Anion Recognition by Uranyl–Salophen Derivatives as Probed by Infrared Multiple Photon Dissociation Spectroscopy and Ab Initio Modeling

The vibrational features and molecular structures of complexes formed by a series of uranyl–salophen receptors with simple anions, such as Cl^?, H^?, and HCOO^?, have been investigated in the gas phase. Spectra of the anionic complexes were studied in the $\tilde \nu $ =800–1800 cm^?1 range by mass‐selective infrared multiple photon dissociation (IRMPD) spectroscopy with a continuously tunable free‐electron laser. The gas‐phase decarboxylation of the formate adducts produces uranyl–salophen monohydride anions, which have been characterized for the first time and reveal a strong U?H bond, the nature of which has been elucidated theoretically. The spectra are in excellent agreement with the results obtained from high‐quality ab initio calculations, which provided the structure and binding features of the anion–receptor complexes.     

A structural study of the retinal photoreceptor, plexiform and ganglion cell layers following exposure to UV-B and UV-C radiation in the albino rat

Over the last two decades, ultraviolet radiation levels (UV), reaching the Earth's surface, have been increasing at a rate of 1.5% per each 1% loss of the ozone layer. Moreover, artificial UV-sources have also proliferated and contributed to the rising UV-stress that many organisms have to face. To assess how the vertebrate retina responds to an exposure of short wavelength UV, we focused our attention on the rat retina, observing photoreceptor (containing outer and inner segments of rods and cones), inner plexiform, and ganglion cell layers by light and transmission electron microscopy using conventional and cytochemical techniques. We analyzed how cells of the layers in question responded to a 30 min exposure to UV-C and UV-B radiation with doses of 7200 and 590 J/cm², respectively. The results show that there are significant changes in the nuclei and cytoplasmic organelles of the exposed retinae when compared with those of the unexposed controls. The changes include an increase in heterochromatin, distension of rough endoplasmic reticulum, mitochondrial disruptions, and increases in the number of myelin bodies. The recorded morphological changes, especially those of the ganglion cells, are suggestive of apoptotic processes and show that the exposure of vertebrate retina to wavelengths ranging from 254 to 312 nm can produce alterations that are likely to impact negatively on the retina's proper functioning.     

Environmental application of pulsed laser-induced incandescence

A portable instrument based on two-color laser-induced incandescence (LII) technique has been designed and developed for the detection of carbonaceous particles for environmental applications. The instrument has been calibrated by performing LII measurements at the exhaust of a home-made soot generator. The incandescence signal from particles sampled into the instrument has been compared with in situ, calibrated, LII measurements to correlate the incandescence signal by the instrument with particles concentration. Measurements of particulate with the LII instrument were then conducted in different environmental conditions, covering a wide range of concentration (from ambient air to cars’ exhaust). The detection limit of the LII instrument has been estimated to be in the range of 200 ng/m³. These measurements have been also compared with results obtained with a commercial aethalometer. The results show a linear relationship between the two sets of measurements, also in the case where significant variation of the carbon particles concentration has been observed over time. These observations allow us to infer that the two instruments are responding in the same way to different carbon particles load, size and nature.     

Isothermal pressure dependence of the solubility of some triazines in supercritical CO2

 The solubility of two related series of each three triazine compounds has been determined in supercritical CO₂, at 40 °C and pressures between 80 and 220⋅10⁵ Pa. A high pressure small volume cell with adjustable optical path and built-in circulation pump has been constructed to be used with quartz windows in a Perkin Elmer UV-VIS spectral photometer. At 220 bar solubilities are between 0.3 and 25 g/L, dependent on the specific triazine structure. The isothermal pressure dependence of the solubility could be modelled using the pressure dependent dielectric constant of CO₂ as the only variable. Received: 30 October 1996/Revised: 3 April 1997/Accepted: 13 April 1997     

Synthesis, characterization and reactivity of new complexes of titanium and zirconium containing a potential tridentate amidinato-cyclopentadienyl ligand

