Oxidative addition - reductive elimination sequences in the photochemistry of some bis(phosphine)platinum complexes

The photolysis of [L₂Pt(C₂H₄)] (L = PPh₃, P(p-C₆H₄CH₃)₃ complexes in halocarbon solvents (CH₂Cl₂, CH₂Br₂) gives C₂H₄ and the coordinatively unsaturated species [L₂Pt]. Photolysis of platinum metallacycles [L₂Pt(CH₂)₄] (L = PPh₃, P(n-Bu)₃) generates alkanes, alkenes and [L₂Pt]. The [L₂Pt] centers are very reactive, and under prolonged photolysis undergo oxidative addition of CH₂Cl₂ forming the trans-[L₂Pt(CH₂Cl)Cl] complexes. Under appropriately controlled conditions the trans complexes isomerize to cis species before bimolecular C₂H₄ elimination occurs and [L₂PtCl₂] is formed as the final product. The oxidative addition-reductive elimination mechanism is discussed on the basis of spin-trapping experiments, quantum yield values, and the sensitivity to radical inhibitors and to solvents.     

Synthesis and characterization of palladium–tin bicolloids prepared by chemical liquid deposition

Palladium–tin bicolloids have been prepared by chemical liquid deposition. The metals were cocondensed at 77 K with ethanol, 2-propanol, 2-methoxyethanol, 2-butanone and acetone. The distribution of particle sizes was determined by transmission electron microscopy of the stables dispersions. The sizes ranged from 3.8 nm for 2-methoxyethanol to 8.3 nm for acetone colloids. Electrophoretic measurements such as colloid charge and zeta potential were achieved. lt was found that the colloids possess electrical charge; therefore, it is postulated that their stability is by simple solvatation. The colloids showed stability over 1 week at room temperature. The zeta-potential values are in agreement with the stability and electrophoretic mobility. The highest zeta potential was obtained for PdSn–2-methoxyethanol colloids with 379 mV and the lowest for 2-butanone with 114 mV. The colloids exhibit absorption bands in the UV region. In the visible region no plasma absorption was found. Active solids obtained by evaporation of the solvent contain a certain amount of the solvent incorporated, and owing to their reactivity they produce a mixture of tin oxide with palladium and tin. The presence of solvents can be observed by Fourier transform IR incorporation in the finely divided solids. Characteristic bands for each solvent were measured. By means of thermogravimetric analysis and differential scanning calorimetry the thermal stability of the solids and the transition heat give us the carbonaceous residues in the films. The elemental analysis of the powders was carried out.     

Mass spectra of picolinic,nicotinic and isonicotinic acids and of their respective N-oxides

Mass spectra of picolinic, nicotinie and isonicotinie acids and of their respective N-oxides are presented. Picolinic acid and its N-oxide demonstrate the facile loss of carbon dioxide and low-intensity molecular ions. Fragmentation is relatively simple. Nicotiuic and isonicotinic acids show very similar fragmentation patterns. For both, the molecular ion is the base peak. The fragmentations of nicotinic acid N-oxide and of isonicotinic acid N-oxide are more complex than those of the previous four compounds, but are similar to each other.     

Chiral optical absorption bands in [ ]Cd1−x Mnx Ga2S4 semiconducting compound

[ ]Cd_1−x Mn_x Ga₂S₄ is a semimagnetic semiconductor and it has revealed an exceptional property namely ‘optical activity‘. Therefore, a spectroscopic investigation of chiral absorption bands has been carried out with the view to examine the role of d*-d states of manganese atoms. It has been found that inner transitions of Mn⁺⁺ dominate the spectral region with a special feature, indicating that these transitions show the presence of a substantial contribution from the magnetic dipole moment which rotates the electric vector of the incident polarized radiation. The origin is associated to the lack of a symmetry center caused by the ordered vacancies in this defect compound.     

The Computational Complexity of Extrapolation Methods

This paper analyzes the cost of extrapolation methods for non-stiff ordinary differential equations depending on the number of digits of accuracy requested. Extrapolation of the explicit midpoint rule is applied for various number sequences. We show that for sequences with arithmetic growth, the cost of the method is polynomial in the number of digits of accuracy, while for sequences of numbers with geometric growth, the cost is super-polynomial with respect to the same parameter.     

