Covalency Effects Research Articles

The covalence effect of the gyromagnetic factor of [formula omitted

In this paper, formulas for the optical spectrum and the gyromagnetic factor for d 7 ions in T d symmetry with a slight triangle perturbation are established by using a covalent model, in which the covalence contribution has been considered. Using the model, we have calculated the optical spectra and the g -factor of CdS : Co 2 + . The calculation results agree well with the experimental findings. It suggests that covalent model should be adopted for the calculation of the optical spectrum and the EPR parameters for Co 2 + ion in covalent crystal.

Theoretical studies of the Spin Hamiltonian Parameters for the orthorhombic Pr4+ ion in SrCeO3 crystal

Abstract The perturbation formulae of the Spin Hamiltonian Parameters (EPR parameters anisotropic gyromagnetic factors g x , g y , g z and hyperfine structure constants A x , A y , A z ) for the lowest Kramers doublet of 2 F 5/2 of 4f 1 ion in orthorhombic symmetry are established. In these formulae, the contributions of the covalence effects, the admixture between J = 5/2 and J = 7/2 states as well as the second-order perturbation are all included. The crystal field parameters for the studied Pr 4+ center are calculated from the superposition model and the crystal structure data. Based on the perturbation equations and related parameters, the g factors g x , g y , g z and hyperfine structure constants A x , A y , A z for orthorhombic Pr 4+ ion in SrCeO 3 are calculated. The results are in reasonable agreement with the observed values. The results are discussed.

Copper(II) and palladium(II) complexes of 2-amino-5-chlorobenzophenone and 2-(2-hydroxybenzylidene)amino-5-chlorobenzophenone-S-methyl-thiosemicarbazones

The S-methyl thiosemicarbazone derivatives of 2-amino-5-chlorobenzophenone and 2-(2-hydroxybenzylidene)amino-5-chlorobenzophenone (LI and LII) and their copper(II) and palladium(II) chelates were synthesized. The solid compounds, [Cu(LI)Cl]Cl · 2H2O, [Cu(LII)Cl]Cl · H2O, [Pd(LI)Cl]Cl, and [Pd(LII)Cl2], were characterized by elemental analysis, thermogravimetric measurements, electronic, i.r., 1H-n.m.r. and e.p.r. spectroscopy. The ligand behavior of the LI and LII explained by means of spectroscopic data. The experimental X-band e.p.r. spectra recorded from two different copper(II) ion coordinated powder complexes have characteristic line shapes that exhibit the axial symmetry around the paramagnetic ions. The theoretically well fitted spectra prove this case too. The absence of hyperfine signals has been attributed to the strong covalency effects of copper electrons with neighbouring N atoms. Orbital energy levels for magnetic electrons were determined from Spin Hamiltonian parameters. The anisotropic Lande splitting factors were specified in an order of \( {g_{| |}>g_{\bot}>2}\) and this case indicates that the unpaired electrons are located mainly in the \( {d_{x^{2}-y^{2}}}\) orbital.

Comparative first-principles analysis of crystal field splitting, charge transfer energies and covalent effects for Cr2+ and Fe2+ ions in II–VI and III–V compounds

Detailed comparative first-principles study of crystal field splitting, covalent effects and “ligand–metal” charge transfer (CT) energies in ZnS, ZnSe, ZnTe, and CdTe crystals doped with Cr2+ and Fe2+ ions and GaAs, GaP, and InP crystals doped with Fe2+ ions is reported. It is shown that the contribution of ligands to the 3d orbitals of central ion is greater in case of Fe2+ ion. As a consequence, CT energies in the Fe2+-doped crystals are lower than in the Cr2+-doped ones. In addition, theoretical absorption spectra were calculated and compared with experimental data.

High pressure study of ZnSe:Cr2+ crystals: the origin of the 1.25 eV luminescence

High pressure studies of Cr2+ luminescence in ZnSe crystals were performed at low temperatures. Inaddition to the mid-infrared luminescence transitions from the first excited5E level, higher energy luminescence was also observed. These two luminescencebands exhibit similar pressure coefficients but of opposite signs, which identifythe origin of the higher energy band as the transitions from the second excited3T1 level. At higher pressures, increased covalence effects are observed, which result inthe appearance of an additional peak in the luminescence due to changes in the spin–orbit coupling strength.

