Multiple imputation in principal component analysis

The available methods to handle missing values in principal component analysis only provide point estimates of the parameters (axes and components) and estimates of the missing values. To take into account the variability due to missing values a multiple imputation method is proposed. First a method to generate multiple imputed data sets from a principal component analysis model is defined. Then, two ways to visualize the uncertainty due to missing values onto the principal component analysis results are described. The first one consists in projecting the imputed data sets onto a reference configuration as supplementary elements to assess the stability of the individuals (respectively of the variables). The second one consists in performing a principal component analysis on each imputed data set and fitting each obtained configuration onto the reference one with Procrustes rotation. The latter strategy allows to assess the variability of the principal component analysis parameters induced by the missing values. The methodology is then evaluated from a real data set.     

Direct polymerization of surface‐tethered polyelectrolyte layers in aqueous solution via surface‐confined atom transfer radical polymerization

The direct polymerization of deprotonated acidic monomers in aqueous solutions was achieved via surface‐confined atom transfer radical polymerization (SC‐ATRP) to produce surface‐tethered polyelectrolyte brushes. Layers of poly(itaconic acid), poly(methacrylic acid), and sodium poly(styrene sulfonate) were grown by SC‐ATRP from self‐assembled initiator monolayers of [BrC(CH₃)₂COO(CH₂)₁₁S]₂ on gold substrates. The polymer layers were characterized with variable‐angle ellipsometry and external‐reflection Fourier transform infrared spectroscopy. Without intervention, atom transfer radical polymerization catalysts were deactivated by complexation with the deprotonated acidic monomers, disproportionation, and dissociation during the polymerization of these monomers in water; the result was the cessation of polymer growth. The addition of an alkali salt to the reaction media suppressed catalyst deactivation, allowing polymer layers to increase in thickness linearly for longer periods of time with respect to salt‐free conditions. This result suggested an improved degree of polymerization control. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 566–575, 2007     

Zur Erkennung von Arsen- und Antimonwasserstoff

Ohne Zusammenfassung     

An improved preparation of tetrahydropyridines from 1-alkyl-3-acylpyridinium salts

The hydrogenation of 1-alkyl-3-acylpyridinium salts in methanol in the presence of 10% palladium-charcoal/disodium hydrogen phosphate-sodium dihydrogenphosphate suspension leads to the corresponding tetrahydropyridines. This represents an improvement over the Wenkert method.     

Etudes récentes des alcaloïdes du Lunaria biennis Moench,Crucifères—II : Structure de la lunaridine

The structure of lunaridine (8), alkaloid isolated from the seeds of Honesty, Lunaria biennis Moench, Cruciferae, has been established on the basis of its spectral and chemical properties and confirmed by a comparative mass spectrometry study of methylated derivatives, deuterated or not, of lunaridine and lunarine 1.     

Low temperature oxidation of benzene and toluene in mixture with n-decane

The oxidation of two blends, benzene/n-decane and toluene/n-decane, was studied in a jet-stirred reactor with gas chromatography analysis (temperatures from 500 to 1100 K, atmospheric pressure, stoichiometric mixtures). The studied hydrocarbon mixtures contained 75% of aromatics in order to highlight the chemistry of the low-temperature oxidation of these two aromatic compounds which have a very low reactivity compared to large alkanes. The difference of behavior between the two aromatic reactants is highly pronounced concerning the formation of derived aromatic products below 800 K. In the case of benzene, only phenol could be quantified. In the case of toluene, significant amounts of benzaldehyde, benzene, and cresols were also formed, as well as several heavy aromatic products such as bibenzyl, phenylbenzylether, methylphenylbenzylether, and ethylphenylphenol. A comparison with results obtained with neat n-decane showed that the reactivity of the alkane is inhibited by the presence of benzene and, to a larger extent, toluene. An improved model for the oxidation of toluene was developed based on recent theoretical studies of the elementary steps involved in the low-temperature chemistry of this molecule. Simulations using this model were successfully compared with the obtained experimental results.     

A short and efficient synthesis of 4a-substituted cis-hexahydro-1,2,3,4,4a,9a-carbazol-4-ones

Photocyclization of tertiary aryl enaminones 1 gave trans-hexahydrocarbazol-4-ones 2 which were subsequently alkylated with a series of electrophiles via the corresponding thermodynamic anion. The 4a-substituted derivatives formed were shown to have the cis-hexahydrocarbazol-4-one structure by comparison with the previously prepared trans isomer.