Generalized van der Waals theory VIII. An improved analysis of the liquid/gas interface

We extend a preceding application of the GvdW theory to the prediction of interface profiles and surface tension in simple fluids by incorporating a variational determination of the effective hard-sphere diameter. This has previously been found to improve the predicted equation of state. Here we find that it also improves the prediction of interface profiles and surface tension in LJ(12-6) fluids. The agreement with experiment and simulation when these quantities are considered as functions ofT/T _c is to within about 5%. As in our earlier calculations the nonlocal entropie effects are found to reduce the surface tension by 5%–10%. In the present conceptually more accurate theory this significantly improves the agreement with experiment and simulation.     

Unusual temperature effects on the non-equivalence of geminal methyl groups

The geminal anisochronism (Δδ) of the isopropyl methyl groups in the ¹H n.m.r. spectrum of N-[2-methyl-1-(1-naphthyl)propylidene]benzylamine showed unusually pronounced temperature and solvent effects. It is noteworthy that both accidental and dynamic equivalences were encountered. Slow rotation around the naphthyl–imino bond is responsible for the diastereotopic nature of the geminal methyl groups, and possible reasons for the temperature and solvent effects are considered. Surprisingly, the temperature effect was not reflected in the ¹³C anisochronism of the methyl carbon nuclei. These observations underline the need for caution in drawing firm conclusions from variable temperature studies on geminal non-equivalence.     

The synthesis of 8-hydroxy- and 8-methoxy-ellipticines

The first synthesis of 8-hydroxyellipticine is described and its identity with an ellipticine metabolite from $\text{Aspergillus}$$\text{alliaceus}$ confirmed.     

The elusive 8-fluoroadenosine: a simple non-enzymatic synthesis and characterization

The only successful synthesis of 8-fluoroadenosine reported until now relied on an enzymatic removal of the acetate protecting groups using thermally resistant hydrolases. In the present communication we describe the first non-enzymatic synthesis of 8-fluoroadenosine. According to this, the C₈-fluorine atom was introduced in a halogen-exchange process performed at elevated temperature. The chief obstacle in the synthesis of 8-fluoroadenosine, the removal of the protecting groups in the presence of the labile C₈-F bond, was addressed by judicious choice of acid-labile protecting groups. Their deprotection in the presence of C₈-F is described. Using this newly developed procedure, significant quantities of 8-fluoroadenosine were synthesized and, for the first time, its physicochemical properties including pH-dependent stability, examined in detail. The intermediate generation of 8-fluoroadenosines as a tool to increase the reaction rates of nucleophilic substitutions was briefly examined and successfully demonstrated with the example of 8-cyanoadenosine. The presented procedure is applicable to the synthesis of various adenosine analogs with potential pharmacological significance.     

Maxwell fields onH-space

It is shown that theH space associated with a solutionM of the Einstein-Maxwell equations can be endowed with a self-dual Maxwell field which arises from the radiation component ofM's Maxwell field.     

Absorption and velocity dispersion due to crystallization and melting of emulsion droplets

The influence of droplet crystallization and melting on the ultrasonic properties of oil-in-water emulsions has been investigated. The ultrasonic velocity and attenuation were measured in a series of 3 wt% n-hexadecane-in-water emulsions as a function of frequency (0.3–4 MHz), droplet diameter (0.4 and 1 μm) and temperature (0–25°C). The emulsified n-hexadecane crystallized at about 5°C due to supercooling effects and melted at about 18°C. As solid and liquid n-hexadecane have significantly different ultrasonic properties, an appreciable change in the velocity and attenuation is observed during the phase transition. This behaviour is modified significantly in systems where the emulsion droplets are partially crystalline because the temperature fluctuations associated with the ultrasonic wave can perturb the phase equilibria solid liquid causing excess attenuation and velocity dispersion. The magnitude of this effect depends on the ultrasonic frequency and the average droplet size.     

Computation of Orientational Averages in Solid-State NMR by Gaussian Spherical Quadrature

We investigate Gaussian spherical quadrature as a method for calculating orientational averages in solid-state NMR. For the case of magic-angle-spinning sideband amplitudes of isolated spins-1/2, we demonstrate the superiority of Gaussian spherical quadrature over other orientational averaging methods. Depending on the shift anisotropy parameters and the desired accuracy, the computation speed is enhanced by a large factor (between two and many hundreds). In addition, a method for improving any present sampling scheme is devised. Such schemes are called SHREWD (Spherical Harmonic Reduction or Elimination by a Weighted Distribution). The role of orientational symmetry in solid-state NMR is explored. We also discuss the limitations of the Gaussian spherical quadrature methods.     

Mobile-ion concentrations and mixed-alkali effects in β-alumina solid electrolytes

A theory of the mixed-alkali effect based on the weak electrolyte model is used in the analysis of conductivity isotherms in the Na/K β-alumina system. This gives mobile interstitialcy concentrations, [Na²⁺₂] or [Na⁺_i], which agree remarkably well with those obtained by Wolf from calculations based on changes in the Haven ratio, H_R. The high conductivity of β-alumina arises apparently from a high “defect mobility” rather than from the existence of quasi-liquid motions. Other consequences of the weak electrolyte theory are discussed.