Studies of pyrolysis and air heating of rayon by inverse gas chromatography

Inverse gas chromatography using water probing at 110°C was carried out to characterize rayon yarns after prior heating at 200 and 250°C in both inert (nitrogen) and oxidative (dry air) atmospheres for successively longer time intervals. During the early stages of heating in either atmosphere, the affinity of the rayon for water drops rapidly. However, whereas continued heating in N₂ results in a further loss of activity, data have been obtained showing that after an initial period yarns heated in air at 250°C exhibit an increasing attraction for water even while losing more weight.     

Behaviour of Pluronic P84 block copolymer micelles above the gelification temperature as probed by positron annihilation lifetime spectroscopy

The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores.     

Acid-catalysed formation of tricyclic N,S-acetals in imidazolinone series based on the use of the unprecedented N-acyliminium ion cascade reaction involving transposition, heterocyclisation and pi-cyclisation

4-Hydroxy-5,5-dimethylimidazolines tethered at N-1 to an aryl sulfide undergo an unprecedented acid-catalysed domino reaction, involving double methyl transposition, heterocyclisation, isomerisation of thiazetidinium ion and, finally, pi-cyclisation. In this way a one-pot synthesis of original tricyclic N,S-acetals was developed. The same triheterocyclic products can be prepared also starting from the corresponding 5-hydroxy isomers (in this case the cascade of reactions does not involve methyl transposition).     

Synthesis of new fluorinated 1,5-benzoxazepine derivatives

A simple approach to the fluorinated 1,5-benzoxazepine ring system is described. By reacting commercially accessible aminophenols 1 and the trifluoroacetylvinyl ether 2, high yields of enaminones 3 were obtained. Functionalization of methyl group of compounds 3 gave rise to dieneamines 4 that were cyclized in acidic environment to benzoxazepine derivatives 5.     

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

The reaction of Mo(N)(CH₂tBu)₃ (1) and SiO_2-(700) generates (SiO)Mo(NH)(CHtBu)(CH₂tBu) (2) when performed in C₆H₆ (material [1/SiO_2-(700)]_C6H6). The grafting occurs presumably by protonation of the nitrido ligand to form an intermediate (SiO)Mo(NH)(CH₂tBu)₃ (3), a pentacoordinated complex, which decomposes into 2 and 2,2-dimethylpropane. While [1/SiO_2-(700)]_C6H6 is highly active in olefin metathesis, [1/SiO_2-(700)]_CH2Cl2 and [1/SiO_2-(700)]_THF are poorly active or inactive catalysts respectively. In contrast, when Mo(N)(CH₂tBu)₃ reacts with a molecular silanol derivative, a soluble model of the surface of SiO_2-(700), it yields a very stable complex, (c-C₅H₉)₇Si₇O₁₂SiO-Mo(NH)(CH₂tBu)₃ (3m), which does not spontaneously generate 2,2-dimethylpropane and an alkylidene complex in contrast to the surface complex. Moreover, 3m does not catalyse olefin metathesis at room temperature as it does not already contain the initiating carbene ligand, and it is necessary to heat up the reaction mixture to 110 °C to obtain low catalytic activity. Nevertheless, the complex 3m generates well-defined metallocarbenes when heated in the presence of PMe₃: (c-C₅H₉)₇Si₇O₁₂SiO-Mo(N)(CHtBu)(P(CH₃)₃)₂ (4m) as a 10:1 mixture of its syn and anti rotamers with the loss of 2 equiv. of 2,2-dimethylpropane.     

