Valence Electron Distribution Research Articles

Pseudopotential Study of Bonding in the Superionic Material AgI

The bonding nature of the superionic material AgI is studied by using a pseudopotential theory. The result is compared with that for GaAs, which is a well-known semiconductor. It is found that the real space valence electron distribution between these compounds differs qualitatively, and that the bonding in AgI is determined mainly by the iodine. It is also suggested that the local electronic excitations from the bonding to the antibonding orbitals can trigger the migration of the mobile species, initiating in this way the superionic transport.

Theoretical investigation of the Auger spectra of Mg in various local atomic environments. Part 3. MgCu and MgZn alloys

The energy distribution of valence electrons localized around the atoms of components in the MgCu and Mg&.sbnd;Zn alloys and around the Mg + impurity (with a core hole) is calculated using the linear muffin-tin-orbitals—Green's-function method. For the alloys with different transition-metal content and different lattice structure the theoretical Mg( KL 1 V) and Mg( KL 2,3 V ) spectra are derived from the calculated local partial densities of states. It is shown that the intensity distribution in Auger spectra is more sensitive to the d-metal content than to the geometry of the local environment of the Mg atom which emits an Auger electron. The difference in the shapes of the Mg( KLV) spectra of MgCu and MgZn alloys is due to the fact that the hybridization of the Mg p-states with the d-states of Cu is stronger than it is with the d-states of Zn.

Theoretical investigation of the Auger spectra of Mg in different local atomic environments part 1. Magnesium and aluminium

The energy distribution of valence electrons localized around the Mg and Mg + (with a core hole) impurities in Mg and Al metals is calculated using the linear muffin tin orbital-Green function method. On the basis of calculated results the theoretical Mg KL 1 V and Mg KL 2,3 V spectra are obtained for Mg metal and for Mg-poor MgAl alloy. The partial composition of the spectra and the hybridization of the host and impurity electron states are analyzed. The role of the core hole and the influence of the local atomic environment on the intensity distribution in the Auger spectra are discussed.

Theoretical investigation of the Auger spectra of Mg in different local atomic environments Part 2. Mg-Pd alloys

The energy distribution of valence electrons localized around the atoms of components of Mg-Pd alloys and around the Mg + (with a core hole) impurity is calculated using the linear muffin tin orbital-Green function method. On the basis of calculation results the theoretical Mg KL 1 V and Mg KL 2,3 V spectra are obtained for the MgPd and MgPd 3 alloys and for the Mg impurity in Pd metal. The partial composition and hybridization of the Mg and Pd electron states are analyzed and the interpretation of the intensity distribution in the Auger spectra is given.

Electron distributions in peptides and related molecules. 1. An experimental and theoretical study of N-acetyl-L-tryptophan methylamide

The thermal vibrations and electron density of N-Ac-L-Trp-NHMe have been analyzed using single-crystal X-ray diffraction data measured at 103 K with Mo K alpha radiation to a resolution corresponding to (sin theta max)/ lambda = 1.17 A-1. Measurements of 10,527 reflections gave 4913 unique data [R(int)(magnitude of F2) = 0.019] of which 2641 had I greater than 3 sigma (I). A multipolar atomic density model was fitted [R(magnitude of F) = 0.028] in order to calculate phases for the crystal structure factors and map the valence-electron distribution. The phase problem for determining deformation densities by Fourier synthesis for noncentrosymmetric crystals is discussed. The experimental density agrees well with the theoretical density from an ab initio SCF molecular wave function calculated at the crystallographic molecular geometry with a split-valence basis set. Both the experimental and theoretical analyses confirm that the electron distribution is the same in the two different peptide groups in the molecule. Crystal data: C14H17N3O2, Mr = 259.31, orthorhombic, P2(1)2(1)2(1), Z = 4, F(000) = 522 e from 295 to 103 K; at 295 K, a = 8.152(2), b = 11.170 (2), c = 15.068 (3) A, V = 1372 A3, Dx = 1.26 mg mm-3; at 103 K, a = 8.209 (3), b = 11.016 (2), c = 14.760 (4) A, V = 1135 A3, Dx = 1.29 mg mm-3, mu = 0.083 mm-1 for lambda = 0.7107 A.

