Infrared multiphoton excitation: Energy distribution of reacting molecules from overtone excitation studies

A method is offered to estimate quantitatively the energy distribution of molecules reacting after being excited by multiphoton absorption of infrared radiation. The method is based on the comparison of the real time reaction kinetics obtained in the infrared multiphoton experiment with those observed upon excitation of a vibrational overtone. Results obtained for the dissociation of tetramethyldioxetane under low-pressure conditions are used as a demonstration.     

Infrared Thermal Emission,Reflection, and Transmission of GaAs Periodic Microstructures

Experimental and theoretical study of thermal emission, transmission, and reflection from bare and coated one-dimensional (1D) and two-dimensional (2D) grating structures on GaAs substrates was carried out. Our experiments have revealed strong correlation between the spectra of thermal emission and the microstructure dimensions. The emissivity enhancement and diminution depends on the groove depth and the spectral range (below or above Rayleigh wavelength). The effects can be attributed to diverse mechanisms: emission escape due to emerging diffraction-order beams, vertical-cavity modes, enhanced optical transmission phenomenon, and electron plasma effect.     

Breakdown Criterion for a Conservation Law with Saturating Dissipation

ABSTRACT

A sufficient condition is given for smooth solutions of a scalar conservation law with a saturating viscous term to develop shocks.     

Percolation theory and fragmentation measures in social networks

We study the statistical properties of a recently proposed social networks measure of fragmentation F after removal of a fraction q of nodes or links from the network. The measure F   is defined as the ratio of the number of pairs of nodes that are not connected in the fragmented network to the total number of pairs in the original fully connected network. We compare this measure with the one traditionally used in percolation theory, _P∞$P_{\infty }$, the fraction of nodes in the largest cluster relative to the total number of nodes. Using both analytical and numerical methods, we study Erd?s–Rényi (ER) and scale-free (SF) networks under various node removal strategies. We find that for a network obtained after removal of a fraction q   of nodes above criticality, _P∞≈(1-F)^1/2$P_{\infty }\approx (1-F{)}^{1/2}$. For fixed _P∞$P_{\infty }$ and close to criticality, we show that 1-F$1-F$ better reflects the actual fragmentation. For a given _P∞$P_{\infty }$, 1-F$1-F$ has a broad distribution and thus one can improve significantly the fragmentation of the network. We also study and compare the fragmentation measure F   and the percolation measure _P∞$P_{\infty }$ for a real national social network of workplaces linked by the households of the employees and find similar results.     

Transforming natural amino acids into alpha-alkyl-substituted amino acids with the help of the HOF.CH3CN complex

Alpha-alkyl amino acids can be efficiently prepared in high yields from the respective amino acids themselves. The key step is the oxidation of the amine function to create the corresponding alpha-nitro acid in a fast and very high yield reaction followed by phase-transfer alkylation and finally reduction to the desired alpha-alkyl amino acid. Several such acids containing aromatic rings or additional carboxylic groups and acids with steric hindrance at the alpha-position are suitable substrates. Several alkyl halides were examined as alkylating agents.     

Acceleration and inhibition of amyloid-β fibril formation by peptide-conjugated fluorescent-maghemite nanoparticles

The formation of amyloid aggregates by association of peptides into ordered structures is hallmark of certain neurodegenerative disorders. Exploring the effect of specific nanoparticles on the formation of amyloid fibrils may contribute toward a mechanistic understanding of the aggregation processes, leading to design nanoparticles that modulate the formation of toxic amyloid plaques. Uniform maghemite (γ-Fe₂O₃) magnetic nanoparticles, containing fluorescein covalently encapsulated within (F-γ-Fe₂O₃)_, were prepared. These F-γ-Fe₂O₃ nanoparticles of 14.0 ± 4.0 nm were then coated with human serum albumin (HSA) via a precipitation process. Covalent conjugation of the spacer arm succinimidyl polyethylene glycol succinimidyl ester (NHS–PEG–NHS) to the F-γ-Fe₂O₃~HSA nanoparticles was then accomplished by interacting the primary amine groups of the HSA coating with excess NHS–PEG–NHS molecules. Covalent conjugation of the peptides amyloid-β 40 (Aβ₄₀) or Leu-Pro-Phe-Phe-Asp (LPFFD) onto the surface of the former fluorescent nanoparticles was then performed, by interacting the terminal activated NHS groups of the PEG derivatized F-γ-Fe₂O₃~HSA nanoparticles with primary amino groups of the peptides. Kinetics of the Aβ₄₀ fibrillation process in the absence and presence of varying concentrations of the Aβ₄₀ or LPFFD conjugated nanoparticles were also elucidated. The non-peptide conjugated fluorescent nanoparticles do not affect the Aβ₄₀ fibrillation process significantly. However, the Aβ₄₀-conjugated nanoparticles (F-γ-Fe₂O₃~HSA–PEG–Aβ₄₀) accelerate the fibrillation process while the LPFFD-conjugated nanoparticles (F-γ-Fe₂O₃~HSA–PEG–LPFFD) inhibit it. By applying MRI and fluorescence imaging techniques simultaneously these bioactive fluorescent magnetic iron oxide nanoparticles can be used as an efficient tool to study and control the Aβ₄₀ amyloid fibril formation process.     