Polynomial cost for solving IVP for high-index DAE

We show that the cost of solving initial value problems for high-index differential algebraic equations is polynomial in the number of digits of accuracy requested. The algorithm analyzed is built on a Taylor series method developed by Pryce for solving a general class of differential algebraic equations. The problem may be fully implicit, of arbitrarily high fixed index and contain derivatives of any order. We give estimates of the residual which are needed to design practical error control algorithms for differential algebraic equations. We show that adaptive meshes are always more efficient than non-adaptive meshes. Finally, we construct sufficiently smooth interpolants of the discrete solution. AMS subject classification (2000) 34A09, 65L80, 68Q25     

Adaptivity and computational complexity in the numerical solution of ODEs

In this paper we analyze the problem of adaptivity for one-step numerical methods for solving ODEs, both IVPs and BVPs, with a view to generating grids of minimal computational cost for which the local error is below a prescribed tolerance (optimal grids). The grids are generated by introducing an auxiliary independent variable τ and finding a grid deformation map, t=Θ(τ), that maps an equidistant grid {τ_j} to a non-equidistant grid in the original independent variable, {t_j}. An optimal deformation map Θ is determined by a variational approach. Finally, we investigate the cost of the solution procedure and compare it to the cost of using equidistant grids. We show that if the principal error function is non-constant, an adaptive method is always more efficient than a non-adaptive method.     

Process‐Scale Total Synthesis of Nature‐Identical (−)‐(S,S)‐7‐Hydroxycalamenal in High Enantiomeric Purity through Catalytic Enantioselective Hydrogenation

A process‐scale stereoselective synthesis of nature‐identical (−)‐(S,S)‐7‐hydroxycalamenal (=(−)‐(5S,8S)‐5,6,7,8‐tetrahydro‐3‐hydroxy‐5‐methyl‐8‐(1‐methylethyl)naphthalene‐2‐carbaldehyde; (−)‐ 1a ) in 96% enantiomeric excess (ee) with the aid of chiral Ru complexes has been developed. The key step was the enantioselective hydrogenation of easily accessible 2‐(4‐methoxyphenyl)‐3‐methylbut‐2‐enoic acid ( 10 ) to (+)‐ 11 in a 86% ee (Scheme 5 and Table 1). A substantial increase in optical purity (96% ee) was achieved by induced crystallization of the intermediate (+)‐3,4‐dihydro‐4‐(1‐methylethyl)‐7‐methoxy‐2H‐naphthalen‐1‐one ((+)‐ 3 ). Computational conformation analysis carried out on the analog (−)‐ 9 rationalized the high diastereoselectivity achieved in the catalytic hydrogenation of the CC bond.     

Ultrastructural study of first and second order neurons in the visual system of the crab Ucides cordatus following exposure to ultraviolet radiation

The visual system as an interface between the environment and the living organism can serve as a sensitive indicator especially in studies that deal with effects of radiation. The crab retina as the seat of the photoreceptors and the lamina ganglionaris as the place of second order neurons were the targets of our study. Conventional specimen preparation techniques for transmission electron microscopy (TEM) are adequate to preserve any modifications that may occur as a consequence of the experimental treatment. In this study we analyzed by TEM how retinal and lamina ganglionaris cells of the crab Ucides cordatus responded to a 30 min exposure to ultraviolet C (UV-C) and ultraviolet B (UV-B) radiation with doses of 7200 and 590 J/cm², respectively. The results show that damaged cells occurred in both retina and lamina ganglionaris, but that the retinal cells were affected to a greater extent. Morphological alterations of the pigment granules and an increase in the quantity of lipid droplets of the retinal cells were also observed. Additional changes include an increase in heterochromatin, nuclear karyolyses and karyorrhexes, distention of rough endoplasmic reticulum and mitochondrial disruptions. The observed morphological changes are indicative of apoptotic processes and show that an exposure to light of wavelengths of 254 and 312 nm may be injurious to the visual system of invertebrates.     

Brominaed β-carbolines from the marine hydroid linnaeus

Three new brotminated β-carbolines (1–3) have been isolated from the lipophylic extract of the marine hydroid Linnaeus and their structures determined on the basis of spectral data including two dimensional proton-carbon shift correlation (direct and long range) NMR spectroscopy. The syntheses of compounds 1–3, starting from the appropriate brominated tryptophan derivatives, are also described.