Compton scattering studies of the valence electron density distribution in GaAs

Directional Compton profiles have been measured along the (100), (110), (111), (112) and (221) crystallographic axes in GaAs using 412 keV and 59.54 keV gamma -radiation from 198Au and 241Am radioisotope sources, respectively. The results have been compared with the prediction of a pseudopotential calculation both in momentum space and in position space. The pseudopotential approach predicts the anisotropy of the valence electron distribution and is consistent with an increase in the ionic bond character compared with Ge. On the other hand, the total electron distributions are not well described due to the omission of core orthogonalisation terms.

Charge transfer and the57Fe isomer shift in iron alloys

This is a brief report on the present state of our experimental studies on the influence of the valence electron distribution on the57Fe isomer shift in iron-based alloys. The studies are performed by Mossbauer and positron annihilation spectroscopies.

Electron spectroscopy of adsorbates via autoionization of core-tobound excited states: experiment and theory

We use autoionization Spectroscopy after core-to-bound excitation of adsorbed molecules to get information about the spectral distribution of valence electrons in adsorbates and about the nature of shake up satellite states. Examples are presented for a physisorbed system (CO 2/Ni(110)), a weakly chemisorbed system (N 2/Ni(110)) and a strongly chemisorbed system (CO/Ni(110)). It is shown that in the first case detailed calculations of the autolonization rates of the isolated molecule can basically explain the experimental findings. For N2/Ni(110) autoionization spectroscopy is shown to verify an assignment put forward for the photoemission spectrum, i.e. that satellites contribute heavily to the valence ionization of many systems. In the third example, the angular dependences of the autoionization lines are used to deduce Information on the symmetries of ion states.

On the accurate measurement of low-order structure factor amplitudes and phases by quantitative CBED

Over the past four years we have developed the software and computer control of electron microscope data acquisition needed for quantitative electron crystallography. CBED patterns in the systematics orientation (see Fig. 1) are scanned over the slit of a Gatan Model 607 ELS unit, and the energy filtered elastic scattering rocking curves compared with the results of theoretical calculations (see Figs. 2 and 3). The calculations are based on the Bloch wave method, and treat three-dimensional dynamical diffraction from a non-centrosymmetric crystal with absorption and inclined boundary conditions. The Fortran source code has been published, and is available by Bitnet from ZALUZEC @ANLMST. High voltage, sample thickness, absorption coefficients Vg and selected low order structure factors Vg are adjusted(using the most sensitive portion of the pattern for each)for best fit. The resulting Fourier coefficients of crystal potential may be used (via the Mott Formula) to obtain a charge density difference map, which reveals the distribution of valence electrons involved in bonding, as shown for GaAs in figure 4.The partially ionic character of the bond is seen. We find V(111)1/V(111) = 0.05, V1 (220)/V(220) = 0.035, V1 (400)/V (400) = 0.07 and V1 (333)/V(333)=0.08. Results for work on MgO in progress are shown in figs. 2 and 3.

Hartree-Fock-Slater linear combination of atomic orbital calculations of the valence electron distribution in neutral and charged iridium clusters

Determination des configurations atomiques moyennes des agregats Ir 4 et Ir 10 ainsi que celles des agregats ioniques Ir 4 + et Ir 10 + . Determination des populations electroniques

Activation of CO2coordinated to a Ti atom

The different energy contributions which determine the stability of the MCO2 molecules (M = transition metal atoms) are analysed in detail. These are: (1) the polarization of the CO2 closed-shell electrons by the effective metal core charge, (2) the polarization of the metal s-electron opposite the CO2, (3) the charge transfer from the metal (s or d) to the CO2, and (4) the repulsion between the metal d-electrons and the CO2 closed-shell electrons. The TiCO2 molecule is calculated to be quite stable with respect to dissociation into the ground stste M + CO2. The greater stability of TiCO2 in comparison with CrCO2 (which is weakly bonded) can be explained by the difference in the energy term (4). The valence electron distributions in the lowest states of TiCO2 can be represented as: Ti+…(OĊO)- for the quintet η1 o end-on and η2 (o, o) states, Ti+…(OĊO)- for the quintet η2 (c, o) side-on states, or Ti+…(OCO:)- for the triplet η2 (c, o) side-on states.

