Stereocontrol in DL-Leucine N-Carboxyanhydride Polymerizations

L-Leucine and DL-leucine N-carboxyanhydride were polymerized in 1,4-dioxane at 25°C with either benzylamine or L-a-methylbenzylamine as initiator, using a constant volume reactor for rate studies. A rapid initiation period was followed by two pseudo-first-order propagation periods for the racemic (DL) monomer and by three pseudo-first-order propagation periods for the L-monomer. The ratio of rate constants of the L- to the DL-polymerization is about 2 for any given polymerization period, independent of initiator type. Such a behavior is expected for stereoselective propagations without crossover, leading to blends of isotactic polymers.     

Ageing and thermal stability studies on quasi-solid composite electrolytes for Grätzel-type solar cells

Loss of volatile components from two types of quasi-solid gel electrolytes to be used in Grätzel (or dye-sensitized, DSSC)-type alternative solar cells are checked time to time by thermogravimetry (TG) to follow the stability or changes of composites during storage at ambient. One of the gel samples, labeled as P2000 and based on UreaSil 2000, a 3D-cage precursor compound, has been found quite stable, while the related P230 sample based on UreaSil 230 seems to have a bit higher deviation in heterogeneity and some tendencies losing gradually from its volatile content, time to time. In addition, two online coupled evolved gas analytical tools (TG–EGA–FTIR and TG/DTA–EGA–MS) are applied to check the gel electrolytes for thermal vaporization, degradation, and decomposition processes as a function of temperature during dynamic heating in air. Initial minor elimination of ethyl acetate has been detected by both in situ coupled mass spectroscopy and FTIR spectroscopic gas cell. According to both evolved gas analytical systems, the other liquid ingredients, acetic acid (AcOH) and sulfolane are also released in the order of their volatility. Above 300 °C, an oxidative thermal degradation of UreaSil-type host matrices is indicated by the occurrence of, among others, formaldehyde or ammonia for electrolyte samples P2000 and P230, respectively. Evolution of various alkyl iodides with short chain length has only been detected by the mass spectroscopic evolved gas analysis method only.     

Third-order nonlinear optical properties of silver nanoparticles mediated by chitosan

Silver nanoparticles in chitosan medium were prepared by the chemical reduction method. Silver nitrate and hydrazine were used as the precursor and reducing agent in the present of chitosan as a natural host polymer. The samples are characterized by UV–visible spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The measurements of nonlinear optical properties were defined by Z-scan technique using green CW laser beam operated at 532 nm wavelengths. Thermal effect has a dominant role in the overall material nonlinearity with CW laser. It is shown that the synthesized samples have a negative nonlinear refractive index.     

More on the identity of Chaundy and Bullard

In what follows we present a homogeneous identity which implies a more elementary treatment of the Chaundy–Bullard identity with n variables. In a different direction we bring another ramification of the Chaundy–Bullard identity.     

A broad and tunable 250- to 430-nm source for microscopy and lifetime measurements by frequency doubling of a 78-MHz-picosecond white-light laser

Broadly tunable picosecond pulses in the UV for nonlinear microscopy and lifetime measurements are not yet readily available. Complex synchronously pumped optical parametric oscillators with subsequent frequency doubling are typically used. We show that direct second harmonic generation of a visible picosecond supercontinuum source at 78 MHz renders pulses easily tunable from 250 to 430 nm. We find that an unexpectedly large numerical aperture and the use of thick crystals increase the efficiency of the frequency doubling process dramatically. The observed spectral width and efficiency are nearly two orders of magnitude larger than predicted by conventional theory. With broadband achromatic doubling, a 130 nm wide spectrum is achieved. Pulse durations of 17–35 ps are found in the UV and an average power between 1 and 70 μW. This qualifies the setup for most UV-based microscopic investigations. As first application, the fluorescence lifetime of two differing conformations of 2-(2′-hydroxyphenyl) benzothiazole is measured.     

Investigations of the half-metallic behavior and the magnetic and thermodynamic properties of half-Heusler CoMnTe and RuMnTe compounds: A first-principles study

First-principles spin-polarized density functional theory （DFT） investigations of the structural, electronic, magnetic, and thermodynamics characteristics of the half-Heusler, CoMnTe and RuMnTe compounds are carried out. Calculations are accomplished within a state of the art full-potential （FP） linearized （L） augmented plane wave plus a local orbital （APW ＋ lo） computational approach framed within DFT. The generalized gradient approximation （GGA） parameterized by Perdew, Burke, and Ernzerhof （PBE） is implemented as an exchange correlation functional as a part of the total energy calculation. From the analysis of the calculated electronic band structure as well as the density of states for both compounds, a strong hybridization between d states of the higher valent transition metal （TM） atoms （Co, Ru） and lower valent TM atoms of （Mn） is observed. Furthermore, total and partial density of states （PDOS） of the ground state and the results of spin magnetic moments reveal that these compounds are both stable and ideal half-metallic ferromagnetic. The effects of the unit cell volume on the magnetic properties and half-metaliicity are crucial. It is worth noting that our computed results of the total spin magnetic moments, for CoMnTe equal to 4 ~tB and 3 p-B per unit cell for RuMnTe, nicely follow the rule μ2tot = Zt - 18. Using the quasi-harmonic Debye model, which considers the phononic effects, the effecs of pressure P and temperature T on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature, and heat capacity for these compounds are investigated for the first time.     

Chattering: an overlooked peculiarity of rocking motion

Nonlinear Dynamics - Complete chattering occurs when a structure undergoes a theoretically infinite sequence of impacts in finite time, that eventually bring the structure to the state of...     

Enantiopure Isoindolinones through Viedma Ripening

Here we demonstrate that deracemization of isoindolinones using Viedma ripening is possible starting from a racemic mixture of conglomerate crystals. Crystals of the enantiopure isoindolinones lose their chiral identity upon dissolution even without the need for a catalyst. This enabled complete deracemization of the reported isoindolinones without a catalyst.