Jahn–Teller coupling in spinel-type crystals doped with transition metal ions

In this article, we report the theoretical evaluation of the vibronic constants for the spinel-type crystals ZnAl 2S 4, ZnAl 2Se 4, CdAl 2Se 4, and CdIn 2S 4 doped with the transition metal ions Cr 3+, Cr 4+, Ti 2+, and Ti 3+. In these crystals the ions Cr 3+, Ti 3+ and Cr 4+, Ti 2+ occupy positions with octahedral and tetrahedral surrounding correspondingly. In the case of a tetrahedral complex the interaction with full symmetric A 1, tetragonal E and two trigonal vibrations T 2 ( 1 ) and T 2 ( 2 ) is considered, while for the octahedral cluster along with the A 1 and E vibrations only one trigonal vibrational mode is taken into account. The examined semiconductor systems are mainly covalent and application of the crystal field point charge model loses proper exactity. For this reason, we employ the exchange charge model for the crystal field that accounts for the covalence effects and provides relatively simple expressions for the crystal field and vibronic parameters keeping at the same time a reasonable level of accuracy. The vibronic coupling constants are numerically calculated and for each case an appropriate multi-mode Jahn–Teller vibronic problem is found out.

Comparative study of the absorption spectrum of Li2CaSiO4:Cr4+: First-principles fully relativistic and crystal field calculations

Systematic analysis of the energy level scheme and ground state absorption of the Cr4+ ion in Li2CaSiO4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33–50] and recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multielectron (DVME) method [K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii, I. Tanaka, H. Adachi, Phys. Rev. B 64 (2001) 115413]. Using the former method, the values of parameters of crystal field acting on the Cr4+ ion valence electrons were calculated using the Li2CaSiO4 crystal structure data. Energy levels of the Cr4+ ion obtained after diagonalizing the crystal field Hamiltonian are in good agreement with those obtained from the experimental spectra. The latter method is based on the numerical solution of the Dirac equation; therefore, all relativistic effects are automatically considered. As a result, energy level scheme of Cr4+ and its absorption spectra in both polarizations were calculated, assigned and compared with experimental data; energy of the lowest charge transfer transition was evaluated and compared with theoretical predictions for the CrO44- complex available in the literature. The main features of the experimental spectra shape are reproduced well by the calculations. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalent effects play an important role in formation of the spectral properties of Cr4+ ion in the considered crystal.

Effects of vibronic interaction in cyano-bridged clusters containing Mn(III) and Mn(II) ions

The effects of vibronic interaction in the formation of the barrier for magnetization reversal in the cyanide-based binuclear Mn(III)–CN–Mn(II) clusters containing octahedrally coordinated Mn(III) and Mn(II) ions were examined. Along with the orbitally dependent superexchange between the Mn(II) and Mn(III) ions the model includes an axial component of the crystal field, spin–orbit coupling and vibronic interaction operating within the ground 3 T 1 ( t 2 4 ) term of the low-spin Mn(III) ion. The vibronic coupling parameters for the Mn(III) ion in the octahedral surrounding of six carbon ions have been evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The dominating role of the tetragonal vibrations for the 3T 1 term of a single Mn(III) ion was demonstrated. The ability of the investigated complex to act as a Single-Molecule Magnet was analyzed as a function of both the temperature and the strength of vibronic coupling with the tetragonal vibrations. Very strong vibronic interaction strongly facilitates tunneling through the barrier, i.e., suppresses the barrier for magnetization reversal and prevents from long life time of magnetization.

Complex study of the crystal field splitting, “ligand–impurity ion” charge transfer transitions and high lying 4f–6s intraconfigurational transitions for all trivalent lanthanides in Cs 2NaYCl 6 crystal

First-principles fully relativistic analysis of dependence of the 5d orbitals splitting (10 Dq) and several types of electron transitions (including 4f–6s transition and several types of “ligand–impurity ion” charge transfer (CT) transition) energies on the interionic distance has been performed for all trivalent lanthanides in Cs 2NaYCl 6 crystal. Salient feature of the method is that the four-component molecular orbitals (MO) composed of atomic wave functions are used in the calculations; this allows for a direct consideration of the covalent effects. Without introducing any fitting parameter, the power dependencies for 10 Dq and linear dependencies for all considered electron transitions energies on the distance between the rare earth (RE) ions and ligands were obtained. The 10 Dq parameter was shown to depend on distance as 1/ R n , n ≠ 5 for all considered systems. Consideration of the calculated results revealed several trends based on the RE 3+ atomic number and MO diagram for all RE 3+ in Cs 2NaYCl 6. Obtained results can be used for analysis of the lattice relaxation effects and pressure dependence of the absorption spectra of RE 3+-doped chloride crystals.

Frequency upconversion in Pr 3+–Li 2O–TeO 2 binary glass by decay curve analysis

Using the decay curve analysis frequency upconversion under continuous wave NIR laser radiation (∼890 nm) following the non-resonant excitation at higher energies than 1G 4 level has been reported. The decay curve of the antistokes emission ( 3P 0 → 3H 4) shows similar behavior as that of the stokes emission ( 3P 0 → 3H 4) and does not show any rise time attributing to the excited state antistokes absorption assisted by phonon emission. The covalency, bonding parameters and nephalauxetic effect for the present system has also been determined.

