturbation theory expansion in anion excited states, which expansion is summed by introducing semiempirical average excitation energies. The contribution of direct exchange polarization of the cation orbitals to the cation hyperfine interactions is found to be negligible, due primarily to the very large excitation energies of the cation core orbitals. Calculated results are compared with experiment for the ionic radicals Na(C=CH2) and NaO2.