Thermodynamic Constants of Acid-Base Equilibria in Solutions of Penicillins

The constants of acid-base equilibria in solutions of benzylpenicillin, carbenicillin, and ampicillin at 20°C were determined by pH-metric titration (supporting electrolytes 0.1, 0.4, 0.7, and 1.0 M KCl and KNO₃). The sizes of the ionic species of the penicillins in their energetically favorable conformation were determined by the PM3 semiempirical method. The thermodynamic equilibrium constants were estimated by extrapolation of the concentration constants to zero ionic strength by the Huckel equation, taking into account the calculated sizes of the ions.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005, pp. 1049–1054.Original Russian Text Copyright © 2005 by Alekseev, Demskaya, Dodonova.     

Thermochemical Study of Acid-Base Reactions of L-Valine in Salt Solutions

Heat effects of protonation and neutralization of L-valine were measured at 298.15 K, ionic strengths of the solutions of 0.25, 0.50, and 0.75, and on lithium, sodium, and potassium nitrate backgrounds. The heat effects of dissociation and neutralization of L-valine were extrapolated to zero ionic strength by a one-parameter equation to find standard enthalpies of dissociation and neutralization of L-valine in aqueous solution. Temperature-dependent and temperature-independent components of thermodynamic values were determined.     

The limit of detection in differential scanning calorimetry

A procedure is described to determine the limit of detection of DSC instruments by using tiny signals from spontaneous polymorphic transitions of CsCl, K₂Cr₂O₇ and Na₂SO₄. It is shown how such signals can be found well-resolved in DSC diagrams of powder samples. To distinguish them from the baseline noise they should exhibit a height at least twice that of the baseline width. For the instrument employed the corresponding smallest amount of heat, i.e., the limit of detection, was found to be 0.1 mJ.The authors thank Mr. H. Maltry for technical help and the Deutsche Forschungsgemeinschaft for support.     

Crystallization behavior of a propylene-1-butene random copolymer in its α and γ modifications

A random propylene-based copolymer containing 1.0 mol% 1-butene as co-unit, synthesized with Ziegler-Natta catalyst and then fractionated to make the sample having a uniform in molecular microstructure, was investigated by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXD), and atomic force microscopy (AFM). In the DSC curves, one can see clearly the endothermic peaks corresponding to the melting of α-iPP crystals and a group of broad endothermic peaks associated to the melting of the γ-iPP crystals. Wide-angle X-ray diffraction results indicate that both the α and γ modifications can be formed in the copolymer in a wide temperature range. The γ fraction increases first with increasing the crystallization temperature at the expense of its α component, which has been explained according to crystalline structures of iPP in its α and γ forms, and then decreases with increasing crystallization temperature as the crystallization of iPP in its γ phase has been suppressed at high temperatures. The γ-iPP content in the copolymer reaches maximum at the temperature of 130 °C. The in situ X-ray diffraction characterization on the isothermal crystallization process at 130 °C indicates that, as long as the γ-iPP can be detected, it takes always ca. 25% of the overall crystallinity. This leads to the conclusion that α- and γ-iPP crystals grow simultaneously during the crystallization process. The fact that the α and γ phases cannot be distinguished by morphological observation leads to the conclusion that they may intermix within one spherulite.     

Peculiarities of the effects of trifluoromethyl and alkoxycarbonyl groups on the structure and reactivity of acrylates

Ab initio quantum-chemical calculations of molecules of CF₃-substituted acrylates and their non-fluorinated analogs were carried out by the restricted Hartree-Fock (RHF) method and at the second-order Møller-Plesset (MP2) level of perturbation theory using the 6–31G^* basis set with full geometry optimization. Peculiarities of their molecular and electronic structure were revealed and the dipole moments, the polarizability and first molecular hyperpolarizability tensors, harmonic vibrational frequencies, electrostatic potentials, and local electron densities in the vicinity of the carbon atoms of the C=C bond were calculated. It was shown that CF₃-substituted acrylates are conjugated systems similar to their nonfluorinated analogs. Peculiarities of the structure and properties of CF₃-substituted acrylates are explained by p-π-interaction between the CF₃ group and the conjugated system.     

Interaction of polynitro compounds with difluoroamine

Difluoroamine does not react with tetranitromethane and fluoro-, chloro-, and bromotrinitromethanes in DMF and in acidic media (CF₃COOH, ClSO₃H, FSO₃H, and oleum), but reacts with α-fluoro- and α-(difluoroamino)-α,α-dinitrotoluenes to give substitution products of the difluoroamino group for both the nitro groups,viz., PhC(NF₂)₂F and PhC(NF₂)₃, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1647–1649, August, 1998.     

