Dilute solutions of polyelectrolytes—III. Unperturbed dimensions of poly(methylene) N,N-dimethylpiperidinium chloride) in aqueous and methanolic solutions in presence of tetraethylammonium bromide

The behaviour of poly(methylene N,N-dimethylpiperidinium chloride) in aqueous and methanolic solutions in the presence of tetraethylammonium bromide has been investigated by light scattering; the parameters characterizing the macromolecular chain have been calculated. Peculiar behaviour of the macromolecules in aqueous solutions was attributed to the influence of the simple electrolyte co-ion due to hydrophobic interactions.     

Ion pairing of quaternary salts in solvent mixtures

Conductances at 25.00°C are reported for the following systems: tetrabutylammonium bromide in dimethyl sulfoxide-acetone mixtures (Bu₄NBr in Me₂SO–Me₂CO); tetraphenylphosphonium bromide (Ph₄PBr) in water Me₂SO, Me₂CO, and in the mixtures H₂O–Me₂SO, Me₂SO–Me₂CO and Me₂CO–H₂O; Ph₄PCl in Me₂SO, Me₂CO, H₂O–Me₂SO, and Me₂SO–Me₂CO; and tetrapropylammonium bromide (Pr₄NBr) in Me₂SO and Me₂CO. The data were analyzed using the Fuoss 1978 equation which is based on the coupled equilibria: (unpaired ions)(solvent-separated pairs)(contact pairs). The conductimetric pairing constantK _A =K _R(l+K _s) is the product of two factors:K _R, which describes the first (diffusion controlled) equilibrium andK _s=exp(–E _s/kT), which describes the second (system-specific) equilibrium. Ions with overlapping cospheres (of diameterR) are defined as paired: their center-to-center distancer lies in the rangearR; contact pairs (r=a) are ions which have one ion of opposite charge as a nearest neighbor, all other nearest and next nearest neighbors being solvent molecules. The quantityE _s is the difference in free energy between the states defined byr=R andr=a. For the Me₂SO–Me₂CO systems,E _s is positive for solutions in Me₂SO and decreases through zero to negative values as the fraction of the less polarizable acetone increases. For solutions in waterE _s is also positive. On addition of Me₂SO or Me₂CO,E _s initially increases, goes through a maximum, and then decreases to negative values as the fraction of the less polarizable component increases. The decrease is an electrostatic effect, common to all the systems. The initial increase inE _s appears when the small water molecules surrounding solvent-separated pairs are replaced by organic molecules which have greater volumes than water.     

The free streaming operator: A mathematical model for diffusive multiplying boundary conditions

We show that the free streaming operator with diffusive multiplying boundary conditions is the generator of a quasi-bounded semigroup. We also examine some spectral properties of such an operator.     

A thermodynamic study of two anionic amphiphilic penicillins in aqueous solution by ultrasound speed measurements

The critical concentrations of sodium cloxacillin and sodium dicloxacillin penicillins in aqueous solution were determined over the temperature range 288.15–313.15 K by a method based on deconvolution into Gaussians of the second derivative of the measured ultrasound velocity. The mass action model was used to calculate the thermodynamic standard quantities: free Gibbs energy, enthalpy and entropy of aggregate formation. The results are discussed in terms of the structure of the drug aggregates.     

