Osmotic and activity coefficients of monomethyl-, dimethyl-, and trimethylammonium chlorides at 25°C

The osmotic and activity coefficients of monomethyl-, dimethyl-, and trimethylammonium chlorides in aqueous solution have been determined at 25°C by the gravimetric isopiestic method. The measurements extend from 0.1 molal (m) to 20, 17, and 15 mol-kg^–1, respectively, for the three salts. In the region below 2m, the osmotic coefficients decrease in the order NH₄Cl>MeH₃NCl>Me₂H₂NCl>Me₃HNCl>Me₄NCl, and above 3m the order is reversed. The intermediate members of the series have identical osmotic coefficients at the crossover molality of 3.0. It is suggested that the behavior at low molalities reflects primarily ion-dipole interactions, decreasing as the cation size increases, whereas hydrophobic (structure-making) interactions, increasing in importance with the number of non-polar methyl substituents, are predominant in concentrated solutions of these salts.     

Why are buckyonions round?

Multi-layered round carbon particles (onions) containing tens to hundreds of thousands of atoms form during electron irradiation of graphite. However, theoretical models of large icosahedral fullerenes predict highly faceted shapes for molecules with more than a few hundred atoms. This discrepancy in shape may be explained by the presence of defects during the formation of carbon onions. Here, we use the semi-empirical tight-binding method for carbon to simulate the incorporation of pentagon-heptagon defects on to the surface of large icosahedral fullerenes. We show a simple mechanism that results in energetically competitive derivative structures and a global change in molecular shape from faceted to round. Our results provide a plausible explanation of the apparent discrepancy between experimental observations of round buckyonions and theoretical predictions of faceted icosahedral fullerenes. Received: 27 August 1997 / Accepted: 23 October 1997     

High-performance chromatofocusing using linear and concave pH gradients formed with simple buffer mixtures. II. Separation of proteins

The separation of proteins using high-performance chromatofocusing with linear or concave pH gradients formed using simple mixtures of buffering species in the elution buffer is investigated experimentally. The separation achieved is comparable to that using polyampholyte elution buffers with these types of systems. More specifically, protein band widths at one half of the band height in the range between 0.1 and 0.025 pH units were observed, and good resolution was achieved of protein variants differing by a single amino acid residue in separation times of 30 min or less. An especially useful elution buffer is investigated that contains only four buffering species and that produces a linear pH gradient in the range between pH 9.5 and 6.0 when used together with a particular high-performance column packing made specifically for chromatofocusing. This elution buffer and column packing combination is evaluated by using it for the chromatofocusing of equine myoglobin and human hemoglobin variants. Additional applications are described in which a polyethyleneimine derivatized silica column packing and a pH gradient that is concave in shape are used for the separation of proteins in an E. coli cell lysate.     

Activity coefficients of hydrochloric acid and ionic interactions in the system HCl−CsCl−H2O from 5 to 50°C

Electromotive-force measurements of the cell $$Pt;H_2 (g, 1 atm)|HCl (y_A I), CsCl (y_B I)|AgCl;Ag$$ for five values of the ionic strength (I) between 0.1 and 3.0 mol-kg^?1 and over the temperature range 5 to 50°C are reported. At I=1.0 and above, the activity coefficients of HCl are a distinctly non-linear function of solution composition. This behavior is satisfactorily accounted for in terms of specific ion interactions of the type H?Cs and H?Cl?Cs. The temperature variation of the Harned coefficient supports the suggestion of Robinson that the interaction H?Cs is insensitive to temperature changes.     

A general method for the formation of aryl-sulfur bonds using copper(I) catalysts

[reaction: see text] We report a mild, palladium-free synthetic protocol for the cross-coupling reaction of aryl iodides and thiols using 10 mol % CuI and 10 mol % neocuproine, with NaOt-Bu as the base, in toluene at 110 degrees C. Using this protocol, we have shown that a variety of aryl sulfides can be synthesized in excellent yields from readily available iodides and thiols.     

Macrolides. Recent Progress in Chemistry and Biochemistry

The progress of macrolide chemistry and biochemistry over the past several years has indeed been remarkable. This review attempts to cover the important contributions which have been made in the fields of structural and synthetic chemistry as well as in the biosynthesis and mode of antimicrobial activity of the “polyoxo” macrolide antibiotics. Emphasis has been placed on the recent synthetic achievements in this field.     

Measurement of longitudinal optical mode frequencies of alkali metal and alkaline earth nitrates and carbonates by PSR infrared spectroscopy

Polarized specular reflectance (PSR) infrared spectroscopy was employed to determine the longitudinal optical mode frequencies of alkali metal and alkaline earth nitrates and carbonates. PSR spectra of AgNO₃, TlNO₃, and Pb(NO₃)₂ also are reported. Approximate values of |?μ/?Q| were calculated for the ν₂(A″₂) and ν₃(E′) modes of NO^?₃ and CO^2?₃ ions from the observed frequencies.     

Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid catalysis

Reactions catalyzed within porous inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, collectively referred to as “solvent effects”. Transition state theory treatments define how solvation phenomena enter kinetic rate expressions, and identify two distinct types of solvent effects that originate from molecular clustering and from the solvation of such clusters by extended solvent networks. We review examples from the recent literature that investigate reactions within microporous zeolite catalysts to illustrate these concepts, and provide a critical appraisal of open questions in the field where future research can aid in developing new chemistry and catalyst design principles.

“Solvent effects” at interfaces in heterogeneous catalysts are described by transition state theory treatments that identify kinetic regimes associated with molecular clustering and the solvation of such clusters by extended molecular networks.