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Effects Operating (effects + operating)
Selected AbstractsCollective Grain Interactions II.CONTRIBUTIONS TO PLASMA PHYSICS, Issue 8 2005Non-linear Collective Drag Force Abstract It is found that the collective effects operating at large distances from the grain surface can produce substantial scattering of the ion flux and create an additional collective drag force dominant for large grain densities. The consideration is restricted to large grain charges , = Zde2a /Ti,Di , 1 and Ti /Te , 1 (,eZd being the grain charge in units of electron charge, a being the grain size, ,Di being the ion Debye radius and Te,i being electron and ion temperatures, respectively). For present dusty plasma experiments , , 10,50, the large charges of grains are screened non-linearly and the ion scattering creates non-linear drag force. The present investigation considers effects of scattering by collective grain fields at large distances from the grains. It is found that the physical reason of the importance of collective drag force, calculated in this paper, is related to presence of weakly screened collective field of grains outside the non-linear screening distance depending on grain densities. The amplitude of this collective fields of the grains is determined by non-linear screening at non-linear screening radius. It is shown that for dust densities of present experiments the collective drag force related to this scattering can be of the order of the non-linear drag force caused by scattering inside the non-linear screening radius or even larger. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synthesis of the Cyclopentyl Nucleoside (,)-Neplanocin A from D -Glucose via Zirconocene-Mediated Ring ContractionHELVETICA CHIMICA ACTA, Issue 6 2005Two approaches for the conversion of d- glucose to (,) -neplanocin A (2), both based on the zirconocene-promoted ring contraction of a vinyl-substituted pyranoside, are herein evaluated (Scheme,1). In the first pathway (Scheme,2), the substrate possesses the , - d- allo configuration (see 6) such that ultimate introduction of the nucleobase would require only an inversion of configuration. However, this precursor proved unresponsive to Cp2Zr (=[ZrCl2(Cp)2]), an end result believed to be a consequence of substantive nonbonded steric effects operating in a key intermediate (Scheme,5). In contrast, the C(2) epimer (see 7) experienced the desired metal-promoted conversion to an enantiomerically pure polyfunctional cyclopentane (see 5 in Scheme,3). The substituents in this product are arrayed in a manner such that conversion to the target nucleoside can be conveniently achieved by a double-inversion sequence (Scheme,4). Recourse to palladium(0)-catalyzed allylic alkylation did not provide an alternate means of generating 2. [source] Can macroeconomic policy stimulate private investment in South Africa?JOURNAL OF INTERNATIONAL DEVELOPMENT, Issue 7 2008New insights from aggregate, manufacturing sector-level evidence Abstract This study explores the determinants of investment using both aggregated industry-level data and disaggretated data on 27 sub-sectors of the manufacturing sector for the period 1970,2001. According to the results in this study, the government has potentially powerful means at its disposal to stimulate private investment. In particular, a domestic demand stimulus and public investment expansion will produce large gains in private investment. While the direct effects of lowering the interest rate appear to be quantitatively small, indirect effects operating notably through domestic demand and cheaper credit are likely to be large. The evidence in this study also indicates that it is important to minimise exchange rate instability to encourage investment. Copyright © 2008 John Wiley & Sons, Ltd. [source] Mechanics of column beds: I. Acquisition of the relevant parametersAICHE JOURNAL, Issue 3 2003Bee Gaik Yew The efficiency of chromatographic columns is adversely affected by large-scale radial variations of the packing density or void ratio of the material used to prepare the bed. This heterogeneity is due to wall friction effects that take place during the preparation of the column and to seepage effects operating during the packing process and the subsequent operation of the column. The dependence of the bed's void fraction on the stress applied during its consolidation was determined, as well as its permeability at various stages of the consolidation process and the coefficient of friction between typical packing materials and the stainless steel wall of chromatographic columns. These results are required to develop and use numerical models of the volumetric response to axial compression of the bed and models of the coupled mechanical-seepage rheology of particulate materials. [source] Changes in plant interactions along a gradient of environmental stressOIKOS, Issue 1 2001Francisco I. Pugnaire A combination of competition and facilitation effects operating simultaneously among plant species appears to be the rule in nature, where these effects change along productivity gradients often in a non-proportional manner. We investigated changes in competition and facilitation between a leguminous shrub, Retama sphaerocarpa, and its associate understorey species along an environmental gradient in semi-arid southeast Spain. Our results show a change in the net balance of the interaction between the shrub and several of its associated species, from clearly positive in the water-stressed, infertile environment to neutral or even negative in the more fertile habitat. There was a weakening of facilitation along the fertility gradient as a consequence of improved abiotic conditions. Competition was the most intense for below-ground resources in the less fertile environment while total competition tended to increase towards the more productive end of the gradient. Changes in the balance of the interaction between and among different plant species along the gradient of stress were caused by a decline in facilitation rather than by a change in competition. As both competition intensity and facilitation change along gradients of resource availability, plant interactions are best viewed as dynamic relationships, the outcome of which depends on abiotic conditions. [source] On the Effect of Tether Composition on cis/trans Selectivity in Intramolecular Diels,Alder ReactionsCHEMISTRY - AN ASIAN JOURNAL, Issue 1 2009Michael Abstract Intramolecular Diels,Alder (IMDA) transition structures (TSs) and energies have been computed at the B3LYP/6-31+G(d) and CBS-QB3 levels of theory for a series of 1,3,8-nonatrienes, H2CCHCHCHCH2XZCHCH2 [XZ=CH2CH2 (1); OC(O) (2); CH2C(O) (3); OCH2 (4); NHC(O) (5); SC(O) (6); OC(S) (7); NHC(S) (8); SC(S) (9)]. For each system studied (1,9), cis - and trans -TS isomers, corresponding, respectively, to endo - and exo -positioning of the CXZ tether with respect to the diene, have been located and their relative energies (ErelTS) employed to predict the cis/trans IMDA product ratio. Although the ErelTS values are modest (typically <3,kJ,mol,1), they follow a clear and systematic trend. Specifically, as the electronegativity of the tether group X is reduced (XO,NH or S), the IMDA cis stereoselectivity diminishes. The predicted stereochemical reaction preferences are explained in terms of two opposing effects operating in the cis -TS, namely (1),unfavorable torsional (eclipsing) strain about the C4C5 bond, that is caused by the CXC(Y) group's strong tendency to maintain local planarity; and (2),attractive electrostatic and secondary orbital interactions between the endo -(thio)carbonyl group, CY, and the diene. The former interaction predominates when X is weakly electronegative (XN, S), while the latter is dominant when X is more strongly electronegative (XO), or a methylene group (XCH2) which increases tether flexibility. These predictions hold up to experimental scrutiny, with synthetic IMDA reactions of 1, 2, 3, and 4 (published work) and 5, 6, and 8 (this work) delivering ratios close to those calculated. The reactions of thiolacrylate 5 and thioamide 8 represent the first examples of IMDA reactions with tethers of these types. Our results point to strategies for designing tethers, which lead to improved cis/trans -selectivities in IMDAs that are normally only weakly selective. Experimental verification of the validity of this claim comes in the form of fumaramide 14, which undergoes a more trans -selective IMDA reaction than the corresponding ester tethered precursor 13. [source] | ||||||||||