Induction of a Heterochiral Helix through the Covalent Chiral Domino Effect Originating in the “Schellman Motif”

We report unique phenomena where the transition from a homochiral helix to a heterochiral helix occurs by increasing the chain length of the l-sequence. Peptides composed of the l-Leu sequences with different lengths and the achiral nona-sequence at the C-terminal side were used here. Conformation of their peptides in solution was investigated mainly by using CD analysis in various solvents, or additionally by IR and NMR. When the l-sequence has a sufficient length, a left-handed helicity was induced in the achiral sequence. Notably, the polymeric l-sequence produced a heterochiral helix that switches the helix sense around the boundary of the chiral/achiral sequence. Energy calculation demonstrated that a stable heterochiral helix favors a bending form, while a homochiral helix takes a relatively straight form. Such a bending form was suggested to be advantageous to solvent effects. The "Schellman motif" has been recognized as a local heterochiral structure in protein helices. We propose a nucleation model of a heterochiral helix through the covalent chiral domino effect derived from the Schellman motif. The present findings not only offer us novel design of a heterochiral helix but also support an elementary model for the origins of homochiral-heterochiral structures from primitive chiral/achiral sequences.

Crystal-field theory ofCo2+in doped ZnO

We present a crystal field theory of transition metal impurities in semiconductors in a trigonally distorted tetrahedral coordination. We develop a perturbative scheme to treat covalency effects within the weak ligand field case (Coulomb interaction dominates over one-particle splitting) and apply it to ZnO:Co$^{2+}$ (3d$^7$). Using the large value of the charge transfer energy $\Delta_{pd}$ compared to the $p$-$d$ hoppings, we perform a canonical transformation which eliminates the coupling with ligands to first order. As a result, we obtain an effective single-ion Hamiltonian, where the influence of the ligands is reduced to the one-particle 'crystal field' acting on $d$-like-functions. This derivation allows to elucidate the microscopic origin of various 'crystal field' parameters and covalency reduction factors which are usually used empirically for the interpretation of optical and ESR experiments. The connection of these parameters with the geometry of the local environment becomes transparent. The experimentally known $g$-values and the zero-field splitting 2D are very well reproduced by the exact diagonalization of the effective single-ion Hamiltonian with only one adjustable parameter $\Delta _{pd}$. Alternatively to the numerical diagonalization we use perturbation theory in the weak field scheme (Coulomb interaction $\gg$ cubic splitting $\gg$ trigonal splitting and spin-orbit coupling) to derive compact analytical expressions for the spin-Hamiltonian parameters that reproduce the result of exact diagonalization within 20% of accuracy.

Polarization dependence of x-ray absorption spectra ofNaxCoO2: Electronic structure from cluster calculations

In order to shed light on the electronic structure of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, and motivated by recent $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}L$-edge x-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a ${\mathrm{CoO}}_{6}$ cluster including all interactions between $3d$ orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the ${\mathrm{O}}^{\ensuremath{-}2}$ oxidation state, and ${\mathrm{Co}}^{+3}$ or ${\mathrm{Co}}^{+4}$. Then, all excited states obtained by promotion of a $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ electron to a $3d$ electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large $\mathrm{Co}\text{\ensuremath{-}}\mathrm{O}$ covalency and cubic crystal field effects, that result in low spin $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}3d$ configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.

Fully relativistic analysis of the absorption spectra of Ca3Sc2Ge3O12:Ni2+

AbstractSystematic analysis of the energy level schemes, ground state absorption (GSA) and covalency effects for the Ni2+ ion in Ca3Sc2Ge3O12 was performed. The recently developed first‐principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi‐electron method (DV‐ME) [K. Ogasawara et al., Phys. Rev. B 64, 115413 (2001)] was used in the calculations. As a result, complete energy level schemes of Ni2+ and its absorption spectra at both possible crystallographic positions (distorted octahedral Sc3+ and tetrahedral Ge4+ positions) were calculated, assigned and compared with experimental data. Energies of the charge transfer (CT) transitions for both positions are estimated. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalency of the chemical bonds between the Ni2+ and O2− ions increases in passing from the hexa‐ to the tetra‐coordinated complex. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence Properties of Red‐Emitting M2Si5N8:Eu2+ (M: Ca, Sr, Ba) LED Conversion Phosphors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Synthesis and EPR studies of metallophthalocyanines containing four carbhexyloxybiphenyloxy substituents

