Systematic studies of positron production in heavy-ion collisions near the Coulomb barrier

We present the results obtained from systematic studies of positron creation for a series of heavy-collision systems, with united chargeZ _u =Z ₁ +Z ₂ ranging fromZ _u =164 (Pb + Pb) toZ _u =184 (U+U) at bombarding energies close to the Coulomb barrier, using the Orangeβ-spectrometer at GSI. For each collision system studied, the dominating continuous distributions due to quasiatomic and nuclear positron emission are determined accurately. This is essential in obtaining the characteristics of the still unexplained monoenergetic positron lines which appear in the energy range between 200 keV and 400 keV. Our results are compared with coupled-channels calculations for quasi-atomic positron creation. The latter describe quite well the global features of the measured spectra, but overestimate systematically their absolute values. From the comparison, a common normalization factor of about 0.75 can be established for the calculated spectra. In particular, the dependence onZ _u of the measured emission probabilities was found to follow a power law (∝Z _u ^195±1), in fair agreement with the theoretical prediction.     

Synthesis of the insect antifeedant CDE molecular fragment of 12-ketoepoxyazadiradione and related compounds

A diastereoselective synthesis of the model insect antifeedant 29 a CDE molecular fragment of 12-ketoepoxyazadiradione has been achieved in ten steps from indenone 9 in 44% overall yield. Several of the compounds obtained along the synthesis related to model compound 29 show significant antifeedant activity against Spodoptera littoralis and Spodoptera frugiperda.     

Crystal and molecular structure of thiamine monochloride

The crystal and molecular structure of thiamine monochloride (C₁₂H₁₇N₄OSCl) was determined by X-ray diffraction and refined to a finalR value of 0.042. The compound crystallizes in the monoclinic system, space groupP2₁/a, with cell constantsa=18.929(4),b=11.663(2),c=6.376(2) Å and=96.72(8)°. The thiamine molecule is anhydrous and unprotonated, and the torsion angles at the methylene carbon show that it possesses anF conformation. The dihedral angle value of 84.16(6)° between thiazolium and pyrimidine rings is in the normal range found for the thiamine withF conformation, protonated or not, hydrated or not. Thiamine therefore has the same conformation notwithstanding protonation or hydration, with the rings similarly oriented in all the crystal structures containing thiamine.     

The role of steric and electronic interactions in the stereocontrol of the asymmetric 1,3-dipolar reactions of 5-ethoxy-3-p-(S)-tolylsulfinylfuran-2(5H)-ones with diazoalkanes: theoretical calculations and experimental evidences

The addition of diazomethane and diazoethane to (5S,SS)- and (5R,SS)-5-ethoxy-3-p-tolylsulfinylfuran-2(5H)-ones (1a and 1b) and their 4-methylderivatives (2a and 2b) proceeded in almost quantitative yields and complete regioselectivity. The observed pi-facial selectivity is determined by the configurations at both C-5 and the sulfinyl group, the later being the most important. The syn adducts were almost exclusively obtained from 1a and 2a in apolar solvents but the pi-facial selectivity was strongly decreased in more polar solvents. On the other hand, the major adducts from 1b and 2b were the anti ones and such predominance was slightly increased with solvent polarity. The exo-selectivity was complete in all the cases except for the anti approach to compounds 2a (in polar solvents) and 2b. The role of the sulfinyl group in this behavior was inferred by comparison of these results with those obtained in reactions of diazoalkanes with 5-methoxyfuran-2(5H)-one (3). Steric interactions seem to be the main ones responsible for the observed exo selectivity of reactions with diazoethane, but electronic factors, which can be modulated by the solvent, are also significant in the pi-facial selectivity control. DFT computational methods are able to correctly predict the reactivity, regioselectivity, and pi-facial selectivity exhibited by 5-alkoxyfuranones as well as their changes with the solvent polarity. A C-H.O hydrogen bond, involving the oxygen atom of the 5-alkoxy group at dipolarophiles and the endo-hydrogen atom at dipoles, seems to play a key role in the electronic interactions influencing the stereochemical course of these reactions.     

Some palladium(II) and platinum(II) lead bonded complexes

Complexes of the type M(PPh₃)₂(PbPh₃)₂ [M = Pd, (Ia) and Pt, (Ib)] have been prepared by oxidative addition of hexaphenyldilead to M(PPh₃)₄. The compound Pt(PPh₃)₂(PbPh₃)₂, (Ib), slowly decomposes in dichloromethane to give cis-Pt(PPh₃)₂(PbPh₃)Ph, (II). which can also be obtained by treating (Ib) with the stoichiometric amount of LiPh. Reaction of Pt(PPh₃)₄ with hexamethyldidead gives the complex Pt(PPh₃)₂(PbMe₃)Me directly.The MPb bonds are easily cleaved by bromine, iodine and hydrogen bromide. The X-ray structure of (II) has been determined using three-dimensional counter data and refined by the least-square method (R = 0.07). The crystals are monoclinic a = 22.501, b = 10.502, c = 24.120 Å, β = 113.43°, space group P2₁/c with Z = 4. The complex exhibits a cis configuration, with the coordination around the platinum atom essentially square-planar: the PtPb and PtC(phenyl)bond lengths are 2.698(1) and 2.055(3)Å, respectively.     