Synthesis and characterization of magnetic poly(divinyl benzene)/Fe3O4, C/Fe3O4/Fe, and C/Fe onionlike fullerene micrometer-sized particles with a narrow size distribution

Magnetic poly(divinyl benzene)/Fe(3)O(4) microspheres with a narrow size distribution were produced by entrapping the iron pentacarbonyl precursor within the pores of uniform porous poly(divinyl benzene) microspheres prepared in our laboratory, followed by the decomposition in a sealed cell of the entrapped Fe(CO)(5) particles at 300 °C under an inert atmosphere. Magnetic onionlike fullerene microspheres with a narrow size distribution were produced by annealing the obtained PDVB/Fe(3)O(4) particles at 500, 600, 800, and 1100 °C, respectively, under an inert atmosphere. The formation of carbon graphitic layers at low temperatures such as 500 °C is unique and probably obtained because of the presence of the magnetic iron nanoparticles. The annealing temperature allowed control of the composition, size, size distribution, crystallinity, porosity, and magnetic properties of the produced magnetic microspheres.     

The role of examples in the learning of mathematics and in everyday thought processes

The purpose of this paper is to present a view of three central conceptual activities in the learning of mathematics: concept formation, conjecture formation and conjecture verification. These activities also take place in everyday thinking, in which the role of examples is crucial. Contrary to mathematics, in everyday thinking examples are, very often, the only tool by which we can form concepts and conjectures, and verify them. Thus, relying on examples in these activities in everyday thought processes becomes immediate and natural. In mathematics, however, we form concepts by means of definitions and verify conjectures by mathematical proofs. Thus, mathematics imposes on students certain ways of thinking, which are counterintuitive and not spontaneous. In other words, mathematical thinking requires a kind of inhibition from the learners. The question is to what extent this goal can be achieved. It is quite clear that some people can achieve it. It is also quite clear that many people cannot achieve it. The crucial question is what percentage of the population is interested in achieving it or, moreover, what percentage of the population really cares about it.     

Robustness of a network of networks

Network research has been focused on studying the properties of a single isolated network, which rarely exists. We develop a general analytical framework for studying percolation of n interdependent networks. We illustrate our analytical solutions for three examples: (i) For any tree of n fully dependent Erd?s-Rényi (ER) networks, each of average degree k, we find that the giant component is P∞ =p[1-exp(-kP∞)](n) where 1-p is the initial fraction of removed nodes. This general result coincides for n = 1 with the known second-order phase transition for a single network. For any n>1 cascading failures occur and the percolation becomes an abrupt first-order transition. (ii) For a starlike network of n partially interdependent ER networks, P∞ depends also on the topology-in contrast to case (i). (iii) For a looplike network formed by n partially dependent ER networks, P∞ is independent of n.     

Solidification of non-halogen fire-retardant liquids by encapsulation within porous uniform PDVB microspheres for preparation of fire-retardant polymeric blends

Polystyrene/polydivinyl benzene (PS/PDVB) composite microspheres of narrow size distribution were prepared by a single-step swelling process of uniform PS microspheres with DVB and benzoyl peroxide, followed by polymerization of DVB within the microspheres. Dissolution of the PS template resulted in porous uniformly sized PDVB microspheres. New, solid, non-halogenated, fire-retardant composite microspheres of narrow size distribution were prepared by encapsulation of resorcinol bis (diphenyl phosphate) (RDP) within the pores of the PDVB microspheres. The encapsulation was performed by two different methods as follows: (1) vacuum and (2) heat/cool cycles. The loading capacity of the RDP into the PDVB particles was elucidated. PS/PDVB-RDP blends were prepared by mixing PS with the PDVB-RDP microspheres. Thermogravimetric analysis (TGA) illustrated that the thermal stability of the PS increases as the content (10–40 %) of the PDVB-RDP increased. Polycarbonate/poly(acrylonitrile-butadiene-styrene)/PDVB-RDP (PC/ABS/PDVB-RDP) blends were prepared by melting PC/ABS together with the PDVB-RDP microspheres at 250 °C and then pelleting it in an injection molding machine at 250 °C and 40 t. The improved thermal stability of the PC/ABS by blending it with PDVB-RDP was demonstrated by a vertical burn test on PC/ABS/PDVB-RDP bones.