Hydrogen storage in Chabazite zeolite frameworks

We have recently highlighted that H-SSZ-13, a highly siliceous zeolite (Si/Al = 11.6) with a chabazitic framework, is the most efficient zeolitic material for hydrogen storage [A. Zecchina, S. Bordiga, J. G. Vitillo, G. Ricchiardi, C. Lamberti, G. Spoto, M. Bj?rgen and K. P. Lillerud, J. Am. Chem. Soc., 2005, 127, 6361]. The aim of this new study is thus to clarify both the role played by the acidic strength and by the density of the polarizing centers hosted in the same framework topology in the increase of the adsorptive capabilities of the chabazitic materials towards H2. To achieve this goal, the volumetric experiments of H2 uptake (performed at 77 K) and the transmission IR experiment of H2 adsorption at 15 K have been performed on H-SSZ-13, H-SAPO-34 (the isostructural silico-aluminophosphate material with the same Br?nsted site density) and H-CHA (the standard chabazite zeolite: Si/Al = 2.1) materials. We have found that a H2 uptake improvement has been obtained by increasing the acidic strength of the Br?nsted sites (moving from H-SAPO-34 to H-SSZ-13). Conversely, the important increase of the Br?nsted sites density (moving from H-SSZ-13 to H-CHA) has played a negative role. This unexpected behavior has been explained as follows. The additional Br?nsted sites are in mutual interaction via H-bonds inside the small cages of the chabazitic framework and for most of them the energetic cost needed to displace the adjacent OH ligand is higher than the adsorption enthalpy of the OH...H2 adduct. From our work it can be concluded that proton exchanged chabazitic frameworks represent, among zeolites, the most efficient materials for hydrogen storage. We have shown that a proper balance between available space (volume accessible to hydrogen), high contact surface, and specific interaction with strong and isolated polarizing centers are the necessary characteristics requested to design better materials for molecular H2 storage.     

Niobium(V) oxide coated on thin glass-ceramic rod as a solid phase microextraction fiber

The efficiency of niobium(V) oxide as a sorbent phase for solid phase microextraction (SPME) was investigated. The thin glass-ceramic rod was coated with niobium(V) oxide using chemical vapor deposition and Nb₂O₅ as a chemical precursor. Optimum conditions for the preparation and conditioning of the fibers are presented. The fibers were used for the extraction of a mixture of alcohols and a mixture of phenols from the headspace samples. The results obtained proved the suitability of niobium(V) oxide as a new SPME fiber. The calibration graphs for alcohols and phenols in a concentration range of 50-1000 μg l⁻¹ were linear (r > 0.995) and the detection limits were below 0.8 μg l⁻¹ level. The repeatability for one fiber (n = 6) under similar conditions was between 3 and 10.4%. The fiber-to-fiber reproducibility (n = 6) was between 5 and 15%.     

Reactivity of 1-(2-nitrophenyl)-5-aminopyrazoles under basic conditions and synthesis of new 3-, 7-, and 8-substituted pyrazolo[5,1-c][1,2,4]benzotriazine 5-oxides,as benzodiazepine receptor ligands

The reaction of 1-(2-nitrophenyl)-5-aminopyrazoles under basic conditions has been reinvestigated and the structures of the obtained pyrazolo[5,1-c][1,2,4]benzotriazine 5-oxides confirmed by spectroscopic means. In particular the different aromatic nucleophilic attack on 8-chloro derivatives 4a and 6a and 7-nitro derivatives 11a and 12a was determined. From these latter compounds unexpected (phenyl-ONN-azoxy)pyrazoles were isolated.     

PHOTOPHYSICAL PARAMETERS OF CHLOROPHYLLS a AND b·FLUORESCENCE AND LASER-INDUCED OPTOACOUSTIC MEASUREMENTS

Abstract— The quantum yields of internal conversion (energy loss) of the photoexcited chlorophylls a and b in ethanol at 24°C have been determined by measuring the quantum yields of the competing radiative (fluorescence) and non-radiative (intersystem crossing) deactivation processes (which lead to energy storing species) by means of emission and laser-induced optoacoustic spectroscopy, respectively. The values for internal conversion of chlorophyll a are 10% and 14% in the presence and absence of molecular oxygen, respectively. The corresponding yields for chlorophyll b are negligibly small. The internal conversion in chlorophyll a is attributed to vibronic coupling with the substituents of the porphin system, since internal conversion was found to be substantially reduced for unsubstituted Mg-porphin in ethanol and for chlorophyll a when dissolved in a viscous medium.