SE Model Research Articles

Diiron Dichalcogenolato (Se and Te) Complexes: Models for the Active Site of [FeFe] Hydrogenase

AbstractA short overview of diiron dichalcogenolato (Se and Te) model complexes related to the chemistry of the diiron subsite of [FeFe] hydrogenase is presented. These model complexes allow direct comparison with the diiron dithiolato compound analogues for their ability to catalyze the formation of H2 from weak acids. Few detailed photoelectron spectroscopy (PES) investigations and density functional theory (DFT) calculations have been reported until now, and the results will be summarized here. On the basis of preliminary results, we propose targets for the synthesis of more efficient biomimetic diiron dichalcogenolato catalysts.

Identification of domains for a new conceptual model of strategic entrepreneurship using the configuration approach

PurposeThe aim of this paper is to structure and synthesize the existing scholarly works in the young and emerging field of “strategic entrepreneurship” (SE) as well as to develop a theoretical model, thereby contributing to further theory‐building.Design/methodology/approachAs the results of a review of extant literature, four different elementary domains conceptualizing SE on the basis of the configuration approach could be identified: the strategy, the entrepreneur, the environment and the structure and resources of the firm.FindingsThe SE model contributes to the understanding of how firms can create value. It has been shown beforehand that the configuration approach is suitable for strategic management as well as for entrepreneurship in the sense of new venture creation and arguably can be transferred to the larger level of SME strategy as well.Research limitations/implicationsWith the four dimensions of SE, this paper has so far only been able to deliver a starting point for further research which (empirically) investigates their interplay and attempts to derive “optimal configurations” of the dimensions, e.g. with regard to different firm sizes and environmental conditions. This empirical part is still outstanding.Practical implicationsThe major managerial implication of the SE approach is the possibility to develop more entrepreneurial and innovative thinking, especially in SMEs and young ventures. This stands in contrast with classical strategic management approaches, which characteristically emphasize administrative management and focus on day‐to‐day business.Originality/valueThis paper is the first to model SE by using the configuration approach, thereby providing a solid theoretical foundation for future empirical research.

Sorption of selenium(IV) and selenium(VI) to mackinawite (FeS): Effect of contact time, extent of removal, sorption envelopes

Higher concentrations (127, 253μM) of Se(IV) at pH 8 were completely removed by 0.5g/L FeS within 120min. Removal of Se(VI) by FeS at pH 8 was less extensive than removal of Se(IV). Only 10% of the Se(VI) was removed by 1g/L FeS within 1h. Removal patterns for Se by FeS depend on pH. Removal patterns of Se at pH 7 and pH 8 were best described by BET models for Se(IV) and Freundlich models for Se(VI), while removal patterns of both at pH 9 and 10 were best described by Langmuir models. Sulfate at 1 and 10mM had negligible effect on removal of Se(IV) by FeS, while sulfate had little effect on removal of Se(VI) by FeS, but there was some indication that sulfate promoted removal of Se(VI) at intermediate concentrations. The test for the effect of pH on sorption of Se(IV) by FeS showed nearly complete removal at all but the high initial pH. When pH was raised back to initial value, greater removals were observed than initially. Mixtures of Se(VI) and FeS showed moderate removal at low pH, a minimum removal near pH 6 and nearly complete removal at high pH. Very high stability was observed with negligible release as pH decreased.

On the specification of structural equation models for ecological systems

The use of structural equation modeling (SEM) is often motivated by its utility for investigating complex networks of relationships, but also because of its promise as a means of representing theoretical concepts using latent variables. In this paper, we discuss characteristics of ecological theory and some of the challenges for proper specification of theoretical ideas in structural equation models (SE models). In our presentation, we describe some of the requirements for classical latent variable models in which observed variables (indicators) are interpreted as the effects of underlying causes. We also describe alternative model specifications in which indicators are interpreted as having causal influences on the theoretical concepts. We suggest that this latter nonclassical specification (which involves another variable type—the composite) will often be appropriate for ecological studies because of the multifaceted nature of our theoretical concepts.In this paper, we employ the use of meta‐models to aid the translation of theory into SE models and also to facilitate our ability to relate results back to our theories. We demonstrate our approach by showing how a synthetic theory of grassland biodiversity can be evaluated using SEM and data from a coastal grassland. In this example, the theory focuses on the responses of species richness to abiotic stress and disturbance, both directly and through intervening effects on community biomass. Models examined include both those based on classical forms (where each concept is represented using a single latent variable) and also ones in which the concepts are recognized to be multifaceted and modeled as such. To address the challenge of matching SE models with the conceptual level of our theory, two approaches are illustrated, compositing and aggregation. Both approaches are shown to have merits, with the former being preferable for cases where the multiple facets of a concept have widely differing effects in the system and the latter being preferable where facets act together consistently when influencing other parts of the system. Because ecological theory characteristically deals with concepts that are multifaceted, we expect the methods presented in this paper will be useful for ecologists wishing to use SEM.