Group 4 metal complexes [M(eta(5)-C(5)Me(4)SiMe(2)-eta(1)-N-2R)(NMe(2))(2)] (R = pyridine, pyrazine, pyrimidine, thiazole, M = Ti; R = pyridine, thiazole; M = Zr) containing the tetramethylcyclopentadienyl-dialkylsilyl bridged amidinato as pendant ligand, were synthesized and characterized by elemental analysis, solution (1)H, (13)C and (15)N NMR spectroscopy and experimental (13)C and (15)N CPMAS in the solid state. The crystal structures of [Ti(eta(5)-C(5)Me(4)SiMe(2)-eta(1)-N-2R)(NMe(2))(2)] (R = pyridine, pyrazine, pyrimidine, thiazole) were determined by single crystal X-ray diffraction studies. All compounds exhibit a distorted tetrahedral geometry, with the ansa-monocyclopentadienyl-amido ligands acting in a bidentate mode. The [M(eta(5)-C(5)Me(4)SiMe(2)-eta(1)-N-2R)(NMe(2))(2)] (R = pyridine, thiazole; M = Zr, Ti) complexes are ethylene polymerization catalysts in the presence of MAO and they are active precursors in regioselective catalytic hydroamination operating with an anti-Markovnikov mechanism.     

Spherical aggregates obtained from N-carboxymethylation and acetylation of chitosan

Chitosan is a biopolymer with great industrial potential. However, solubility in aqueous solutions limits some applications. In this paper, the N-carboxymethylation (CM) of chitosan generated five derivatives namely, N-carboxymethylchitosan (NCMC) 1, NCMC2, NCMC3, NCMC4, and NCMC5, from 10.1% to 80% of CM. Afterwards, a new NCMC6 (∼60% of CM) was acetylated to produce the NCMCAc derivate. The modifications were confirmed by ¹³C nuclear magnetic resonance. A globular conformation to NCMC derivatives was confirmed by association of static and dynamic light scattering in which the form factor (ρ) is ∼1.3 in phosphate buffer (pH 7.4). The NCMCAc showed a ρ of 0.75, which is characteristic of dense spheres. Topographical analyses were performed by atomic force microscopy images in which dense spheres were observed. The critical aggregation concentration (CAC) was determined using pyrene as a hydrophobic fluorescent probe. For NCMC1–6 and NCMCAc, a CAC of 1.0 and 0.1 mg mL⁻¹ were, respectively, observed. NCMC1–6 and NCMCAc showed a globular and dense spherical conformation, respectively.     

Effects of natural convection on thermal explosion in a closed vessel

A new way of ascertaining whether or not a reacting mixture will explode uses just three timescales: that for chemical reaction to heat up the fluid containing the reactants and products, the timescale for heat conduction out of the reactor, and the timescale for natural convection in the fluid. This approach is developed for an nth order chemical reaction, A --> B occurring exothermically in a spherical, batch reactor without significant consumption of A. The three timescales are expressed in terms of the physical and chemical parameters of the system. Numerical simulations are performed for laminar natural convection occurring; also, a theoretical relation is developed for turbulent flow. These theoretical and numerical results agree well with previous experimental measurements for the decomposition of azomethane in the gas phase. The new theory developed here is compared with Frank-Kamenetskii's classical criterion for explosion. This new treatment has the advantage of separating the two effects inhibiting explosion, viz. heat removal by thermal conduction and by natural convection. Also, the approach is easily generalised to more complex reactions and flow systems.     

A general approach to the synthesis of 1-deoxy-L-iminosugars

A stereoselective procedure for the preparation of non-naturally occurring deoxy iminosugars belonging to L-series has been developed. The synthesis involves the construction of the key intermediate bicycle pyperidine 8, available in few steps by the coupling of the heterocyclic synthon 3 and the readily available Garner aldehyde 4.     

Impact of Covalent Modifications on the Hydrogen Bond Strengths in Diaminotriazine Supramolecules

Melamine (M) is a popular triamine triazine compound in the field of supramolecular materials. In this work, we have computationally investigated how substituents can be exploited to improve the binding strength of M supramolecules. Two types of covalent modifications were studied: the substitution of an H atom within an amine group −NHR, and the replacement of the whole −NH₂ group (R=H, F, CH₃ and COCH₃). Through our dispersion-corrected density functional theory computations, we explain which covalent modification will show the best self-assembling capabilities, and why the binding energy is enhanced. Our charge density and molecular orbital analyses indicate that the best substituents are those that generate a charge accumulation on the endocyclic N atom, providing an improvement of the electrostatic attraction. At the same time the substituent assists the main N−H⋅⋅⋅N hydrogen bonds by interacting with the amino group of the other monomer. We also show how the selected group notably boosts the strength of hexameric rosettes. This research, therefore, provides molecular tools for the rational design of emerging materials based on uneven hydrogen-bonded arrangements.