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The physisorption of molecules in confined geometry, i.e., in pores of atomic size such as found in zeolites, has been investigated using a simple pairwise-additive Lennard-Jones potential and an effective-medium model. In a spherical geometry, it is found that the equilibrium distance D corresponding to the lowest equilibrium energy is reduced to about 90% of the pair equilibrium distance σe. This originates from the increased dominance of long-range forces in the condensed state. The enhancement of the physisorption energy due to surface curvature and confinement effects reaches its maximum value of 5.05, relative to the flat surface, when D=0.899σe. This value must be compared to the factor of 8 which was derived previously [D. H. Everett and P. C Powl, J. Chem. Soc. Faraday Trans. 1 72, 619 (1976); E. G. Derouane, J.-M. André, and A. A. Lucas, Chem. Phys. Lett. 137, 336 (1987)] using a simple van der Waals model neglecting repulsion forces. It is also concluded that molecules can be strongly trapped in pores which are substantially narrower than their free (gaseous phase) sizes, the situation of lowest energy corresponding to R=D=0.899σe and the sorption energy remaining negative down to R=D=0.749σe (R denotes the pore radius). [ABSTRACT FROM AUTHOR]

The problem of evaluating the van der Waals attraction energy of an atom or microparticle adsorbed near the edge of a straight material wedge of arbitrary opening angle is considered. In order to disentangle more clearly the effect of the wedge geometry from the material parameters, a simple model is used in which the particle is represented by a harmonic oscillator and the substrate by a perfect conductor. By a straightforward extension of the method of images of classical electrostatics, the dipolar coupling of the particle to the substrate is constructed from which new oscillator frequencies can be determined as a function of its position relative to the edge and of the wedge angle. The van der Waals energy is given by the shift in zero-point energy of the oscillator coupled to itself via its images. For a physisorbed atom crossing the edge of a wedge material, the detailed shape of the van der Waals energy barrier (respectively, well) offered by a convex (respectively, concave) wedge is displayed and compared with approximate results deduced from a pairwise summation of Lennard-Jones interactions. The interest of these results for the dynamics and kinetics of sorption by microporous solids is briefly discussed. [ABSTRACT FROM AUTHOR]

Variable angle sample spinning NMR of quadrupolar nuclei in the solid state has been theoretically investigated in order to determine the optimal angles to achieve high-resolved and quantitative spectra. The earlier literature contributions have been included in a more general model. Complete analytical expressions were derived and are reported for all possible cases. It is concluded that magic angle spinning (at 54.7°) is to be used whenever the dipolar interactions and/or the chemical shift anisotropy dominate the spectra. For the other cases, θ=43.5° (which would allow an easy discrimination of the quadrupolar frequency and the asymmetry parameter), and θ=60°–70° (where the linewidth is half of that obtained for MAS conditions) are proposed as preferred spinning angles. [ABSTRACT FROM AUTHOR]

The steady-state kinetics of the one-stage Wacker oxidation of gaseous olefins such as ethene, propene, and 1-butene by oxygen (1:2 stoichiometric mixture of O2 and olefin) to form acetaldehyde, acetone, and methyl ethyl ketone, respectively, was studied in a semicontinuous reactor. The catalytic system involves an aqueous solution of Pd(II) chloride (0.05–2 mM Pd(II), [Pd(II)]/[Cl−]=1:50) and Keggin-type heteropoly anions [PMo9V3O40]6− (50 mM), at 20–50°C. Under these conditions in a steady state, the reactivity of olefins increases in the order ethene ≤1-butene < propene, but the total range is only a factor of 3. The oxidation of 1-butene is accompanied by the double-bond migration to form 2-butene. [Copyright &y& Elsevier]