Major Symmetry Research Articles

Synthesis, Structure, Electrochemistry, and Mössbauer Effect Studies of the Ferraphosphadicarbollides [(C5R5)Fe(PC2B8H10)] (R = H, Me)

AbstractReaction of the phosphadicarbollide anion [7,8,9‐PC2B8H10]– (4) with [CpFe(C6H6)]+ or [Cp*Fe(MeCN)3]+ gives the expected 12‐vertex ferraphosphadicarbollides [1‐C5R5‐1,2,3,4‐FePC2B8H10] [R = H (5a), Me (5b); the metal atom is assigned number 1] in around 35 % yield. Compounds 5a,b undergo selective polyhedral rearrangement at 150 °C to give the corresponding isomers [1‐C5R5‐1,2,4,5‐FePC2B8H10] (6a,b). The structure of 5a has been determined by X‐ray diffraction. An electrochemical investigation has revealed that all the compounds obtained display the FeII/FeIII oxidation process with features of chemical reversibility on the cyclic voltammetric timescale. However, only cation [6b]+ is reasonably stable on the long timescales of electrolysis. Ferraphospadicarbollides 5a and 6a are more difficult to oxidize than the related dicarbollide and tricarbollide derivatives [CpFe(C2B9H11)]–and [CpFe(C3B8H11)], respectively. Temperature‐dependent Mössbauer effect studies show that the recoil‐free fraction over the temperature interval 110–342 K for 5a is well accounted for by a linear regression, and a calculation of the root‐mean‐square‐amplitude‐of‐vibration (rmsav) of the iron atom, based on the Mössbauer effect (ME) data, is in reasonable agreement with the value calculated from the single‐crystal X‐ray study at 120 K. Similarly, the ME and X‐ray rmsav data for 6a at 293 K are in excellent agreement with each other, and the value at 120 K calculated for 6a is in good agreement with that observed for 5a. The metal atom motion is nearly isotropic with respect to the major symmetry axis running through the Cp ring and the Fe atom. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Phonon dynamics of lanthanum manganiteLaMnO3using an interatomic shell model potential

We have investigated the detailed phonon dynamics of the magnetoresistant lanthanum manganite $(\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3})$ using a shell model with pairwise interionic interaction potential. The investigations include the computation of the phonon density of states, the specific heat, the elastic constants, and the phonon-dispersion curves of $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$. The symmetry vectors obtained through detailed group-theoretical analysis at the zone center point were employed to classify the phonon frequencies obtained into their irreducible representations. This analysis enabled direct comparison with the observed Raman and infrared spectra. Furthermore, we have computed the specific heat at constant volume at different temperatures and the phonon-dispersion curves along the three major symmetry directions for $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$. The results have been found to present an overall better interpretation of the available experimental data as compared to those obtained from other theoretical model calculations.

Synthesis, characterization and catalytic activity of the complex titanium bis(dimethylmalonate)–bis(diethylamido) in the polymerization of propylene

In this account we present the synthesis, characterization and catalytic activity in the polymerization of propylene of a bis(dimethyl malonate) titanium bis(diethylamine) complex ( 1). The complex exhibits in solution a dynamical isomerization following an internal Bailar twist. The activation of complex 1 was obtained by its reaction with methylalumoxane (MAO). The activated complex in solution shows a different dynamic process involving an equilibrium between a monodentate η 1 and bidentate η 2 binding of the dimethylmalonate ligand to the metal center. This equilibrium is responsible for the formation of, at least, two active species for the polymerization reaction bearing major symmetry differences. The monodentate coordination (opened form) of the ligand was found to be the major form of the active species of the complex when activated with MAO, most probably due to a strong interaction of the oxygen atoms in the ligand with the strong Lewis acid co-catalyst. The active species in the polymerization were studied by NMR and ESR spectroscopies. The resulting polypropylene showed elastomeric properties with low tacticites.

Calculation of phonon spectrum for noble metals by modified analytic embedded atom method (MAEAM)

In the harmonic approximation, the atomic force constants are derived and the phonon dispersion curves along four major symmetry directions [00ζ], [0ζζ], [ζζζ] and [0ζ1] (or Δ, Σ, Λ and Z in group-theory notation) are calculated for four noble metals Cu, Ag, Au and Pt by combining the modified analytic embedded atom method (MAEAM) with the theory of lattice dynamics. A good agreement between calculations and measurements, especially for lower frequencies, shows that the MAEAM provides a reasonable description of lattice dynamics in noble metals.

