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201.
Forbidden resonant Raman scattering from screened longitudinal optical, LO, phonons has been observed in the back-reflection geometry from n-type EuTe at 2°K. The Raman shift increased with increasing excitation frequency but was always between the LO and TO phonon frequencies. This effect is explained in terms of a varying ‘effective’ carrier concentration as a function of laser penetration depth through the surface depletion layer in the situation of large phonon wave vector. Conduction band and lattice parameters have been calculated from infrared Reststrahlen and plasma edge measurements.  相似文献   
202.
A survey of identities connected with sums of three squares, 2-dimensional subalgebras of the quaternions, and additive commutators is given. The study of the last is linked to a theorem already observed by Gauss and re-proved here in various ways. A study of generalizations in recent years is added.  相似文献   
203.
Previous treatments of surface segregation in metal alloys have all failed in some systems, due either to inadequacies in the theories or in the thermochemical data. A new model is proposed here using only alloy diffusion data. Predicted surface compositions agree quantitatively with ISS measurements and qualitatively AES results.  相似文献   
204.
One of the most frequently occurring integer programming structures is the one which has special ordered sets of variables included in multiple choice constraints. For problems with this structure a set of ideal columns are defined from the linear programming relaxation of the integer program and a reduced integer program is formed by keeping only those columns within a specified distance from the ideal column. Conditions are established which guarantee when the optimal solution to the reduced problem is als optimal for the original problem. When these conditions are not satisfied, bounds on the optimal solution value are provided. Ideal columns are also used to establish weights for the special ordered set variables. This procedure has been implemented through a control program written by the authors for MPSX/370-MIP/370. Computational results are given.  相似文献   
205.
The rates and products of addition of benzeneselenenyl chloride to ethylene and its six Me substituted derivatives have been determined in methylene chloride at 25°. Unlike the addition of 4-chlorobenzenesulfenyl chloride to this same series of compounds, the effect of Me groups on the rates of addition is not cumulative. Also the regiochemistry of the product is different. For arenesulfenyl chloride additions, products of anti-Markownikoff orientation are formed preferentially under conditions of kinetic control. Under similar conditions regiospecific formation of the Markownikoff adduct is observed for the addition of benzeneselenenyl chloride to methylpropene and 2-methyl-2-butene. These data indicate a difference in both rate and product determining transition states between additions of arenesulfenyl and selenenyl chlorides to alkenes.  相似文献   
206.
207.
The chemokines (CXCL9, CXCL10 and CXCL11) and associated CXCR3 receptor are expressed during the inflammatory process from multiple sclerosis, atherosclerosis or organ transplantation resulting in the recruitment of lymphocytes leading to tissue damage. It is hypothesized that blocking of the ligand/CXCR3 receptor interaction has potential to provide opportunity for development of agents that would block tissue rejection. In this paper, four classes of natural product inhibitors (IC50 ranging 0.1–41 M) have been described that block the CXCR3 receptor interaction of IP-10 ligand. These include a cyclic thiopeptide (duramycin), polyketide glycosides (roselipins), steroidal glycosides (hypoglausin A and dioscin) and a novel alkyl pyridinium alkaloid that were isolated by bioassay-guided fractionation of the organic extracts derived from actinomycete, fungal, plant and marine sources and discovered using 125 I IP-10/CXCR3 binding assay. Duramycin was the most potent with an IC50 of 0.1 M. Roselipins 2A, 2B and 1A showed IC50 values of 14.6, 23.5, and 41 M, respectively. Diosgenin glycosides dioscin, hypoglaucin A and kallstroemin D exhibited IC50 values of 2.1, 0.47 and 3 M, respectively. A novel cyclic 3-alkyl pyridinium salt isolated from a sponge displayed a binding IC50 of 0.67 M.  相似文献   
208.
Three chars and an activated carbon were gasified by reaction with CO2 in a fluidised bed of sand, at 800–1050 °C. The chars were produced from (i) dried sewage sludge, (ii) car tyres, and (iii) a bituminous coal. For the conditions used, the rate of CO2 + C → 2CO was largely determined by chemical kinetics; there was a small effect from mass transfer for the most reactive char, derived from sewage sludge. The rate of CO formation, r, differed greatly for these chars, but was well described by:
The reactivity of a char depends on: (i) its pore structure, (ii) catalytic activity of the associated ash, and (iii) the activity of the char’s carbon. The sewage sludge char was the most reactive, on the basis of either BET area or mass by 2 orders of magnitude. The activated carbon had the lowest reactivity per unit BET surface area, indicating that the area in its micropores is comparatively unreactive.  相似文献   
209.
[reaction: see text] We have developed highly versatile, homochiral oxabicyclo[3.2.1]octadiene building blocks for the synthesis of natural products. We have found that these bridged alkenes undergo exceptionally facile Diels-Alder reactions and react faster than several well studied bicyclo[2.2.1]heptene dienophiles. The reaction proceeds with high levels of stereochemical control and in very good to excellent yields, providing access to bicyclo[5.4.0]undecane and bicyclo[5.3.0]decane systems. This reactivity is attributed to strain and homoconjugation effects.  相似文献   
210.
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