Aqueous iodic acid: Conductance and thermodynamics

Precision conductance measurements are reported on aqueous solutions of iodic acid for 16 concentrations between 17 and 0.7 mM and for 20 temperatures between 5° and 100°C. RlnK _a (m) and _o were calculated at each temperature and the data expressed by suitable temperature functions. From RlnK _a (m) as a function of temperature changes in standard enthalpy, entropy, and heat capacity were calculated. C _p proved to be independent of temperature so that H⁰ was a linear function of temperature. Comparisons have been made with other published data for iodic acid. The pattern of variation of Walden products with temperature was similar to that found earlier for substituted benzoic acids.     

Halogenation of vinyl ketoximes. Synthesis of isoxazoles and preparation and silver ion-promoted reactions of 4-halo-2-isoxazolines

Reaction of several α,β-unsaturated ketoximes with N-bromosuccinimide (NBS) gave isoxazoles, but yields were lower and the reaction less general than a similar transformation using iodine under basic conditions. With β,β-disubstituted oximes, 4-halo-5,5-disubstituted-2-isoxazolines were obtained using NBS, iodine, or N-chlorosuccinimide. Treatment of the 4-bromoisoxazo-lines with silver acetate or silver nitrate caused either elimination with rearrangement to give isoxazoles or substitution at C-4, depending upon the nature of the substituents at C-5.     

Oligomerization of vinyl monomers 20. Carbon-13 NMR of oligomers of 2-vinylpyridine. Studies of labelled end groups

Oligomers of 2-vinylpyridine initiated with Li, Na, K, and Rb salts of ¹³C-labeled or-unlabeled 2-ethylpyridine and terminated with ¹³C-labeled or -unlabeled methyl iodide were separated by liquid chromatography and analyzed by ¹³C-NMR of the initial or terminal CH₃ group. The stereochemistry of the three dyads which flank the initial or terminal methyl group could be determined and this allowed us to evaluate the stereochemistry of both methylation and vinyl addition. The methylation of the Li and Na salts of the oligomer anions is highly (> 95%) stereoselective and independent of chain length, whereas the stereoselectivity of the K and Rb salts is substantially less. These data indicate that the Li and Na oligomer and polymer anions exist in a chelated form in which the metal ion is complexed with the penultimate 2-pyridine group, the other counterions being chelated to a lesser extent. The stereochemistry of monomer addition was found to be less stereoselective than that of methylation for the Li and Na salts. In addition, the apparent meso-stereoselectivity of monomer addition of the Na, K, and Rb systems was greater than that of the Li ion. For the latter system, in particular, the mechanism appeared to be Markoff-like and consistent with different reactivities of anions flanked by a distinct dyad or triad stereochemistry. These data appear to be consistent with the intramolecular coordination that tends to hold the third asymmetric center close to the propagating carbanion.     

Drag coefficients of spherical liquid droplets Part 2: Turbulent gaseous fields

The drag of non-evaporating, spherical, liquid droplets was measured in turbulent flow fields at parametric ranges relevant to spray combustion, characterized by the droplet Reynolds number, and the intensity and spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure was to inject water droplets of uniform size co-currently and continuously with vertical turbulent air flows while droplet velocity was measured at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injection. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were investigated in the range 10–100, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than unity so droplets were effectively spherical. Relative intensities of turbulence were investigated in the range 20–65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in comparison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11–38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experimental data showed that the drag in turbulent fields under these conditions is not significantly different than that of solid spheres in a quiescent field at the same Reynolds number.The financial support of the Natural Sciences and Engineering Research Council of Canada and the Manufacturing Research Corporation of Ontario is gratefully acknowledged.     

Volumetric properties of some methyl substituted benzoic acids and their sodium salts in water as a function of temperature

Density data are presented for benzoic, the toluic and 2,6-dimethylbenzoic acids and their sodium salts at temperatures from 5 to 65°C. Apparent and partial molar volumes have been calculated from the experimental data. From the partial molar volume data the volume change on substitution of a methyl group has been calculated as has the volume change on ionization of the acids. It is found that the volume contribution of a methyl group is somewhat dependent on the substituent position and on the temperature. The relation between volume change on ionization and the corresponding entropy change is presented for a range of temperature. The data support the idea that electrostriction is an important feature of these systems and that its importance increases with rising temperature.     

The crystal structure and molecular conformation of threo 5-methylmetadone in relation to opioid receptor binding

Crystals of the hydrochloride salt of the biologically inactive threo isomer of 5-methylmethadone, C₂₂H₃₀ONCl, are monoclinic space group P2₁ with unit cell dimnsionsa = 11.019 Å, b = 8.6153Å, c = 10.680Å and β = 93.026°. The observed conformation is one in which the nitrogen bearing chain is extended with the substituents on C(5) and C(6) nearly eclipsed, a feature compatible with NMR studies and molecular mechanics calculations. The very potent agonist (5S, 6S)-erythro-5-methylmethadone has a solid state conformation in which the N atom is rotated back toward the phenyl rings [C(4)-C(5)-C(6)-N = 97°]in agreement with molecular mechanics calculations. The fact that the more potent enantiomers, (6R)-methadone and (5S)-isomethadone, and the inactive threo isomer are observed in the extended solid state conformation in contrast to (5S, 6S)-erythro-5-methylmethadone is consistent with three different models for their interaction with opioid receptors. It is proposed that the more likely of these involves a receptor bound conformation of (6R)-methadone and (5S)-isomethadone that resembles the conformation of (5S, 6S)-erythro-5-methylmethadone or that opioid receptors recognize both gauche-like and extended conformations.     

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.