Strategic entrepreneurship: origins, core elements and research directions

PurposeThe question of how to integrate strategic and entrepreneurial management to achieve a better balance between advantage‐ and opportunity‐seeking behaviours has received increased academic and practitioner interest in recent years. However, little consensus exists over the meaning of this concept of “strategic entrepreneurship” (SE), its constituents and its operation. This paper aims to address these issues.Design/methodology/approachIn response, the paper reports a thorough review of SE's origins and current conceptualizations to map its core components and charts critical research directions for this exciting emerging field. Analysis of the terrain of SE reveals eight core components drawn from entrepreneurship and strategic management that captures conditions necessary for its application.FindingsFrom this analysis, the paper offers an alternative model of SE, and charts four key research areas with accompanying research questions to inspire future research. These outcomes offer avenues to further sharpen the understanding of how SE might operate both in theory and practice.Research limitations/implicationsA relative lack of analysis and integration of external factors influencing and shaping the process of SE represents a limitation. Also, whilst the authors have attempted to review, assess and understand its position in the entrepreneurship and strategic management literature, the full range of its strengths and weaknesses cannot be grasped.Originality/valueThe paper illustrates how these four question areas pose significant promise to better understand the development and application of SE in research and in practice.

Идентификация параметров математической модели авиационного двигателя Д-436 по результатам наземных стендовых испытаний

Methodic and identification parameters and characteristics results of mathematical model (MM) for aviation gas-turbine engine D-436 SE ‘Ivchenco – Progress” by experimental data are presented

Interfacial area based variable tortuosity‐connectivity for unsaturated media: A comparison using Miller–Miller scaling and Arya–Paris model

The tortuosity and/or pore connectivity factor, τ, in unsaturated media is typically represented by Seɛ, where Se = effective saturation. The exponent ɛ is treated as an optimized parameter with typical values of 0.5 and 2, respectively, for Mualem and Burdine‐type models. In this study, the tortuosity and/or connectivity term in Mualem and Burdine models is replaced by a normalized ratio, τ(Se = 1) / τ(Se); τ(Se) is the ratio of air–water interfacial area for a soil sample to the interfacial area for the corresponding idealized capillary bundle. The interfacial area for a soil sample is based on Leverett's (1941) thermodynamic considerations and is obtained by integrating the area under the laboratory‐measured moisture retention curve. In addition to Miller–Miller scaling used in an earlier work, the idealized medium retention curve in this study is based on translation via the Arya–Paris (AP) model of the particle size distribution data for a soil sample into its pore size distribution. The saturation‐dependent, variable tortuosity concept is tested for 24 fine‐textured samples in addition to 22 coarse‐textured samples considered in the earlier study. For both fine‐ and coarse‐textured samples, the interfacial area based variable τ(Se) formulation provides at least as good a fit of the measured and predicted unsaturated hydraulic conductivities, K, as a function of moisture content, θ, as the standard Brooks–Corey–Burdine and van Genuchten–Mualem models. Specifically, for the fine‐textured samples, Brooks–Corey–Burdine K(θ) predictions based on the variable τ(Se) model compare well with K(θ) measurements, with an r2 of 0.86, a regression slope of 1.02, and a relatively low residual mean squared deviation of 1.13. Unlike the Burdine (Se2) and Mualem (Se0.5) empirical models, the τ(Se)‐based model parameters have explicit physical significance because the interfacial area in unsaturated media can be measured using interfacial tracers. The use of the AP model to represent the idealized medium retention data suggests its broader applicability for both fine‐ and coarse‐textured media.

Body size and host range co‐determine the altitudinal distribution of Neotropical tephritid flies