Electron distribution in silver halides

Solid fast ion conductors such as silver halides and silver chalcogenides have been recently drawing attention, because of their unusually high ionic conductivity in the high-temperature phase at about 200°C. This peculiar property is of particular importance for both research field of codensed matter physics and practical application as solid state sensors. The information of the bonding nature relevant to the electron distribution is desirable, in order to obtain insights into their characteristic property of solid fast ion conductors. An attempt will be made in this paper to review the current information of the bonding nature of silver halides in terms of the electron density distribution. Thus, this paper is primarily concerned with some physical properties of silver halides including the characteristic features of the crystal structure and the phase diagram. The data of alkali halides are also described for comparison. The experimental results of the magnetic susceptibilities of silver halides are summarized from the phenomenological point of view. The magnetic susceptibilities of silver halides were found to be sensitive to the change of temperature and crystal structure. It implies that the valence electrons of silver halides are loosely bound by the ion core. The electron density distribution in Agl and AgBr is also discussed using the results by single crystal X-ray diffractometry and the following points are suggested; in AgBr, the deformation of the charge distribution in bromine ion is much larger than that of silver ion and the radial distribution of valence electrons in bromine ion is enlarged in comparison with that of free ion state. In Agl, charge accumulation is feasible only on the c-directed bond and the difference Fourier map on the (1 1 0) plane indicates the deficiency of electrons around the silver site.

ChemInform Abstract: 99Ru Moessbauer Study of Ruthenium Red and Its Derivatives.

AbstractThe 99Ru Moessbauer spectra of (Ru3(O)2(NH3)14)Cln (n = 6 or 7) and their ethylenediamine substituted derivatives, (Ru3(O)2(NH3)10(en)2)Cln, are recorded at 4.2 K. The results support the previously proposed distribution of valence electrons on the Ru centers for the reduced species (n = 6).

Is the Isotropic Atom—Atom Model Potential Adequate?

Abstract We discuss some of the problems that have frustrated the development of reliable model intermolecular potentials for polyatomic molecules. In particular, the usual assumption of an isotropic atom-atom model potential is analysed, and evidence for its inadequacies is presented. A new approach to designing model potentials, an anisotropic site—site model, is introduced by describing several applications to both small and organic molecules, including molecular dynamics and Monte Carlo simulations. The anisotropy required in an atom—atom potential can be directly linked to the non-spherical features in the valence electron distribution, such as lone pairs and π electrons. An accurate electrostatic model for these effects can be constructed from a distributed multipole analysis of the ab initio wavefunction. The empirically required forms of anisotropy in the repulsion potential can also be qualitatively linked to the molecular electron density difference map. Thus, consideration of the molecular bond...

Experimental electron-density-distribution study of potassium iron disulfide, a low-dimensional material.