4-(4′-Carbhexyloxybiphenyloxy)phthalonitrile was synthesized by esterification of 4-(4′-carboxybiphenyloxy)phthalonitrile with hexanol in the presence of dicyclohexylcarbodiimide (DCCI). Zn(II), Co(II) and Cu(II) phthalocyanines with four carbhexyloxybiphenyloxy pendant groups on the periphery were prepared from 4-(4′-carbhexyloxybiphenyloxy)phthalonitrile and the corresponding divalent metal salts (Zn(CH 3COO) 2, CoCl 2 and CuCl 2). These new compounds have been characterized by 1H NMR, 13C NMR, FT-IR, UV–vis and mass spectroscopies. The EPR spectra obtained at room temperature indicate axially symmetric character of ligand field at the site of Co(II) and Cu(II) paramagnetic ions. Especially, the Cu(II) coordinated compounds show stronger covalency effects with neighboring N atoms with respect to Co(II) coordinated compounds. Computer programs have been written to determine Spin–Hamiltonian parameters for solid powder form of ion coordinated complexes. Orbital energy levels for magnetic electrons were determined from Spin–Hamiltonian parameters as well.

Fully relativistic calculations of the L2,3-edge XANES spectra for vanadium oxides

Fully relativistic multielectron method based on the numerical solution of the Dirac equation was used to calculate the L2,3-edge X-ray absorption near edge structure (XANES) spectra of VO2, V2O3, and V2O5 crystals. The key-points of the method are: i) usage of the molecular orbitals (MO); ii) absence of any fitting parameters; iii) wide area of application: to any ion in any symmetry; iv) possibility of numerical analysis of the MO composition. The calculated spectra are in a satisfactory agreement with experimental data available in the literature, including the absolute values of the transitions energy, the shape of the absorption bands, and polarization dependence. The assignment of the absorption bands in terms of the electronic configurations was done. The structure of the absorption bands is attributed to the splitting of the vanadium p- and d-orbitals; the magnitude of this splitting is estimated from the spectra. Covalency effects were considered for all hosts; it was shown that the contribution of the oxygen wave functions increases with increasing the vanadium oxidation state. Dependence of the relative positions of the vanadium 3d and oxygen 2p levels and energies of the “ligand–metal” charge transfer transitions on the vanadium oxidation state was analysed.

Stepwise Cope Rearrangement of Cyclo-biphenalenyl via an Unusual Multicenter Covalent π-Bonded Intermediate

Multicenter covalent pi-bonding between pi-conjugated radicals has been recently recognized as a novel and important bonding interaction. The Cope rearrangement of cyclo-biphenalenyl 9 is studied by exploring its potential energy surface with density functional theory (DFT), and it is found that pi-bonding plays a critical role in the rearrangement process. Affected by this, the rearrangement of 9 takes place by a stepwise mechanism through an unusual pi-intermediate 10, of C2h symmetry, which can be characterized as a 2 x 13pi + 2 x 2pi system. The pi-intermediate has a long inter-phenalenyl distance of R approximately 2.8 angstroms, which is shorter than the sum of the van der Waals radii displaying multicenter covalent pi-bonding between the two phenalenyl units. The energy of the pi-intermediate 10 is higher than that of the sigma-bonded reactant 9 by approximately 2 kcal/mol according to the employed spin-restricted DFT. NMR chemical shift calculations support the sigma-bonded 9 as the global minimum. The calculated activation barrier of approximately 6 kcal/mol for the Cope rearrangement is consistent with the stepwise mechanism. A covalent pi-bonding effect in the pi-intermediate 10 is demonstrated indirectly by the shortening of inter-naphthalene distance of the dianion and dication of the cyclophane 14 compared to that of its neutral counterpart. The unusual pi-bonded structure with a long inter-phenalenyl distance becomes the most stable structure for the ethano-bridged derivative 13, which should have observable paramagnetism according to the calculated paramagnetic susceptibility.

Comparative first-principles study of the Ni 2+ absorption spectra and covalence effects in isostructural crystals NiCl 2, NiBr 2 and NiI 2

Systematic ab initio calculations of the ground-state absorption spectra and covalence effects in three isostructural anisotropic crystals with layered structure (NiCl 2, NiBr 2 and NiI 2) are presented. The recently developed first-principles approach to the analysis of the absorption spectra based on the discrete variational multi-electron method (K. Ogasawara et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, energies of the Ni 2+-ion multiplets and absorption spectra of all above-mentioned crystals were calculated, assigned and compared with experimental spectra. By performing analysis of the molecular orbital population, it was shown that the degree of covalence of the chemical bond between Ni 2+ ion and ligands is in the following order: NiI 2>NiBr 2>NiCl 2.

Effects of covalency on thep-shell photoemission of transition metals:MnO

This article discusses the effects of covalency on the p-shell photoemission of transition metals. The treatment of covalency has not been included previously in ab initio theoretical studies of the 2p-shell XPS of transition-metal complexes. In this work, covalent interactions between the metal and ligands are treated on an equal footing with spin-orbit splittings.