Phonon anomalies and elastic constants of cubicNiAlfrom first principles

The phonon dispersion relation of $\mathrm{NiAl}$ in the cubic $B2$ structure is calculated using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. Anomalies are present in acoustic branches along three major symmetry directions: $\ensuremath{\Gamma}\text{\ensuremath{-}}X$, $\ensuremath{\Gamma}\text{\ensuremath{-}}M$, and $\ensuremath{\Gamma}\text{\ensuremath{-}}R$, with the positions being in excellent agreement with experiment. Analysis of the Fermi surface and the generalized susceptibility shows that these are Kohn anomalies. Overall, the computed phonon frequencies significantly decrease with increasing lattice parameter. This unusual sensitivity is attributed to a two-dimensional van Hove singularity in the electronic density of states near the Fermi level. The phonon dispersion is compared with that of $B2$ $\mathrm{NiTi}$, and the origin of phonon anomalies in the high-temperature phase is found to be different in the two systems.

Phonon assignments in GaN bulk

The measured phonon-density of states of bulk GaN by time-of-flight neutron spectroscopy has been recently reported by Nipko et al. [1]. The authors have also calculated the true partial and total DOS as well as the phonon dispersion curves along major symmetry directions in the Brillouin zone. However, the group-theoretical phonon assignments have not been provided. Based on calculated symmetry allowed modes spanned by displacement representation and on the derived connectivity relations along the major directions in the Brillouin zone we have assigned Nipko's phonon dispersion curves to irreducible representations (species) of the C 4 6c (P6 3 mc) space group of GaN.

Major histocompatibility complex genes, symmetry, and body scent attractiveness in men and women

Previous research indicates that the scent of developmental stability (low fluctuating asymmetry, FA) is attractive to women who are fertile (at high-conception risk points in their menstrual cycles), but not to other women or men. Prior research also indicates that the scent of dissimilarity in major histocompatibility complex (MHC) genes may play a role in human mate choice. We studied the scent attractiveness to the opposite sex of t-shirts worn for 2 nights' sleep. Our results indicate that the two olfactory systems are independent. We repeated previous results from studies of the scent of symmetry. We repeated previous results from MHC research in part; men, but not women, showed a preference for t-shirts with the scent of MHC dissimilarity. Women's scent ratings of t-shirts were uncorrelated with the wearer's MHC dissimilarity and allele frequency, but positively correlated with the wearer's MHC heterozygosity. Fertile women did not exhibit any MHC trait preferences. Women's preference for the scent of men who were heterozygous for MHC alleles may be stronger in women who are at infertile cycle points. Men preferred the scent of common MHC alleles, which may function to avoid mates with rare alleles that exhibit gestational drive. Men also preferred the scent of women at fertile cycle points. The scent of facially attractive women, but not men, was preferred. Neither FA nor facial attractiveness in either sex correlated with MHC dissimilarity to others, MHC heterozygosity, or MHC allelic rarity. Copyright 2003.

Shape selectivity in outer-sphere electron transfer reactions

Chiral induction has been examined in the four diastereomeric products formed in a series of outer-sphere electron transfer reactions between the oxidants [Co(ox) 3] 3−, [Co(edta)] −, [Co(gly)(ox) 2] 2−, C 1 -cis( N)-[Co(gly) 2(ox)] −, [Co(en)(ox) 2] −, C 2 -cis( N)-[Co(gly) 2(ox)] − and trans( N)-[Co(gly) 2(ox)] − with [Co(( RR, SS)-chxn) 3] 2+ and [Co(( R, S)-pn) 3] 2+ as reductants. The products; [Co(( RR, SS)-chxn) 3-lel 3] 3+, [Co(( RR, SS)-chxn) 3-lel 2ob] 3+, [Co(( RR, SS)-chxn) 3-lelob 2] 3+, [Co(( RR, SS)-chxn) 3-ob 3] 3+ and corresponding species for [Co(( R, S)-pn) 3] 3+ show patterns of selectivity which are analyzed in terms of the size and structure of the reactants. The presence of a pseudo- C 3 carboxylate face on the oxidant enhances selectivity but the pattern is quite different for those oxidants that contain oxalate as one of their ligands compared with non-oxalate containing species such as [Co(edta)] −. A very simple model is developed in which the reductant employs a limited set of interactions corresponding to the major symmetry axes. The unrestricted reductant has very low aggregate selectvity. Steric and hydrogen bonding patterns in both oxidant and reductant enhance individual interactions resulting in the observed selectivities.