ABSTRACTAim We addressed the following questions: (1) Does tephritid body size tend to increase in species found at higher elevations, as predicted by Bergmann's rule? (2) Do tephritids conform to Rapoport's rule, so that species found at higher elevations tend to have broader altitudinal ranges? (3) More generally, how do body size and host range jointly affect the patterns of altitudinal distribution among Neotropical tephritid flies?Location The Mantiqueira mountain range, south‐eastern Brazil, at sites ranging from c. 700 to 2500 m a.s.l.Methods At each site we collected flower heads of all Asteraceae species to rear out endophagous immatures (from January to June in 1998 and 1999). We used structural equation models (SEM) to evaluate jointly the relationships between body size, host range and altitudinal distribution (range and mid‐point).Results Neotropical tephritid body size showed a negative relationship with altitudinal distribution. SE modelling showed no significant direct effect of body size on altitudinal range; however, it had significant indirect negative effects through host range and altitudinal mid‐point. The SE model was a good predictor of observed correlations and accounted for 84% of the variation in tephritid altitudinal range.Main conclusions The altitudinal range of flower‐head‐feeding tephritids is related to host range and is indirectly affected by body size via host range and altitudinal mid‐point. As predicted by Rapoport's rule, tephritids that occur at higher elevations also present wider altitudinal ranges. Bergmann's rule does not apply to Neotropical tephritids along a tropical elevational gradient, but rather its converse was found. Body size may determine host range by imposing a restriction upon large individuals using small flower heads. Host species turnover along the altitudinal gradient may be the main factor explaining the strong relationship between host range and insect elevational distribution.

Purification, crystallization and preliminary X-ray diffraction analysis of RafE, a sugar-binding lipoprotein from Streptococcus pneumoniae.

Streptococcus pneumoniae contains a large number of sugar-transport systems and the system responsible for raffinose uptake has recently been identified. The substrate-binding protein component of this system shares strong sequence homology with the multiple sugar metabolism substrate-binding protein MsmE from S. mutans and contains a lipoprotein-attachment site at cysteine residue 23. A truncated form (residues 24-419) of RafE from S. pneumoniae was cloned and overexpressed in Escherichia coli. Native and selenomethionine-labelled protein have been crystallized in the hexagonal space group P6(1)22. Diffraction data have been successfully phased to 2.90 angstroms using Se SAD data and model building is in progress.

Concurrent and Predictive Validity of Two Scales to Assess the Fidelity of Implementation of Supported Employment.

The ability of the Individual Placement and Support (IPS) and the Quality of Supported Employment Implementation Scale (QSEIS) fidelity scales to predict independently rated SE adherence and percentage of clients closed by the Office of Vocational Rehabilitation (OVR) was tested at 23 SE sites. QSEIS Total score (r = .47), Integration subscale (r = .53), and Support subscales (r = .61) correlated with SE adherence. Total QSEIS score also predicted OVR closure rate (r = .42). Correlations between IPS (sub)scale scores and the criterion measures were non-significant. The results support QSEIS validity and provide evidence that fidelity to the SE model is associated with better employment outcomes.

Reactivity of 1,3,5-trithiacyclohexane and 1,3,5-triselenacyclohexane towards molecular diiodine. Crystal structures of the diiodine adducts

The reactions between I 2 and the donors (D) 1,3,5-trithiacyclohexane ( 1) and 1,3,5-triselenacyclohexane ( 2), in methylene chloride and hot benzene respectively (1:1 I 2:D molar ratio) give the solid adducts 1·I 2 ( I) and 2 · I 2 ( II) whose structures have been solved by X-ray diffraction. The use of an excess of diiodine up to the molar ratio 2:1 always gives II from 2, and 1·2I 2 ( III) from 1. Compound I consists of polymeric chains in which each diiodine bridges between different 1 molecules, while II consists of discrete molecular units. The Raman ν(II) frequency and II bond distance in I fits well with the previously found linear correlation. This correlation suggests two different II bond distances for III. The comparison of the FTIR and FT-Raman spectra for II is consistent with the three-body model for SeII. The association constant between 1 and I 2 in methylene chloride has been also studied both by UV-visible and 13C NMR spectroscopy, giving K values at 21°C in good agreement.

Mesobonding (Se,Se) pairs in Si

Using symmetry, qualitative analyses of g-shift data for the (Se,Se) + deep level in Si, and the general features of the electronic structure of Si, we find the mesobonding model of (Se,Se) substitutional pairs in Si to be preferred over the ordinary model -- without relying on any detailed model Hamiltonian.

Shallow structure of part of northwestern Iberia from short-period Rayleigh-wave observations

Propagation of 0.2–2.5 s short-period Rayleigh-waves across a part of northwestern Iberia is investigated. Analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot-point, provides group-velocities and attenuation coefficients of the fundamental mode. First higher-mode group-velocity has been obtained for only one of the studied cases. The studied region has been divided into two subrogions, the northwestern and the southeastern side from the shot-point. Inversion procedures applied to the velocity and attenuation data yield the shear velocity and the Q−1β models up to a depth of about 1.5 km for the NW, and 2.4 for the SE side. From the shear-wave obtained models corresponding to the NW and to the SE sides, it is possible to infer the existence of some degree of lateral variation of S velocity. The NW side presents velocities varying between 1.8 km · s−1 (at the surface) and 3.3 km · s−1 (at 1.2 km depth). The velocities in the SE model, for the same depths, are 2.2 and 3.4 km · s−1, respectively. Using only the fist six traces of the SE side, it is possible to infer a low-velocity region located between 0.4 km and 0.7 km depth. It seems that the velocities are higher in the complex geological region of the SE side than in the NW one, which is in agreement with P velocity models found by others. From the tentative anelastic attenuation coefficients, Q−1β models have been inferred for both sides of the seismic refraction profile. Although the obtained Q−1β models are somehow different in shape, the maximum Q−1β value is about 0.05 in both cases.