Determination of α-aminonitriles, α-amino acid amides and α-amino acids by means of HPLC,post-column reaction and fluorescence detection

Summary An HPLC method was developed for the simultaneous assay of intermediate (α-aminonitrile and α-amino acid amide) and end products (α-amino acid) in process streams of α-amino acid synthesis. Applications are given for Ala, Val and Leu. α-Aminonitriles were stable in a phosphate buffer pH 3, which was subsequently used for sample handling and chromatography. The α-aminonitrile, the corresponding acid amide and α-amino acid were separated using a buffered ion-pair mobile phase on an RP column and were detected fluorimetrically after post-column reaction with fluorescamine. Linearity and precision of the method are given.     

Solid-Solute Phase Equilibria in Aqueous Solutions XIV [1]. Thermodynamic Analysisof the Solubility of Hellyerite in Water

Summary.  The solubility of hellyerite, NiCO₃ · 6H₂O, in water was studied at different temperatures. From the experimental data obtained, a preliminary set of the thermodynamic quantities Δ_f G ^⊖, Δ_f H ^⊖, and S ^⊖ for hellyerite was derived using the ChemSage optimizer routine. Received January 16, 2001. Accepted January 18, 2001     

A correction function to determine the β-fusion heat in a mixture of α- and β-PP

Blends of known amounts of α- and β-PP crystals were prepared from pure α- and pure β-PPsamples. Their fusion behaviours were studied by differential scanning calorimetry (DSC).The fusion heats of the α- and β-crystals were approximated from the DSC curves and compared with those calculated on the basis of the compositions of the blends. A correction function was found which improved the accuracy of the respective fusion heats significantly from the DSC analysis. The correction function can be used to determine the respective fusion heats of a PP sample which contains an unknown mixture of the α- and β-crystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigations on the Decomposition of Carboplatin in Infusion Solutions II. Effect of 1,1-Cyclobutanedicarboxylic Acid Admixture

 Eleven infusion solutions containing carboplatin ( ≈ 10 mg/ml) stored for 5 to 78 months at room temperature were investigated for the content of carboplatin and the profile of decomposition products by means of atomic absorption spectrophotometry (AAS) and high performance liquid chromatography (HPLC). Carboplatin loss was between 3.09% and 7.28%. Carboplatin degradation was exponential and depended only on the time of storage. For seven of the investigated products an admixture of 1,1-cyclobutane-dicarboxylic acid (CBDCA) (0.75%–2%) was shown. This admixture did not suppress the break-down of carboplatin, but changed the quantity of certain degradation products. Besides hydrophilic aqua- or hydroxyplatinum(II)-species also hydrophobic side products were detectable. Especially in infusion solutions with CBDCA admixture a side product (SP) developed after the 55^th week of storage. The peak area of this SP amounted to more than 1% of the peak area of carboplatin by the time of expiry. Received December 11, 2001; accepted February 27, 2002; published online July 15, 2002     

Thermal transitions and state of water in binary mixtures of water andn-decanephosphonic acid or its sodium salts

The state of water and several transitions were examined in the systemsn-decanephosphonic acid (DPA)—water and the sodium salts of DPA—water. Temperature — composition phase diagrams are reported. The results show that several liquid crystalline phases plus isotropic liquid, and two solid phases (a waxy solid phase and a crystalline phase) are formed. Several types of water were detected: bulk-like water, interfacial water and hydration water. This work was supported by the Consejo Nacional de Ciencia y Technología de México (grant # 3319-E) and by the Consejo Nacional de Investigaciones Científicas y Técnicas de la República de Argentina.     

Gelation on Heating of Supercooled Gelatin Solutions

Diluted (1.0–1.5 wt%) aqueous gelatin solutions have been cooled to –10 °C at a cooling rate 20 °C min⁻¹ without freezing and detectable gelation. When heated at a constant heating rate (0.5 –2 °C min⁻¹), the obtained supercooled solutions demonstrate an atypical process of gelation that has been characterized by regular and stochastically modulated differential scanning calorimetry (DSC) as well as by isoconversional kinetic analysis. The process is detectable as an exothermic peak in the total heat flow of regular DSC and in the nonreversing heat flow of stochastically modulated DSC. Isoconversional kinetic analysis applied to DSC data reveals that the effective activation energy of the process increases from approximately 75 to 200 kJ mol⁻¹ as a supercooled solution transforms to gel on continuous h eating.     

Reaction of N-Acyl-α-triphenylphosphonio-α-amino Acid Esters with Organic Bases: Mechanism of the Base-Catalyzed Nucleophilic Substitution of the Triphenylphosphonium Group

Summary.  Reactions of N-acyl-α-triphenylphosphonio-α-amino acid methyl esters with organic bases (triethylamine or DBU) were investigated as the crucial step of the base-catalyzed displacement of the triphenylphosphonium group by nucleophiles. It was proved that N-acyl-α-triphenylphosphonioglycinates are transformed to an equilibrium mixture of the corresponding N-acyliminoacetates and N-acyl-α-triphenylphosphoranylidene glycinates by bases. In the case of N-acyl-α-triphenylphosphonio-α-amino acid esters with quaternary α-carbon, the α-substituted homologues of the N-acyliminoacetates were detected to be the only primary reaction product which, however, can undergo further tautomerization to the corresponding α,β-dehydro-α-amino acid derivatives. In both these cases the reaction of N-acyl-α-triphenylphosphonio-α-amino acid esters with nucleophiles proceeds via the addition of a nucleophile to the activated C=N double bond of the N-acylimino intermediate. Corresponding author. E-mail: romanm@zeus.polsl.gliwice.pl Received October 31, 2001. Accepted (revised) December 17, 2001     