Fermi energy and Fermi surface distortion of the Cs–K, Cs–Rb and Rb–K binary systems

Systematic theoretical investigation of the Fermi energy (FE) and the Fermi surface distortion (FSD) of Cs 0.7Rb 0.3, Cs 0.3Rb 0.7, Cs 0.7K 0.3, Cs 0.3K 0.7, Rb 0.71K 0.29 and Rb 0.3K 0.7 disordered alloys have been reported for the first time by using the pseudo-alloy-atom model. A local pseudopotential consisting of a Coulombic term outside the core and varying cancellation of interaction within the core is proposed to describe the electron–ion interaction in each of these systems. The influence of various local-field correction functions on the aforesaid properties is 64–93%. The distortion of the Fermi surface (FS) along major symmetry directions obeys the metallic behavior. For the Cs 1− x Rb x and Cs 1− x K x systems, the FSD along the zone boundaries increases with the atomic volume. For Rb 1− x K x , a reverse trend is observed. The maximum anisotropy in the FS for the six binaries is observed at the [1 1 0] point.

Irreversibility of time and symmetry property of relaxation function in linear viscoelasticity

We prove, without using the Onsager's Reciprocity Principle and the time-reversal hypothesis, that the relaxation function of a linear viscoelastic three-dimensional material satisfies the property of major symmetry.

The interatomic interaction and the phonondispersion relations of magnesium

In the present paper, the interatomic interaction between atoms inmagnesium is calculated and the phonon dispersion relations foralong the major symmetry directions, T, T', Σ, and Δ,in this metal are computed by use of a method different from thetraditional ones. Instead of being fitted using the elasticconstants or determined from the phonon frequencies at some specialpoints in the first Brillouin zone, the force constants are directlycalculated using the calculated potential, which is obtained fromthe cohesive energy by use of the Möbius theorem of numbertheory. This calculated interaction does not have any of theadjustable parameters that are present in the previous modelpotentials. The calculated phonon dispersions are compared withexperiments and good agreement found.

The nature of lead–quasicrystal interfaces and its effect on the melting behaviour of lead nanoparticles embedded in quasicrystalline matrices

Nanoparticles of lead were embedded in Al–Cu–Fe icosahedral matrix by rapid solidification. The embedded particles showed definite orientation relationship with the matrix and the interface, facetted on the major symmetry planes of the matrix, showed coherency. Annealing treatments at 600 and 800°C showed that this particle–matrix interface is unstable. The coherency at this interface is supported by the phason strains formed on rapid quenching. This coherency breaks down on annealing in the stability region of the icosahedral phase, leading to a lowering in the melting temperature of the lead particles.

Refractive index profiles in LiNbO 3 waveguide formed by 3.2 MeV He ions

Optically polished y-cut LiNbO 3 was implanted at 3×10 16 ions/cm 2 at room temperature. In order to minimize any channeling effects, the sample was implanted at 7° off the major symmetry axis. The He beam current was approximately 700 nA and the scanning area was 0.8 cm 2. The prism coupling method was carried out to measure the dark modes in the LiNbO 3 waveguide. In the present work, an isosceles prism (rutile) was used. The dark modes were measured at 632.8 nm (He–Ne). Fifteen TM modes and 13 TE modes in the LiNbO 3 waveguide were observed. We have used the reflectivity calculation method (RCM) developed by Chandler and Lama to fit the refractive index profile. This method has been used for ion-implanted waveguides based on dark mode lines by the prism coupling method. Also we have used the TRIM (transport of ions in matter) code to simulate the damage profile in LiNbO 3 by 3.2 MeV He ions which is helpful for choosing the analytic function in RCM calculations.

A influência da canonicidade de linking sintático-semântico na construção do significado

The aim of this work was to investigate to what extent the canonicalcorrespondence (canonical linking) between the syntactic structureand thematic roles of the direct objects, subjects and prepositionalphrases affect the construction of meanings of the texts. In a secondlevel of analysis, the effects of canonical linking were investigated ininteraction with processing capacity and prior knowledge.Firstly, two preliminary tests were applied: a test of span reading toclassify the readers according to high, middle and low processing capacityand a test to measure the reader’s prior knowledge about the texts (thefunctioning of a washing machine and the functioning of a scanner).The results revealed that canonical linking, isolated or in interactionwith the other two factors, have a systematic effect; however, its effectsare diversified in the comprehension of the direct objects, subjectsand prepositional phrases.These results suggest, among other things, the existence ofsimilarities and differences in the processing of these sentenceconstituents. A major symmetry in the processing of the direct objectsand subjects in comparison with prepositioned phrases was observed.

Stability of the interface between lead particles and quasicrystals and its effect on the melting temperature of the lead particles

Lead nanoparticles were embedded in an icosahedral Al-Cu-Fe alloy by rapid solidification, forming coherent interfaces on the facets on major symmetry planes of the matrix. The lattice mismatch at the interface led to considerable strain in the matrix, which reduced the stability of the icosahedral phase in its metastable state. On annealing in the icosahedral stability temperature region, the strain in the matrix resulted in the formation of small-angle boundaries in the matrix and a change in the shape of the particles on these boundaries to round ones. Simultaneous with this, a lowering of the melting point of the lead by about 6oC occurred. The role of the interfacial energy in the melting temperature is discussed.