The experimental electron-density distribution of potassium iron disulfide, ${\mathrm{KFeS}}_{2}$, has been determined from high-resolution single-crystal x-ray intensity measurements collected with Ag K\ensuremath{\alpha} radiation to a resolution of (sin\ensuremath{\theta})/\ensuremath{\lambda}=1.30 A${\r{}}^{\mathrm{\ensuremath{-}}1}$. The structure of ${\mathrm{KFeS}}_{2}$ consists of parallel chains of edge-sharing ${\mathrm{FeS}}_{4}$ tetrahedra extending along the c-axis. In addition to the conventional structural parameters, multipole deformation functions and third- and fourth-cumulant anharmonic thermal parameters were included in some least-squares refinements. In all cases, inclusion of anharmonic thermal vibration parameters yielded significantly better fits to the x-ray data. From the experimental multipole population parameters of the iron atom, apparent d-orbital occupancies and the number of unpaired 3${d}_{\ensuremath{\uparrow}}$ electrons have been estimated within the crystal-field approximation and the assumption that a minimal basis set of atomic d orbitals is adequate for the d electrons. The values obtained for the number of 3${d}_{\ensuremath{\uparrow}}$ electrons from various refinements of the x-ray data agree well with values deduced from previous magnetic-susceptibility measurements and from theoretical calculations. The crystal-field splitting of the iron d orbitals and the ensuing asphericity is clearly evident in the experimental electron-density maps. The electric field gradient at the iron nucleus, calculated from the x-ray multipole parameters due to the slightly distorted (${D}_{2d}$) tetrahedral crystal field, predicts a quadrupole splitting of \ensuremath{\Delta}${E}_{\mathrm{QS}=3.3(10}$) mm/sec, somewhat larger than observed in M\ossbauer experiments (\ensuremath{\Delta}${E}_{\mathrm{QS}=0.53}$ mm/sec). The major contribution to the electric field gradient comes from the valence-electron distribution, which is consistent with the observed asphericity of electron density around iron.

Ruthenium-99 Mössbauer study of ruthenium red and its derivatives

A 99Ru Mossbauer study of the ions [Ru3(O)2(NH3)14]n+(n= 6 or 7) and their ethylenediamine (en) substituted derivatives, [Ru3(O)2(NH3)10(en)2]n+, has been carried out. The results support earlier proposals regarding the distribution of valence electrons on the ruthenium centres for the reduced species (n= 6). For the oxidised complexes (n= 7), however, they reveal a delocalisation of the odd electron amongst the three ruthenium centres.

The Mössbauer effect and positron annihilation studies of Fe-Cr alloys

Room temperature measurement of 57Fe Mössbauer spectra as well as positron lifetime spectra and angular distributions of photons coming from the two-photon annihilation of an electron-positron pair were performed for Fe-Cr alloys containing 1-15 at.% of chromium. The data were analysed in terms of a rearrangement of valence electrons due to alloying of Fe with Cr. It was shown that the data can be used for verification of theoretical computations concerning the influence of the valence electron distribution on the 57Fe isomer shift in alloys.

Non-additive interactions between noble gas atoms by the model potential method

Non-additive interactions in homonuclear trimers composed of Ar, Kr and Xe atoms have been examined using the model potential SCF (MP) method of Huzinaga and co-workers. During optimization of the atomic valence pseudo-orbitals and MP parameters great emphasis has been put on maintaining a reliable description of the long-range valence electron distribution. For Ar 3 very accurate all electron SCF calculations have also been performed. The present MP values compared excellently with the all electron energies. For Xe 3 relativistic effects have been estimated within MP quasi-relativistic approach.

The structure of the homonuclear diatomic solids revisited

The relative stability of various crystal packings of a homonuclear diatomic molecule is investigated as a function of the anisotropy in the intermolecular atom-atom potential. It is shown that different crystal structures can be predicted if the electrostatic interaction arises from a quadrupolar rather than dipolar distortion of the atoms from spherical, for the same total quadrupole moment, and that the corresponding anisotropy in the repulsion-dispersion potential also plays a significant role in determining the minimum energy crystal structure. These results are used to account for the diverse experimental crystal structures of the homonuclear diatomics in terms of their valence electron distribution, and provide a starting point for the development of realistic intermolecular model potentials.

ENDOR spectroscopy

Electron-nuclear double resonance (ENDOR) spectroscopy is a high resolution technique by which electron-nuclear hyperfine interactions are measured and thus is the method of choice to determine the valence electron distribution in complex paramagnetic biomolecules. The interaction provides a sensitive probe of the electronic structure of the radical site and its surroundings, and can often be related to functional properties of the system. The application of this technique to electron donor/acceptor species in bacterial photosynthetic reaction centres is discussed as an example.