A modulated dsc study on the strain-induced βα-transformation in a β-form isotactic polypropylene

The mechanical strain-induced βα-transition of a β-phase isotactic polypropylene (β-iPP) was studied by modulated differential scanning calorimetry (MDSC). Samples were taken after tensile fracture of a double notched specimen from its process and plastic zones, respectively, and the related calorimetric response was compared to that of the bulk material. In contrast to conventional DSC results, it was found that the βα-transformation was not completed in the process zone. Furthermore, the melting of the α-iPP showed both non-reversing and reversing characteristics, whereas the melting of the β-phase proved to be a reversing process. Therefore, it was recommended to consider the conversion grade of the βα-transformation by the relative change in the melt flux of the reversing β-melting peak. This work was supported by the German Science Foundation (DFG-Ka 1202/4-1).     

Phase Equilibria in the Binary System Lead Fluoride [PbF2]-Cadmium Fluoride [CdF2]

Phase dependences in the binary system lead fluoride [PbF₂]—cadmium fluoride [CdF₂] were examined and the phase diagram of this system was established. The occurrence of solid continuous solutions with a minimum melting point of 750°C and a CdF₂content of 35 mol% was confirmed. Thermal, dilatometric, microscopic and X-ray analytical methods were used during the investigations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of α- and β-trifluoromethylsubstituted acrylic acids in their reactions with thiols

α-(Trifluoromethyl)acrylic acid (1) and γ,γ,γ-trifluorocrotonic acid (2) add AcSH (exothermally and at 100 °C, respectively) in the absence of a catalyst to form products of β-thiolation, which can be easily hydrolyzed to the corresponding β-mercaptoalkanoic acids. Thiols also add regiospecifically to acids1 (in the absence of a catalyst) and2 only in the presence of trifluoromethanesulfonic acid as the catalyst) when heated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1230–1232, June, 1997.     

Solid-Solute Phase Equilibria in Aqueous Solutions XVII [I]. Solubility and Thermodynamic Data of Nickel(II) Hydroxide

Summary.  The solubility of nickel(II) hydroxide (theophrastite) in water was determined as a function of temperature and ionic strength by the pH variation method. In each experiment the inert electrolyte medium was made up with sodium perchlorate. The experimental data were thermodynamically analyzed, and the standard solubility constant was extrapolated to zero ionic strength with the specific ion-interaction equation. Furthermore, the standard molar Gibbs energy and the enthalpy of formation for theophrastite, β-Ni(OH)₂, were evaluated. For all calculations, ChemSage and its optimizer routine were used. Received October 25, 2001. Accepted October 29, 2001     

Role of Acid-Base Equilibria in the Reduction of Selenious(IV) Acid

ABSTRACT

Electroanalytical methods have been widely used for determination of Se(IV), but the nature of the reduction processes involved is not well understood. Polarographic reduction occurs in three waves (i₁, i₂, and i₃) the height of which changes with pH. We proved that in wave i₁, H₃SeO₃ ⁺ is reduced, in i₂ H₂SeO₃, and in i₃ HSeO₃ ^-. SeO₃ ^2? is not reducible. All reductions involve a transfer of six electrons and yield selenides. Limiting currents are controlled by the rate of protonation. As proton donors, in addition to H₃O⁺, the acid forms of the buffer present also act. Limiting currents increase markedly with increasing concentration of the buffer. Tenfold increase in buffer concentration can result in up to 200% increase in limiting current.     

Thermodynamics of Protolytic Equilibria in Aqueous Solutions of Biologically Active Ligands

The protolytic equilibria in aqueous solutions of different amino acids and dipeptides were studied by a direct calorimetric method at 288–328 K. The measurements were made with an isothermal-shell calorimeter with automatic temperature recording. The thermodynamic characteristics of the processes of acid-base equilibria were found from the combined results of thermochemical measurements and equilibrium constant data. Gurney concepts were used to analyse the thermodynamic characteristics of reactions. Comparisons of the temperature-dependent and temperature-independent contributions to the Gibbs energy and enthalpy change afford the criteria of the variability in sign of the ΔH of the process, i.e. they allow prediction of the magnitude of the temperature θ at which ΔH = 0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reaction of 5-allyl-2,5-dichloro-4,4-dimethoxy-3-morpholinocyclopent-2-enone with Me3SiI

5-Allyl-2,5-dichloro-4,4-dimethoxy-3-morpholinocyclopent-2-enone reacts with Me₃SiI in MeCN to give the corresponding product of reductive vinylic monodechlorination. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1660–1661, August, 1998.