Quantitative photoacoustic spectroscopy of cataractous human lenses

Quantitative photoacoustic spectra of the nuclei of cataractous human lenses with various degrees of colouration and opacification were measured in the spectral range 250-600 nm. The lens nuclei were obtained from 20 cataractous patients through extracapsular cataract extraction. These measurements yield the light loss per unit path length in the nucleus of cataractous lenses.     

Mechanical properties and molecular mobility in oriented polyolefines

The static modulus of elasticity (E) and the correlation time of rotation ( _c ) of 2,-2,6,6-tetramethylpiperidine-1-oxyl are studied as a function of the temperature (210<T <350°K) for oriented films of isotactic polypropylene and polyethylene of high and low density.E and _c change both upon heating and polymer orientation; this result indicates that sample properties are influenced by the microstructure of the amorphous phase where probes are localized.     

On the applicability of the HSAB principle through the use of improved computational schemes for chemical hardness evaluation

Finite difference schemes, named Compact Finite Difference Schemes with Spectral-like Resolution, have been used for a less crude approximation of the analytical hardness definition as the second-order derivative of the energy with respect to the electron number. The improved computational schemes, at different levels of theory, have been used to calculate global hardness values of some probe bases, traditionally classified as hard and soft on the basis of their chemical behavior, and to investigate the quantitative applicability of the HSAB principle. Exchange acid-base reactions have been used to test the HSAB principle assuming the reaction energies as a measure of the stabilization of product adducts.     

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

We report the structural properties of ultra-small ThO₂ and UO₂ nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO₂ and UO₂ NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO₂ and UO₂ materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th–O and U–O bond distances despite retaining average bond lengths that are characteristic of bulk ThO₂ and UO₂. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

ThO₂ and UO₂ nanoparticles synthesized using a COF-5 template exhibit unpassivated surfaces and provide insight into nanoscale properties of actinides.     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Bond energies and attachments sites of sodium and potassium cations to DNA and RNA nucleic acid bases in the gas phase

Gas-phase metal affinities of DNA and RNA bases for the Na(+) and K(+) ions were determined at density functional level employing the hybrid B3LYP exchange correlation potential in connection with the 6-311+G(2df,2p) basis set. All the molecular complexes, obtained by the interaction between several low-lying tautomers of nucleic acid bases and the alkali ions on the different binding sites, were considered. Structural features of the sodium and potassium complexes were found to be similar except in some uracil and thymine compounds in which the tendency of potassium ion toward monocoordination appeared evident. B3LYP bond energies for both metal ions were in agreement with the available experimental results in the cases of uracil and thymine for which the most stable complex was obtained starting from the most stable tautomer of the free nucleic acid base. For adenine, although the interaction of the ions with the most stable free tautomer generated the least stable molecular complex, the best agreement with experiment was found in just this case. For the remaining cytosine and guanine bases, our calculations indicated that the metal ion affinity value closest to experiment should be determined taking into account the role played by the different tautomers of the free bases with similar energy and all the possible complexes obtained by them.     

Condensed heterocycles containing the pyrimidine nucleus

Continuing earlier studies designed to obtain derivatives of 1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one and of the isomeric 7-one of pharmacological interest, some novel compounds 2 and derivatives of 6,7,8,9-tetrahydro-5H-1,3,4-thiadiazolo[2,3-b]quinazolin-5-one ( 3 ) were prepared. Derivatives of pyrimido[2,1-b]benzothiazol-2-one ( 6 ) and of the isomeric 4-one derivatives 8 were also synthesized. Structural identification was obtained by ¹H-nmr, ir and mass spectra.     

Rhodium and platinum complexes as catalysts for the polymerization of phenylacetylene

In polymerization reactions of phenylacetylene three different types of polyphenylacetylene (PPA) were prepared by using Rh and Pt complexes as catalysts in different reaction conditions. Type I PPA is obtained with [Rh (COD) Chel] PF₆ complexes (COD = cis,cis-cycloocta 1,5-diene; chel = 2,2′-bipyridine, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline) in bulk, benzene methanol, while type II PPA is obtained with the same catalysts in p-dioxane and type III PPA in the presence of [Pt (? C?CPh)₂(PPh₃)₂] in bulk. Type I, II, and III PPA exhibit different IR and ¹H-NMR spectra, which have been compared with literature data. Correlations proposed by different Authors between spectral properties of PPA and chain structures are also discussed.     

The influence of the ligands on the catalytic activity of a series of RhI complexes in reactions with phenylacetylene: Synthesis of stereoregular poly(phenyl) acetylene

The catalytic activity of a series of [Rh L-L chel]X complexes, in which we have varied the unsaturated ligand [L-L = cis, cis-cycloocta 1,5-diene(cod) or 2,5-norbornadiene(nbd) the nitrogen chelating ligand [chel = 2,2′-bipyridine(bipy), 2,2′-dipyridylamine(dipyam), 2,2′-bipyrazine (bipz), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me₂bipy)] and the counter ion [X = PF₆, ClO₄, BPh₄], has been examined in reactions with phyenylacetylene (PA). The catalytic behaviour of the [Rh(cod)Cl₂],tmeda (tmeda = N,N,N′,N′tetramethylethylendiamine), [Rh(cod)Cl₂],teda] (teda = triethylendiamine), of the dimer [Rh(cod)Cl]₂, and the use of NaOH as cocatalyst in different reaction conditions was also examined. The influence of the ligands on the catalytic activity of these Rh^I complexes is discussed. ¹H and ¹³C NMR spectra have shown that highly stereoregular polyphenylacetilene can be obtained. Conditions for homogeneous doping of PPA, to obtain materials whose conductivity varies over 10–11 magnitude orders, are proposed. The stability of the doped polymers is also discussed.