Vector charge density wave model of metallic Te and Se

High pressure metallic Te and Se are studied by the vector charge density wave (VCDW) model. They can be described by a VCDW state with the wave number (0, π/2, π/2) on a cubic lattice. In the VCDW state, Te or Se atoms make stacked zigzag chains. The period of orientational arrangement of p lone pairs in this state is two along a chain, four along two square edges including a chain plane and two along the stacking direction. The electronic and lattice structures of the metallic phases are calculated by the temperature-pressure Hartree-Fock (HF) method. The mixing between p x , p y and p z atomic orbitals is included so as to consider a general distortion including shearing displacements. This model can explain the characteristics features of the lattice structure such as the dimerization and staggered stacking of chains and the electronic energy band structure of metallic Te.

Raman spectra of chalcogen chains isolated in zeolite matrixes

Raman spectra are reported for single Se and Te chains stabilised in the regular system of parallel channels of a mordenite dielectric matrix. The symmetric bond-stretching mode frequency is determined for Se chains (260 cm-1) and for Te chains (164 cm-1). The distinct bond-bending mode bands associated with ring-like and chain-like fragments and the Fermi resonance structure are found in the Se chain spectrum. A comparison of the spectra of chains with those of bulk materials is made. It is shown that the interchain interaction in trigonal Se and Te crystals weakens significantly the intrachain bonding. Confirmation is given for the amorphous Se and Te models that are based on slightly interacting chains with varying signs and magnitudes of dihedral angles.

Electronic structure of trigonal and amorphous Se and Te

The electronic structure of trigonal and amorphous Se and Te is investigated using the empirical pseudopotential method (EPM), charge-density calculations, and simple tight-binding models. Band structures and electronic densities of states are obtained which are in excellent agreement with recent photoemission measurements. The tight-binding models are used to obtain analytic expressions for the energy bands and to interpret the EPM band structures in terms of real-space orbital-orbital interactions. Charge-density calculations obtained as a function of energy and evaluated within specific energy intervals are used to interpret various structure in the density of states. Specifically certain easily resolvable peaks in the experimental photoemission spectra are associated with intrachain and interchain localized states, respectively. By taking only short-wavelength components of the charge density, a bonding charge can be defined which gives an estimate of the intrachain vs interchain bonding strengths. The trigonal results along with model calculations to investigate the effects of bond-angle variations on chains and the presence of eight- and six-fold rings of bonds are used to interpret the changes observed in the experimental spectra of amorphous Se and Te. A new model of amorphous Se is proposed.

Amorphous and crystalline selenium - a Mössbauer effect study

Te 125 Mössbauer spectra of Se 0.98 Te 0.02 alloy in bulk amorphous and trigonal phases reveal partially resolved quadrupole doublets. The average quadrupole splitting in the amorphous phase is found to be 10 per cent larger. This observation, in the framework of the ring-chain model of amorphous Se suggests a substantially larger quadrupole interaction at Te 125 sites in mixed rings than in copolymeric chains. The strength of the quadrupole interaction in mixed rings and copolymetric chains is found to be 12.72 ± 0.40 mm/sec and 10.33 ± 0.08 mm/sec. Implications of these results on the chemical bonding are discussed.

Density of States of Amorphous Selenium by Vacuum Photoemission

The entire valence band of amorphous Se has been probed by vacuum photoemission. The valence-band density of states consists of a large peak 1.7 eV below the upper edge of the band, and a triplet structure with shoulder at 3.8 and peaks at 4.4 and 5.4 eV below the band edge. Although the upper peak is in good agreement with the results of Kramer's calculation, the lower triplet is not. Comparison with molecular-orbital calculations of Chen and Tutihasi and Chen, reveals better but not perfect agreement. It is suggested that a model of amorphous Se as a mixture of rings and chains will produce better agreement with experiment than the disordered trigonal lattice employed in the calculation of Kramer. The interface between Se and various metal electrodes was investigated and an upper limit on the density of states near the Fermi level of 2 \ifmmode\times\else\texttimes\fi{} ${10}^{14}$ e${\mathrm{V}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ determined by a new technique. This limit on the density of states is consistent with that obtained by Lanyon from space-charge-limited-current studies.