Efficiency of face-plated underwater acoustic transducers

The possibility of obtaining the efficiency of simple disc transducers, broad-banded by use of face-plates, from input electrical conductance measurements under air-loading is explored. For an air-backed ceramic disc with a Perspex face-plate it was found that the main limitation to the accuracy of efficiency estimates is due to departures from uniformity of the face-plate thickness.     

Prediction of excess enthalpies for ternary aqueous solutions of nonelectrolytes

The excess enthalpiesH _xy ^E of ternary aqueous solutions of nonelectrolytes are used to test the possibility of making predictions for ternary solutions from the properties of the binary solutions only. Two methods are proposed: one is based on the empirical rule (h _xx ·h _yy )^1/2=h _xy . Another leads to the numerical prediction of the overallH _xy ^E . Both are successful for most of the pairs of solutes for whichH _x ^E ,H _y ^E >0.     

The Skyrmion Skyrmion interaction and the meson exchange model for the NN potential

The properties of nucleon-nucleon (NN) potentials derived from the Skyrme model are investigated compared to those of typical One-Boson-Exchange (OBE) models, For that purpose the NN as well as the nucleon-antinucleon (N¯N) potentials are evaluated for the isoscalar central channel, the isovector spin channel, and the isovector tensor channel. A comparison of NN and N¯N interactions allows a separation into terms with positive and negativeG-parity and therefore a detailed comparison with the properties of the OBE model. This analysis is performed using the classical Lagrangian of the Skyrme model and considering an extension of this Lagrangian by a term of sixth order, which has recently been proposed. The results indicate that the extended Lagrangian yields an interaction which is quite consistent with the OBE picture. The extended model, however, is much more sensitive to the values of the parameters and the approximations being used for the evaluation of the field for two baryons.     

1-Azetines

The synthesis, characteristics and chemical behaviour of some 3,3-disubstituted-2-ethoxy- and 2-ethy lthio-1-azetines, the first examples of a new class of heterocyclic compounds, are described.     

Gel-type polyacrylic resins cross-linked with trimethylolpropanetrimethacrylate: the issue of their nanostructure and molecular accessibility unveiled with a combination of inverse steric exclusion chromatography (ISEC), and ESR and CP-MAS 13C NMR spectroscopy

Six gel-type functional resins, that is, three poly-DMAA-co-TMPTP (DMAA = N,N-dimethylacrylamide, TMPTP = trimethylolpropyltrimethacrylate) samples with different degrees of cross-linking (0.6, 1.2, 1.7 % mol) and three poly-DMAA-co-MA-co-TMPTP (MA = methacrylic acid, ca. 5.5 % mol) samples with 1.7, 3.5, and 7 % mol cross-linking were investigated with ISEC (inverse steric exclusion chromatography), and ESR and CP-MAS (cross polarization magic angle spinning) 13C NMR spectroscopy after swelling in water and other solvents. This unprecedented combination of conceptually independent physicochemical techniques provides a thorough overall consistent picture of the morphology of the resins on the nanometer scale and of the molecular accessibility of the swollen polymer framework to the paramagnetic probe TEMPONE (2,2,6,6-tetramethyl-4-oxo-1-oxypiperidine) and to selected solvents.     

Effect of solute polarity on the performance of porapak type porous polymers

Summary The breakthrough volume of ådsorbent traps made with porous polymer beads Porapak N, P. Q, R, S was evaluated by analysis of polarity reference substances proposed by Rohrschneider (ethanol, methyl ethyl ketone, nitrometane, benzene, pyridine) by McReynolds (butanol, methyl propyl ketone) and of the first term of some homologous series: alcohols, aldehides, carboxilic acids, nitrocompounds, iodoalkanes and nitriles.Adjusted retention times of the compounds were measured at 200°C and correlated with the physical properties and with polarity class according to Ewell.Trapping efficiency of the various Porapak types was evaluated and the theoretical breakthrough volumes at 25°C was calculated for the used test substances.Dedicated to Prof. Dr. A. Liberti on the occasion of his 70th birthday.     

Application of a classical least-squares regression method to the assay of 1,4-dihydropyridine antihypertensives and their photoproducts

A multicomponent UV spectrophotometric method using a classical least-squares (CLS) algorithm has been developed for the quantitative determination of 1,4-dihydropyridine (DHP) calcium antagonists and respective photoproducts. The procedure was optimized by defining a fractionation scheme for selecting the more useful wavelength ranges to be used in the calibration model. The method is potentially able to be extended to the other drugs of the same family. The recovery values from synthetic mixtures and commercial formulations were verified to be 99.09 and 97.85% for drug parents and degradation products, respectively. The sensitivity for the photoproducts determination was found to be below 0.8%. The results obtained from laboratory mixtures and commercial formulations were compared with those provided by UV derivative spectrophotometry.     

Constitution moléculaire d'α,ω-difluoropolydiméthyl-N-méthylsilazanes. Equilibres obtenus par une réaction de redistribution

Unlike the α,ω-dihalogenopolydimethylsiloxanes, the α,ω-dichloropolydimethyl-N-methylsilazanes show a net preference for cyclic species with respect to linear structures at equilibrium. The aim of this study is to evaluate the perturbations in the molecular constitution of these α,ω-dihalogenopolydimethyl-N-methylsilazanes resulting from the substitution of the terminal chlorine atoms by fluorine atoms. This polymeric family was prepared by reacting (CH₃)₂SiF₂ with nonamethylsilazane [(CH₃)₂SiNCH₃]₃. The redistribution of the fluorine atoms with the bridging methylimino groups reached an equilibrium after about 5 months' heating at 150°C for all the samples prepared. The relative abundance of the various molecular species and fragments at equilibrium was deduced from the quantitative analysis of the proton nuclear magnetic resonance (NMR) spectra. The molecular constitution at equilibrium is described by two constants. The first, K = [neso] [middles in chains]/[terminal moieties]² = (2.8 ± 0.8) 10^?2, shows that the presence of terminal fluorine atoms is unfavorable to the formation of short chains. On the other hand, the trimeric cyclic species [(CH₃)₂SiNCH₃]₃ are found to be highly favored (K°₃ = 550 ± 100 mole/liter). These observations further confirm that the equilibrium constants which control the noncyclic part of polymeric families depend little on the functionality of the substituents exchanged [for example, on changing from ? N(CH₃)₂ to ? NCH₃? ] when the reorganization heat order is one.     

Asymmetric dihydroxylation of allenes

We have used asymmetric dihydroxylation (AD) of allenes in order to synthesize chiral α-hydroxy ketones. This methodology has been applied to several aryl-substituted allenes. We have found that electron donating groups on the aromatic ring increase the efficiency of the reaction.     

Chiral recognition in aqueous solutions between enantiomeric α-amino acids bearing substituted alkyl chains at 25°C: The peferential configuration model

Calorimetric, densimetric, and nuclear relaxation time measurements were carried out at 25°C on binary and ternary aqueous solutions containing L and D forms of the following amino acids bearing substituted alkyl chains: lysine, glutamine, arginine, asparagine, serine, and homoserine. For three of them (lysine, glutamine, and arginine), very high differences were found between the values of the homochiral and heterochiral pairwise enthalpic interaction coefficients. Volume and spectroscopic data showed that it is possible to detect chiral recognition by techniques other than calorimetric. The role of the zwitterionic interaction and of the substituted side chain is discussed to explain the enhanced chiral recognition of these -aminoacids in respect to those bearing unsubstituted alkyl chains (alanine, -aminobutyric acid, valines and leucines).