Study of the dynamic structure factor of strong glasses

The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm⁻¹. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q² dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.     

1‐(9H‐Carbazol‐4‐yloxy)‐3‐{[2‐(2‐methoxyphenoxy)ethyl]amino}propan‐2‐ol hemihydrate: a carvedilol solvatomorph

In the title racemic hemihydrated solvatomorph of carvedilol (carv), C₂₄H₂₆N₂O₄·0.5H₂O, the asymmetric unit contains two independent organic moieties and one water molecule. Within this 2(carv)·H₂O unit, the molecular components are strongly linked by hydrogen bonds and the unit acts as the basic building block for the crystal structure. Interactions parallel to (10) generate hydrogen‐bonded layers which are further linked by much weaker C—H...N/O interactions. The conformations of the organic molecules, as well as the hydrogen‐bonding interactions connecting them, are compared with other related structures in the literature.     

Determination of hydroxycarbamates in aqueous matrices by direct derivatization and GC-MS analysis in chemical ionization mode

A rapid and effective procedure, developed for the determination of polar compounds in water, has been applied to hydroxycarbamates. In few minutes, it is possible to perform a catalyzed derivatization of such compounds directly in the aqueous medium, using n-hexyl chloroformate. Extraction with n-hexane and injection into a benchtop GC-MS system are the only two further steps to complete the analysis. Chemical ionization proved to be more effective than electron impact to produce valuable mass spectra in terms of selectivity and sensitivity. Linear quantitative responses were demonstrated over a two-order-of-magnitude range. The minimum detectable concentration was 3 ppb for acetohydroxamic acid and 10 ppb for hydroxyurethane, obtained from Po river water samples spiked with the two hydroxycarbamate standards.     

Non-hemolytic α-AApeptides as antimicrobial peptidomimetics

We report a new class of peptide mimetics, α-AApeptides, that display broad-spectrum activity against both Gram-negative and Gram-positive bacteria and fungi. With non-hemolytic activity, resistance to protease hydrolysis, and easy sequence programmability, α-AApeptides may emerge as a novel class of antibiotics.     

Capillary electrophoresis analysis of inorganic cations in post-blast residue extracts applying a guanidinium-based electrolyte and bilayer-coated capillaries

A new CE method was developed for the identification and quantitation of inorganic cations in post‐blast residues. The simultaneous analysis in 20 min total runtime of eight cations in post‐blast residues (ammonium, potassium, monomethylammonium, calcium, sodium, magnesium, strontium), plus lithium cation as the internal reference, was carried out with a BGE involving a non‐CMR (carcinogenic, mutagenic, and harmful to reproduction) chromophore (guanidinium cation) and a double‐layer modified capillary (hexadimethrine bromide/polyvinylsulfonate). A study of UV detection conditions using guanidinium ion as the probe led us to set the analysis and reference wavelengths and their associated bandwidths as well as the probe concentration fixed at 15 mM. The successive multiple ionic‐polymer layer approach limited the cation adsorption on capillary wall and improved the EOF stability. These caused a significant improvement in method repeatability. Intermediate precisions were 2.4% for corrected areas and 1.3% for normalized migration times. Limits of detection close to 1 mg/L for all cations were obtained. The matrix effects were studied with chemometric approach for different matrices representative of those collected after explosion. Tests with blank matrix extracts of soil, cloth, and with simulated matrix extract containing 800 mg/L Ca²⁺ and 500 mg/L Fe²⁺ were carried out and no significant matrix effects were observed. Finally, analyses of real residues collected after cash dispenser and homemade firework explosions demonstrate excellent correlation between the CE results and those obtained with the ion chromatography method used routinely.     

Exocyclic push–pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene- and 2-cyanoimino-quinazolines and -benzodiazepines

The structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, 4 (X=–CN,–COOEt) and their 2-cyanoimino substituted analogues 2, 3c,d (X=–CN,–SO₂C₆H₄–Me(p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods. The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push–pull effect in this part of the molecules the restricted rotation about the partial C²,C¹¹ and C²,N¹¹ double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1–4, to calculate the barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. In the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism.     

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor–π–acceptor)⁺ systems with a methyl pyridinium or quinolinium as the electron‐deficient group, a dimethyl amino as the electron‐donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD‐DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited‐state optimized geometries (planar in low‐polarity media and twisted in high‐polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.