Theoretical studies of stereoselectivities of intramolecular aldol cyclizations catalyzed by amino acids

The effects of different amino acid catalysts and substrate substituents on the stereoselectivity of the title reactions have been studied with the aid of density functional theory methods. Experimental data available in the literature have been compiled. B3LYP/6-31G(d) calculations match the general experimental trends and provide useful insights into the origins of the variations in stereoselectivities. Acyclic primary amino acids allow a greater conformational flexibility in the aldol transition states compared with proline. This makes them poorer enantioselective catalysts with triketone substrates with a methyl ketone side chain. The steric repulsion upon substitution at the terminal methyl group increases the energy difference between anti- and syn-chairs with primary amino acid catalysts and, consequently, the stereoselectivities. Proline, in contrast, is a poor catalyst for the latter reactions because the substituent's steric bulkiness raises the activation energy of the favored C-C bond-forming pathway.     

Determination of Benzo[a]pyrene in smoking-flavour agents (water-soluble liquid smoke) by second derivative constant-wavelength synchronous spectrofluorimetry

We have developed a simple, rapid, inexpensive method for the determination of benzo[a]pyrene (BP, a known carcinogen) in smoking-flavour agents (water-soluble liquid smoke; WSLS). After purification of the WSLS by a single passage through a Sep Pak C18 Plus cartridge, BP in the hexane eluate was determined by second derivative constant-wavelength synchronous spectrofluorimetry. Method precision (RSD < 6%) and recovery ( approximately 92%) were satisfactory, and the detection and quantification limits (1.05 and 2.28 mug kg(-1) respectively) indicated that the current maximum permissible concentration of BP in smoke flavourings (10 mug kg(-1)) can be monitored by this method.     

NO Release from trans-[Ru(NH3)4L(NO)]3+ Complexes Upon Reduction (L = 1-Methylimidazole or Benzoimidazole)

The synthesis, characterization and reactivity of trans-[Ru(NH₃)₄(L)NO](PF₆)₃(L = benzoimidazole or 1-methylimidazole in trans position to NO) are presented. ¹H-n.m.r. spectroscopy data indicate that the benzoimidazole and 1-methylimidazole ligands are coordinated to Ru^II through carbon and nitrogen, respectively. The nitrosyl stretching frequencies [(NO) > 1900 cm^–1] suggest that the coordinated nitrosyl has substantial NO⁺ character. The complexes undergo a single-electron reduction (E ⁰–0.245 versus NHE), which involves the coordinated nitrosyl. Dissociation of NO in the reduced species is facilitated by the 1-methylimidazole ligand, which is not observed for the benzoimidazole species. The complex with 1-methylimidazole does not suffer hydroxide attack on the NO⁺, at least at pH values lower than 11.     

Occurrence and analysis of estrogens and progestogens in river sediments by liquid chromatography-electrospray-mass spectrometry

In this study, an analytical procedure for the determination in sediment of the most abundant and/or physiological active estrogens (estradiol, estriol, estrone, ethynyl estradiol, and diethylstilbestrol) and progestogens (progesterone, norethindrone. and levonorgestrel) is described. The procedure includes ultrasonic extraction of the lyophilized sediment, clean-up with octadecylsilica cartridges, and analysis by liquid chromatography-diode array detection-mass spectrometry (LC-DAD-MS). MS detection is performed with an electrospray interface in the positive ion mode for determination of the progestogens and in the negative ion mode for determination of the estrogens. The method was applied to the determination of the target compounds in river sediments from the area of Catalonia. Estrogens and progestogens were found at concentrations usually in the low ng g(-1) range. Estriol and norethindrone were the compounds most frequently found whereas maximum concentrations in all sediment samples were obtained for ethynyl estradiol (22.8 ng g(-1)) and estrone (11.9 ng g(-1)). Detection limits were in the range of 0.04-1.00 ng g(-1). Preliminary conjectures with regards to the environmental behavior and impact of estrogens and progestogens in rivers are made. To the authors' knowledge, this is the first work reporting a detailed method for the analysis of estrogens and progestogens in river sediments and data on the environmental occurrence of both groups of compounds.     

Unified and generalized Fresnel numbers

The concept of Fresnel number is discussed and expressions are derived for misaligned optical systems. For the case of perfectly aligned optical elements, the usual number,N, based on a Fresnel zone concept is found to be given byN = (a ²/)(D ₁/B ₁ +A ₂/B ₂), whereB ₁,D ₁ andB ₂,A ₂ are the transfer matrix elements of the optical systems before and after a circular aperture of radiusa respectively. A modified definition of the Fresnel number is proposed for Gaussian beam propagation. This parameterN _G, is related to the complex beam parameter and may be represented by the angle = tan^–1 N _G, found in the familiar Collins chart and its dual representation. A general relation for the transformation of this Fresnel number is found. The expressions for Gaussian beam transformation are thus simplified, since the waist-waist transform is given byN _G1 =N _G2 = 0. Finally, these two kinds of Fresnel numbers are written as tensors when applied to cases involving elliptical apertures, astigmatic beams and nonsymmetrical systems.