Pharmacokinetics of the nitroxide PCA measured by in vivo EPR

PCA (2,2,5,5-tetramethylpiperidine-1-oxyl-3-carboxylic acid) is a relatively stable free radical which has been shown to be useful as a contrast agent for nuclear magnetic resonance imaging and as an imaging/spectroscopy agent for EPR. In an effort to determine the role of the liver and kidney in the pharmacokinetics of PCA, using low frequency in vivo EPR spectroscopy, we followed the clearance of PCA after intravenous injection in mice: under normal conditions, with a restricted blood supply to the kidneys, after exposure to an acute hepatotoxin CCl₄, and after exposure to lipopolysaccharide (endotoxin). The observed pharmacokinetics fit a two-component model. The fast component was dramatically affected when the renal vessels were restricted, while CCl₄ and endotoxin had a smaller but significant effect. The half times of the slow components were not significantly different (p>0.05) in the groups treated by renal blood flow occlusion, CCl₄, or LPS, compared with the control group. In conclusion, we find that the pharmacokinetics of PCA need to be completely described in term of a two component model: the fast component of the decay is mainly due to the elimination by the kidneys and also is affected by the time for the initial distribution; the slow component is related to the bioreduction of the nitroxide. In addition to the liver other tissues can also effectively metabolize PCA. The effect of oxygen on the rate of metabolism is modest at most.     

Acidobasic properties of molten potassium tetrahalogenogallates (X=Cl,I)

Molten potassium tetrachlorogallate and potassium tetraiodogallate were studied in terms of halogenoacidity, based on X^? ion-exchange. Titration of KX solution with GaX₃ were achieved and characterized by the shift of cathodic voltammetric curves. Autodissociation constants K_i,X/mol² kg^?1 were determined: ?log K_i,Cl=4.25±0.05 and ?log K_i,I=2.6±0.05, as well as the solubility values of KX: 0.41±0.02 and 0.80±0.02 mol kg^?1 for KCl and KI respectively.     

Interaction between zigzag single-wall carbon nanotubes and polymers: a density-functional study

Density-functional all-electrons calculations within local-density approximation show that the two isoelectronic polymers poly(para-phenylene) and poly(para-borazylene) weakly interact with zigzag single-walled carbon nanotubes. The analysis of the electronic properties of the joint systems, both with the polymer inside and outside the nanotubes, reveals a physisorption process with small changes in band structures and densities of states with respect to the constituents. We evaluate the potential barrier arising between polymers and nanotubes. Finally, we remark a generic selectivity of poly(para-phenylene) with respect to the electronic behavior of nanotubes, leading to a change in the density of states of metallic tubules.     

Mesoionic compounds VIII. Synthesis of 3-phenylsydnonyl substituted alkenes. Some new photochromic sydnones

The reactions of 3-phenylsydnonyllithium with a variety of ketones to afford tertiary carbinols 4a-4i and their subsequent dehydration to 4-(3-phenylsydnonyl) alkenes 5a-5i are discussed. Among the alkenes, 2-[4-(3-phenylsydnonyl)]propene ( 5b ) and 1-[4-(3-phenylsydnonyl)]-1-cyclohexene ( 5i ) were photochromic. A Wittig reaction between ethyl-4-(3-phenylsydnonyl)-ketone and methylenetriphenylphosphorane afforded the photochromic 2-[4-(3-phenylsydnon-yl)]-1-butene(5j).     

Comparison of tridentate ligands in competition experiments for their ability to form a [Tc(CO)3] complex

In this study we have compared different ligands containing three or more hetero-atoms (N, O and/or S) with respect to their ability to form tridentate complexes with a Tc-tricarbonyl moiety. Comparison of each ligand in a competition reaction with histidine first and then with each other compound allowed to rank the ligands according to their ability of complex formation with the [^99mTc(CO)₃]⁺ precursor from diethylenetriamine (most efficient of the studied ligands) to nitrilotriacetic acid (weakest complexing properties). The results provide insight in the structural requirements for the formation of stable Tc-tricarbonyl complexes and suggest preferred combinations and arrangements of the hetero-atoms involved in the complex formation. They also give a good indication which type of ligand is most appropriate to modify biomolecules for an efficient and stable labelling with a Tc-tricarbonyl moiety.