Quantum yield of Eu2+ emission in (Ca1−xSrx)S:Eu light emitting diode converter at 20–420 K

A series of red-emitting light converters Ca_1?xSr_xS:Eu²⁺, with tunable composition-dependent emission maxima were synthesized and characterized concerning their photoluminescent (PL) properties. X-ray diffraction patterns, photoluminescence spectra, luminosities and quantum yields were compared for phosphors with strontium concentrations varying from 0 to 100%. The maxima wavelength of emission shifts from 663 down to 619 nm, originating from the dependence of Eu²⁺ 5d state energy on the surrounding crystal field. Upon increasing the temperature from 20 to 420 K, a broadening of emission spectra along with thermal quenching of emission intensity and quantum yield occurs. Satisfying PL properties and their thermal stability demonstrate that the phosphors could be used as light converters in light emitting diodes (LEDs).     

Bifurcation analysis of 4-axle rail vehicle models in a curved track

The dynamics of a diffusive predator–prey model with time delay and Michaelis–Menten-type harvesting subject to Neumann boundary condition is considered. Turing instability and Hopf bifurcation at positive equilibrium for the system without delay are investigated. Time delay-induced instability and Hopf bifurcation are also discussed. By the theory of normal form and center manifold, conditions for determining the bifurcation direction and the stability of bifurcating periodic solution are derived. Some numerical simulations are carried out for illustrating the theoretical results.     

Studies on the reaction of 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone with tribenzylamine and diazabicyclo[2.2.2]octane in acetonitrile solvent

The reaction of chloranilic acid with two nitrogen nucleophiles, tribenzylamine and diazabicyclo[2.2.2]octane has been studied. Careful spectroscopic analysis showed that the product in both cases is a radical ion pair of Q^.-, A^.+ type. Stoichiometry of the complex formed is of 1:1 type. Mechanism of the reaction is proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reconsideration of solvent effects calculated by semiempirical quantum chemical methods

AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2-pyridone/2-hydroxypyridine and 4-pyridone/4-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AM1, method A) tautomeric equilibrium constants (log K₁) for 2-pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are ?0.3, 0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data (?0.4, 0.24, 0.78, and 2.17, respectively). For 4-pyridone/4-hydroxypyridine differences between calculated log K₁ for the gas phase, chloroform and acetonitrile (?6.0, ?2.6, and ?1.2, respectively) and experimental data (< ?1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed. © 1993 John Wiley & Sons, Inc.     

Linear upper bounds for local Ramsey numbers

A coloring of the edges of a graph is called alocal k-coloring if every vertex is incident to edges of at mostk distinct colors. For a given graphG, thelocal Ramsey number, r _loc ^k (G), is the smallest integern such that any localk-coloring ofK _n , (the complete graph onn vertices), contains a monochromatic copy ofG. The following conjecture of Gyárfás et al. is proved here: for each positive integerk there exists a constantc = c(k) such thatr _loc ^k (G) cr ^k (G), for every connected grraphG (wherer ^k (G) is theusual Ramsey number fork colors). Possible generalizations for hypergraphs are considered.On leave from the Institute of Mathematics, Technical University of Warsaw, Poland.While on leave at University of Louisville, Fall 1985.     

X-ray structures of 1,3-dithiolo[4,5-c]quinoline and 2,3-dihydro-1,4-dithiino[5,6-c]quinoline

The title compound I (C₁₀H₇NS₂) is orthorhombic: Pbca,a=16.223(6),b=15.065(5),c=7.356(4) Å. The conformation of 1,3-dithiol ring is of an envelope-type. A very short intramolecular sulfur-sulfur distance of 2.94 Å was observed. The compoundII (C₁₁H₉NS₂) is monoclinic: P2₁/c,a=5.113(2),b=11.978(4),c=16.444(5) Å, =98.00(5) deg. The conformation of the 1,4-dithiin rings is a half-chair. As compared to other 3,4-quinolinediyl bis-sulfides an elongated intramolecular sulfur-sulfur distance of 3.437(4) Å in dihydrodithiinoquinolineII was observed.Part XXXI in the series of Azinyl Sulfides.     

Fourier and Hermite series estimates of regression functions

Summary In the paper we estimate a regressionm(x)=E {Y|X=x} from a sequence of independent observations (X ₁,Y ₁),…, (X _n, Y_n) of a pair (X, Y) of random variables. We examine an estimate of a type , whereN depends onn andϕ _N is Dirichlet kernel and the kernel associated with the hermite series. Assuming, that E|Y|<∞ and |Y|≦γ≦∞, we give condition for to converge tom(x) at almost allx, provided thatX has a density. if the regression hass derivatives, then converges tom(x) as rapidly asO(nC^{−(2s−1)/4s}) in probability andO(n ^{−(2s−1)/4s} logn) almost completely.     

Characterization of a discontinuous epitope of the HIV envelope protein gp120 recognized by a human monoclonal antibody using chemical modification and mass spectrometric analysis

A subset of the neutralizing anti-HIV antibodies recognize epitopes on the envelope protein gp120 of the human immunodeficiency virus. These epitopes are exposed during conformational changes when gp120 binds to its primary receptor CD4. Based on chemical modification of lysine and arginine residues followed by mass spectrometric analysis, we determined the epitope on gp120 recognized by the human monoclonal antibody 559/64-D, which was previously found to be specific for the CD4 binding domain. Twenty-four lysine and arginine residues in recombinant full-length glycosylated gp120 were characterized; the relative reactivities of two lysine residues and five arginine residues were affected by the binding of 559/64-D. The data show that the epitope is discontinuous and is located in the proximity of the CD4-binding site. Additionally, the reactivities of a residue that is located in the secondary receptor binding region and several residues distant from the CD4 binding site were also altered by Ab binding. These data suggest that binding of 559/64-D induced conformational changes which result in altered surface exposure of specific amino acids distant from the CD4-binding site. Consequently, binding of 559/64-D to gp120 affects not only the CD4-binding site, which is recognized as the epitope, but appears to have a global effect on surface exposed residues of the full-length glycosylated gp120.     

Beam propagation method in rectangular structures with a high step index

In this paper a simple and effective method to analyze rectangular media with a high step of the refractive index is shown. Comparison of the method with a classical Beam Propagation Method and with analytical solutions for the planar waveguides in the (1+1)dimensional case are presented. There is also shown an influence of a high step of refractive index in (2+1)dimensional structures on the field distribution of guided beams as well as on the far field diffraction. The presented results show that the proposed method is very exact and relatively fast in comparison with other methods.