Highly resolved photoemission on High- Tc's of the BiSrCaCuO-family

The critical temperature T c attainable with BiSrCaCuO high-temperature superconductors depends on the number of CuO 2 layers per unit cell ( n=1–4). Here we give an overview of our results achieved so far on single crystalline material with n=1 (T c =29 K at optimal doping) and n=3 (T c =108 K) such layers in comparison to the extensively studied n=2 compound ( T c=89 K). The investigations involve highly resolved photoemission spectra along major symmetry lines and polarization studies. The polarization studies were performed at a crossed undulator synchrotron beamline enabling an in situ variation of light polarization without moving the sample.

Experimental and Theoretical Characterization of the High-Affinity Cation-Binding Site of the Purple Membrane

Binding of Mn 2+ or Mg 2+ to the high-affinity site of the purple membrane from Halobacterium salinarium has been studied by superconducting quantum interference device magnetometry or by ab initio quantum mechanical calculations, respectively. The binding of Mn 2+ cation, in a low-spin state, to the high-affinity site occurs through a major octahedral local symmetry character with a minor rhombic distortion and a coordination number of six. A molecular model of this binding site in the Schiff base vicinity is proposed. In this model, a Mg 2+ cation interacts with one oxygen atom of the side chain of Asp 85, with both oxygen atoms of Asp 212 and with three water molecules. One of these water molecules is hydrogen bonded to both the nitrogen of the protonated Schiff base and the Asp 85 oxygen. It could serve as a shuttle for the Schiff base proton to move to Asp 85 in the L-M transition.

Phonon excitations and related thermal properties of aluminum nitride

The phonon density of states of aluminum nitride was determined by time-of-flight neutron spectroscopy using a polycrystalline sample. The observed phonon excitation spectrum consists of a broadband centered at about 35 meV, a small gap in the 75--80 meV region, and two sharp bands at approximately 85 and 92 meV. A rigid-ion model was applied to the interpretation of the data. After optimization, the model provided a satisfactory description of the neutron results as well as the Raman and IR data, sound-velocity measurements, and the lattice specific heat reported in the literature. The partial and total DOS and the phonon-dispersion curves along major symmetry directions of the Brillouin zone were calculated, and the contribution of phonons to the Debye behavior of the low-temperature thermal conductivity was discussed.

The dynamics of H absorption in and adsorption on Cu(111)

We have studied theoretically the dynamics of H adsorption on and absorption in Cu(111) using a classical molecular dynamics approach. A key ingredient of this study is plane-wave and pseudopotential calculations of potential energy curves for the major high symmetry sites. These calculations are based on density functional theory and the generalized gradient approximation. The extracted chemisorption bond parameters from the energy curves are in good agreement with available experimental data. We find that the calculated energy barriers for absorption of an adsorbed atom are lowered dramatically by relaxations of the Cu atoms; these barriers are so low that, even in the rigid surface lattice situation, the absorption of an incident H atom is non-activated for impacts close to the so-called fcc and hcp hollow sites. The model interaction potential that we have used in the dynamics calculations is determined from the calculated potential energy curves and its form is taken from a semi-empirical effective medium theory for binary compounds. The main results of the dynamics calculations are: the relaxations and thermal fluctuations of the Cu atom do not affect the absorption of H in the surface; the energy transfer to the phonons is rather inefficient so the H atom has to make a large number of collisions with the surface atoms before it sticks either in the surface adsorption well or in the subsurface absorption well; a simple estimation shows that the energy transfer to electron-hole pairs can be as efficient as the energy transfer to phonons; our results are consistent with experiments, which indicate that subsurface sites can be populated by an incident atomic H beam and show that the scattering probability is small.

Three-Dimensional Cryoelectron Microscopic Reconstruction of the 2.25-MDa Homomultimeric Phosphoenolpyruvate Synthase fromStaphylothermus marinus

The phosphoenolpyruvate synthase (EC 2.7.9.2) of the hyperthermophilic archaeonStaphylothermus marinusforms a 2.25 MDa homomultimeric complex of 24 subunits. Here, computational analysis of low-dose cryoelectron micrographs was used to ascertain that the major rotational symmetry axes were 2-fold, 3-fold, and 4-fold. These symmetry considerations were used to perform a three-dimensional reconstruction to a spatial resolution of 4 nm. This assembly has an octahedral architecture with a solvent accessible interior and with ill-defined yet seemingly flexible appendages on the periphery. This macromolecular assembly is unusually large in mass compared with most other known globular proteins - especially other identified phosphoenolpyruvate synthases which are usually dimeric - and its elaborate quaternary arrangement might represent an adaptation to its extreme environment.