The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Microchromatography of hemoglobins. VIII. A general qualitative and quantitative method in plastic drinking straws and the quantitative analysis of Hb-F

The microchromatographic procedure for the quantitative analysis of the hemoglobin components in a hemolysate uses columns of DEAE-cellulose in a plastic drinking straw with a glycine-KCN-NaCl developer. Not only may the method be used for the quantitative analysis of Hb-F but also for the analysis of the varied components in mixtures of hemoglobins.     

Multivariate calibration techniques applied to the spectrophotometric analysis of one-to-four component systems

The UV spectrophotometric analysis of a multicomponent mixture containing paracetamol, caffeine, tripelenamine and salicylamide by using multivariate calibration methods, such as principal component regression (PCR) and partial least-squares regression (PLS), was described. The calibration set was based on 47 reference samples, consisting of quaternary, ternary, binary and single-component mixtures, with the aim to develop models able to predict the concentrations of unknown samples containing as many as one-to-four components. The calibration models were optimized by an appropriate selection of the number of factors as well as wavelength ranges to be used for building up the data matrix and excluding any information about the interfering excipients included in pharmaceutics. The PCR and PLS models were compared and their predictive performance was inferred by a successful application to the assays of synthetic mixtures and pharmaceutical formulations.     

Fluorinated heterocyclic compounds. An effective strategy for the synthesis of fluorinated Z-oximes of 3-perfluoroalkyl-6-phenyl-2h-1,2,4-triazin-5-ones via a ring-enlargement reaction of 3-benzoyl-5-perfluoroalkyl-1,2,4-oxadiazoles and hydrazine

The reaction of 3-benzoyl-5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine has been investigated, evidencing the possibility of competitive reaction paths. Nucleophilic addition of the hydrazine to the electrophilic C(5) of the 1,2,4-oxadiazole ring, followed by ring opening and ring closure with enlargement, leads with high yield and in very mild experimental conditions to the formation of Z-oximes of 3-perfluoroalkyl-6-phenyl-2H-1,2,4-triazin-5-ones (11a-c) as major products of the reaction. In turn, the hydrazine can attack the electrophilic carbonyl carbon giving 4-perfluoroacylamino-5-phenyl-2H-1,2,3-triazoles (13a-c) through the well-known Boulton-Katritzky rearrangement of the intermediate hydrazones.     

Influence of CaCO<Subscript>3</Subscript> nanoparticles shape on thermal and crystallization behavior of isotactic polypropylene based nanocomposites

Summary The influence of calcium carbonate nanoparticles with different shapes (spherical and elongated) on the thermal properties and crystallization behavior of isotactic polypropylene was investigated. CaCO₃ nanoparticles were covered by an appropriate coating agent to improve the interfacial adhesion between the filler and the polyolefin matrix. The nanocomposites were prepared by melt mixing and subsequent compression molding. A remarkable effect of CaCO₃ on the thermal properties of iPP was observed. Moreover, the analysis of crystallization kinetics showed that CaCO₃ nanopowder coated with PP-MA are efficient nucleating agents for iPP, and the overall crystallization rate results higher than plain iPP.     

Simultaneous determination of solubility, dissolution and dilution enthalpies of a substance from a single calorimetric experiment

The thermodynamics of dissolution in water of a set of substances has been studied calorimetrically. The examined substances were: potassium chloride, (glycyl-glycyl)diketopiperazine, (alanyl-alanyl)diketopiperazine, (leucyl-glycyl)diketopiperazine. They were chosen on the basis of their solubilities, going from a highly soluble electrolyte to the sparingly soluble diketopiperazines. It is shown that, using a commercially available calorimeter, it is possible to perform in a single calorimetric experiment the simultaneous determination of all thermodynamic parameters characterizing dissolution of a substance in a given solvent, i.e. solubility, dissolution enthalpy and dilution enthalpy. The solubility values in water obtained through the proposed method are in good agreement with those reported in the literature and obtained by other techniques.     

Structures of Highly Twisted Amides Relevant to Amide N−C Cross‐Coupling: Evidence for Ground‐State Amide Destabilization

Herein, we show that acyclic amides that have recently enabled a series of elusive transition‐metal‐catalyzed N?C activation/cross‐coupling reactions are highly twisted around the N?C(O) axis by a new destabilization mechanism of the amide bond. A unique effect of the N‐glutarimide substituent, leading to uniformly high twist (ca. 90°) irrespective of the steric effect at the carbon side of the amide bond has been found. This represents the first example of a twisted amide that does not bear significant steric hindrance at the α‐carbon atom. The ¹⁵N NMR data show linear correlations between electron density at nitrogen and amide bond twist. This study strongly supports the concept of amide bond ground‐state twist as a blueprint for activation of amides toward N?C bond cleavage. The new mechanism offers considerable opportunities for organic synthesis and biological processes involving non‐planar amide bonds.