Structure électronique de quelques cyclophosphazènes halogénés de symétrie Dnh

The electronic structure of some D_nh cyclophosphazenes is investigated by the /2 approximation of the method. The results allow a critical analysis of ideas previously developed by Craig et al. and Dewar et al. This work supports the localised model of Dewar and tests the Faraday effect experimentally.     

Length dependence of excitation energies in linear polyenes: Localized and delocalized descriptions

The length dependence of the lowest allowed transition energy of linear polyenes is studied using delocalized SCF and localized excitonic approaches. Within the PPP SCF approximations the calculated transition energies converge to a finite values as N^?1 as the number of double bonds (N) becomes large, when the excited state contains all singly excited configurations. On the other hand, the fully localized excitonic method at the level of single excitations, although it predicts a gap in the excitation spectrum of an infinite polyene, gives results which converge to this value as N^?2. The inclusion of double and triple excitations into the excitonic method by means of perturbation theory does not appear to change this behavior. The reasons for the discrepancy between the two approaches is analyzed. Experimentally, the transition energies in solution converge to a finite value as N^?1 to a good degree of approximation. If it is assumed that the solvent shift is constant for long polyenes, the available experimental results favour the delocalized approach as the starting point in describing the length dependence of the excitation energy of long polyenes.     

Application of capillary electrophoresis with field-amplified sample injection for the detection of new adrenoreceptor antagonist enantiomers in plasma in the low ng/mL concentration range

Throughout the separation of chiral basic drugs by capillary electrophoresis (CE) with neutral hydroxypropyl-beta-cyclodextrin (HP-beta-CD) as chiral selector, the sensitivity of detection has been improved by using field-amplified sample injection (FASI). In the present work, this on-line stacking method has been used to detect low ng/mL levels of cationic enantiomers of a new adrenoreceptor antagonist in plasma. A systematic study of the parameters affecting on-line concentration of these enantiomers (nature of the preinjection plug, composition of sample solvent, injection times of water and sample plugs) has been performed enabling the detection sensitivity of antagonist enantiomers to be improved by 180 times compared with usual hydrodynamic injection. The quantification of each adrenoreceptor antagonist enantiomer in plasma samples was then performed in the 2-100 ng/mL (or 8-400 nM) concentration range after a solid-phase extraction step. Using this FASI-CE-UV procedure, the limit of quantification (LOQ) for each enantiomer was in the low ng/mL concentration range (3 ng/mL or 10 nM).     

A versatile access to unsymmetrical and symmetrical α-diketones via organotin reagents

Unsymmetrical α -diketones have been obtained via cross coupling of acyltins with acyl halides under PdCl₂(PPh₃)₂ catalysis while symmetrical α -diketones have been readily obtained via “in situ” formation of acyltins using hexabutylditin and acyl chlorides under similar experimental conditions.     

Polymer-supported organotin reagents for regioselective halogenation of aromatic amines

[reaction: see text] Polymer-supported triorganotin halides were used in the halogenation reaction of aromatic amines. Treatment of aromatic amines with n-butyllithium and polymer-supported organotin halides gave the corresponding polymer-bound N-triorganostannylamines, which by treatment with bromine or iodine monochloride gave the para-halogenated aromatic amines with high yields and high selectivities. The polymer-supported organotin halides reagents regenerated during the course of the halogenation reaction can be reused without loss of efficiency. The presence of tin residues in halogenated aromatic amines was also investigated and evaluated at under 20 ppm after three runs.     

Delocalized and localized pictures of excited and ionized states

The valence shell repolarization under a core ionization in a diatomic molecule as N₂ is analyzed through localized and delocalized pictures, showing their total equivalence. The interpretation is easier in the localized model, appearing mainly through local single excitation processes; in the delocalized model the repolarization effect is hidden (partly or totally) under double excitation processes involving simultaneous excitations in the valence shell and hole change in the core level, appearing therefore as a “correlation effect”. This effect is analyzed forn Be atoms, showing an ^?1 behaviour of the single excitations effect in the delocalized model, and explains the Hartree-Fock unstability numerically verified on O ₂ ⁺ , but it prevents to give any physical meaning to the “localization of the core hole in a diatomic molecule”.     

A multi-residue method for characterization and determination of atmospheric pesticides measured at two French urban and rural sampling sites

The extensive use of pesticides to protect agricultural crops can result in the transfer of these compounds into the atmosphere and their diffusion towards urban areas. Precise evaluation of the geographic impact of this type of pollution is important environmentally. In this paper, analytical methods for the sampling, characterization, and determination of agricultural pesticides in air were developed; the methods were then applied in the Paris and Champagne regions. Sixteen pesticides belonging to nine chemical families were monitored. Sampling was carried out in urban (Paris) and rural (Aube district) sites, utilizing either a high-volume pump (12.5 m³ h^–1) (urban site) or a low-volume pump (2.3 m³ h^–1) for the rural site. Quartz filters and polyurethane foams (PUF) were used for sampling in all cases. After extracting the samples and concentrating the recovered solutions, high-performance liquid chromatography (HPLC) analysis with UV detection was performed. Identification of the pesticides was confirmed by applying to the HPLC measurements a novel UV-detection procedure based on the normalized absorbance variation with wavelength (Noravawa procedure). The presence of metsulfuron methyl, isoproturon, linuron, deltamethrin (and/or malathion), and chlorophenoxy acids (2,4-D and MCPP) was found at the urban sampling site at levels ranging from about 1 to 1130 ng m^–3 of air, depending on the compound and sampling period. On the rural sampling site residues of isoproturon, deltamethrin (and/or malathion), MCPP, and 2,4-D were generally detected at higher levels (19–5130 ng m^–3) than on the urban site, as expected. The effects of the weather conditions and agricultural activity on the atmospheric concentrations of pesticides are discussed, as are long-range atmospheric transfer processes for these pesticides.     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

Total Syntheses of (−)-Conduritol B ((−)-1L-Cyclohex-5-ene-1,3/2,4-tetrol) and of (+)-Conduritol F((+)-1D-Cyclohex-5-ene-1,2,4/3-tetrol). Determination of the Absolute Configuration of (+)-Leucanthemito

The ‘naked sugar’ (+)-(1R,2R4R)-2-endo-cyano-7-oxabicyclo[2.2.1]hept-5-sn-2-exo-yl acetate ((+)- 4 ) was converted (7 steps, 45% overall) with high stereoselectivity into (?)-(4R,5S,6R)-4,5,6-tris{[(tert-butyl)dimethylsilyl]oxy}cyclohex-2-en-1-one ((?)- 11 ). Reduction of (?)- 1 with NaBH₄- CeCl₃ · 7 H₂O, followed by deprotection of the silyl ether moieties gave (+)-conduritol F ((+)- 1 ; 47%) whose characteristics were identical to those of natural (+)-leucanthemitol. Reduction of (?)- 11 with DIBAH, followed by deprotection of the silyl ether moiety led to (?)-conduritol B ((?)- 3 ; 51 %).