Optical absorption in vitreous GeSb2Se4

Optical absorption of vitreous GeSb₂Se₄ was studied in spectral region 0.7–25 μm. At low absorption levels near the edge the absorption coefficient K depends exponentially on energy. At high absorption levels the quadratical energy dependence of K is observed. In the present work we determined the optical energy gap E^opt_g = 1.29 eV and discussed the temperature dependence of the absorption coefficient. Measured reflectivity curves were used to estimate the value of the refractive index of GeSb₂Se₄ ($\text{n = 3.13–3.56}\text{at}\text{h}\text{?}\text{ω = 1.00–1.70}\text{eV}$). Vitreous GeSb₂Se₄ is also transparent in the spectral interval 2–15.7 μm.     

Functionalisation of ethereal-based saturated heterocycles with concomitant aerobic C–H activation and C–C bond formation

With an ever-growing emphasis on sustainable synthesis, aerobic C–H activation (the use of oxygen in air to activate C–H bonds) represents a highly attractive conduit for the development of novel synthetic methodologies. Herein, we report the air mediated functionalisation of various saturated heterocycles and ethers via aerobically generated radical intermediates to form new C–C bonds using acetylenic and vinyl triflones as radical acceptors. This enables access to a variety of acetylenic and vinyl substituted saturated heterocycles that are rich in synthetic value. Mechanistic studies and control reactions support an aerobic radical-based C–H activation mechanism.

Herein we disclose a novel method for the aerobic C–H activation of ethereal-based heterocycles to generate various α-functionalised building blocks.     

A structure determination protocol based on combined analysis of 3D-ED data,powder XRD data,solid-state NMR data and DFT-D calculations reveals the structure of a new polymorph of l-tyrosine

We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic ¹³C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state ¹³C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs.

A new polymorph of l-tyrosine is reported, with the crystal structure determined by combined analysis of 3D-ED data and powder XRD data, augmented by information from periodic DFT-D calculations and solid-state ¹³C NMR data.     

Offline and online capillary electrophoresis enzyme assays of β-N-acetylhexosaminidase

Enzyme assays of β-N-acetylhexosaminidase from Aspergillus oryzae using capillary electrophoresis in the offline and online setup have been developed. The pH value and concentration of the borate-based background electrolyte were optimized in order to achieve baseline separation of N,N′,N″-triacetylchitotriose, N,N′-diacetylchitobiose, and N-acetyl-d-glucosamine. The optimized method using 25 mM tetraborate buffer, pH 10.0, was evaluated in terms of repeatability, limits of detection, quantification, and linearity. The method was successfully applied to the offline enzyme assay of β-N-acetylhexosaminidase, which was demonstrated by monitoring the hydrolysis of N,N′,N″-triacetylchitotriose. The presented method was also utilized to study the pH dependence of enzyme activity. An online assay with N,N′-diacetylchitobiose as a substrate was developed using the Transverse Diffusion of Laminar Flow Profiles model to optimize the injection sequence and in-capillary mixing of substrate and enzyme plugs. The experimental results were in good agreement with predictions of the model. The online assay was successfully used to observe the inhibition effect of N,N′-dimethylformamide on the activity of β-N-acetylhexosaminidase with nanoliter volumes of reagents used per run and a high degree of automation. After adjustment of background electrolyte pH, an online assay with N,N′,N″-triacetylchitotriose as a substrate was also performed.

The role of nanocellulose fibers, starch and chitosan on multipolysaccharide based films

Thin nanocomposite films of thermoplastic starch, chitosan and cellulose nanofibers (bacterial cellulose or nanofibrillated cellulose) were prepared for the first time by solvent casting of water based suspensions of the three polysaccharides. The role of the different bioploymers on the final properties (thermal stability, transparency, mechanical performance and antimicrobial activity) of the films was related with their intrinsic features, contents and synergic effects resulting from the establishment of interactions between them. Thermoplastic starch displays an important role on the thermal stability of the films because it is the most stable polysaccharide; however it has a negative impact on the mechanical performance and transparency of the films. The addition of chitosan improves considerably the transparency (up to 50 % transmittance for 50 % of chitosan, in respect to the amount of starch), mechanical performance and antimicrobial properties (at least 25 % of chitosan and no more than 10 % of cellulose nanofibers are required to observe bacteriostatic or bactericidal activity) but decrease their thermal stability. The incorporation of cellulose nanofibers had the strongest positive impact on the mechanical properties of the materials (increments of up to 15 and 30 MPa on the Young′s modulus and Tensile strength, respectively, for films with 20 % of BC or NFC). Nonetheless, the impact in thermal stability and mechanical performance of the films, promoted by the addition of chitosan and cellulose nanofibres, respectively, was higher than the expected considering their percentage contents certainly because of the establishment of strong and complex interactions between the three polysaccharides.     

Evidence for symplectic symmetry in Ab initio no-core shell model results for light nuclei

Clear evidence for symplectic symmetry in low-lying states of 12C and 16O is reported. Eigenstates of 12C and 16O, determined within the framework of the no-core shell model using the J-matrix inverse scattering potential with A相似文献   

On the strength of hydrogen bonding within water clusters on the coordination limit

Hydrogen bonds (HB) are arguably the most important noncovalent interactions in chemistry. We study herein how differences in connectivity alter the strength of HBs within water clusters of different sizes. We used for this purpose the interacting quantum atoms energy partition, which allows for the quantification of HB formation energies within a molecular cluster. We could expand our previously reported hierarchy of HB strength in these systems (Phys. Chem. Chem. Phys., 2016, 18 , 19557) to include tetracoordinated monomers. Surprisingly, the HBs between tetracoordinated water molecules are not the strongest HBs despite the widespread occurrence of these motifs (e.g., in ice I_h). The strongest HBs within H₂O clusters involve tricoordinated monomers. Nonetheless, HB tetracoordination is preferred in large water clusters because (a) it reduces HB anticooperativity associated with double HB donors and acceptors and (b) it results in a larger number of favorable interactions in the system. Finally, we also discuss (a) the importance of exchange-correlation to discriminate among the different examined types of HBs within H₂O clusters, (b) the use of the above-mentioned scale to quickly assess the relative stability of different isomers of a given water cluster, and (c) how the findings of this research can be exploited to indagate about the formation of polymorphs in crystallography. Overall, we expect that this investigation will provide valuable insights into the subtle interplay of tri- and tetracoordination in HB donors and acceptors as well as the ensuing interaction energies within H₂O clusters.