The interaction between an atom and a surface at large separation

A simple expression is generated for the coefficient C₃ of the asymptotic dispersion interaction ~z^?3 between an atom and a surface. The basis of the derivation is the assumption of simple forms for the frequency dependence of the polarizability of the atom and the dielectric function of the solid. The expression yields the known value of C₃ within a few percent for all cases for which it has been calculated previously. We use it to generate C₃ values for a large number of systems not previously treated.     

Crystal and molecular structure of bis(tropolonato)-dioxo-(ethanol)uranium(vi)

Crystals of bis(tropolonato)-dioxo(ethanol)uranium(VI) are orthorhombic:a = 12·495(8),b = 8·817(7),c = 15·459(9) Å,Z = 4, space groupPnam. The structure was determined with MoK diffractometer data by standard Patterson and Fourier methods, and was refined by full-matrix least-squares methods toR = 0·049 for 1289 observed reflexions.The coordination plane consists of the four tropolonato oxygen atoms and the oxygen from the ethanol ligand; the uranyl group is perpendicular to it. The molecule lies across the mirror plane, consistent with a boat conformation for the tropolone rings; the dihedral angle between the plane of tropolone ring and the coordination plane is 22·5 °. The deviations of the tropolonato oxygen atoms from the tropolonato plane are 0·08 Å (above) and 0·16 Å (below). It is almost certain that the ethyl group of the ethanol ligand does not lie in the symmetry plane; C(8) is about 0·3 Å on one side of this plane and C(9) is about the same distance on the other, within a satisfactory bonding scheme.     

Crystal structure of 1-(2-hydroxyphenyl)-3-(2-thiomethoxyphenyl)-1,3-propanedione,C16H14O3S

The crystal structure of the enol tautomer of 1-(2-hydroxphenyl)-3-(2-thiomethoxyphenyl)-1,3-propanedione C₁₆H₁₄O₃S, was determined from X-ray crystallography. The finalR value was 5.99% based on 1945 unique observed reflections. The crystals are monoclinic,P2₁/c, witha=7.406(6),b=8.585(6),c=21.992(4) Å,=93.42(2),D _c =1.36 g cm^–3 forz=4. The compound is present in the solid state as a unique enol tautomer.There are two short, intramolecular OO contacts of 2.553(4) and 2.577(4) Å. Both the phenol and the enol H atoms are asymmetrically located with respect to the O atoms.     

Asymptotic interaction between He,H, H2 and a graphite surface

An atom or molecule far from a surface experiences an interaction V ~ ?C₃z^?3, where z is the distance to the surface. We compute the coefficient C₃ for He, H, and H₂ interacting with a graphite surface. The calculation utilizes measured frequency dependent values of the polarizabilities and dielectric function. Comparison is made with C₃ values deduced from surface scattering and adsorption experiments.     

Restrictions on classical distance-regular graphs

Let \(\varGamma \) be a distance-regular graph with diameter \(d \ge 2\). It is said to have classical parameters \((d, b, \alpha , \beta )\) when its intersection array \(\{b_0,b_1,\dots ,b_{d-1};c_1,c_2,\dots ,c_d\}\) satisfies

$$\begin{aligned} b_i= & {} ([d]_b - [i]_b)(\beta - \alpha [i]_b) \qquad \text {and} \qquad c_{i+1} = [i+1]_b (1 + \alpha [i]_b)\\&\quad (0 \le i \le d-1), \end{aligned}$$

where \([i]_b := 1 + b + \cdots + b^{i-1}\). Apart from the well-known families, there are many sets of classical parameters for which the existence of a corresponding graph is still open. It turns out that in most such cases we have either \(\alpha = b-1\) or \(\alpha = b\). For these two cases, we derive bounds on the parameter \(\beta \), which give us complete classifications when \(b = -2\). Distance-regular graphs with classical parameters are antipodal iff \(b=1\) and \(\beta =1+\alpha [d-1]_b\). If we drop the condition \(b=1\), it turns out that one obtains either bipartite or tight graphs. For the latter graphs, we find closed formulas for the parameters of the CAB partitions and the distance partition corresponding to an edge. Finally, we find a two-parameter family of feasible intersection arrays for tight distance-regular graphs with classical parameters \((d,b,b-1,b^{d-1})\) (primitive iff \(b \ne 1\)) and apply our results to show that it is realized only by d-cubes (\(b = 1\)).

     

Interaction of hydrogen with surfaces of silicates: single crystal vs. amorphous

We have studied how the formation of molecular hydrogen on silicates at low temperature is influenced by surface morphology. At low temperature (<30 K), the formation of molecular hydrogen occurs chiefly through weak physical adsorption processes. Morphology then plays a role in facilitating or hindering the formation of molecular hydrogen. We studied the formation of molecular hydrogen on a single crystal forsterite and on thin films of amorphous silicate of general composition (Fe(x)Mg((x-1)))(2)SiO(4), 0 < x < 1. The samples were studied ex situ by Atom Force Microscopy (AFM), and in situ using Thermal Programmed Desorption (TPD). The data were analysed using a rate equation model. The main outcome of the experiments is that TPD features of HD desorbing from an amorphous silicate after its formation are much wider than the ones from a single crystal; correspondingly typical energy barriers for diffusion and desorption of H, H(2) are larger as well. The results of our model can be used in chemical evolution codes of space environments, where both amorphous and crystalline silicates have been detected.     

Novel stereocontrolled approach to syn- and anti-oxepene-cyclogeranyl trans-fused polycyclic systems: asymmetric total synthesis of (-)-Aplysistatin, (+)-Palisadin A, (+)-Palisadin B, (+)-12-hydroxy-palisadin B, and the AB ring system of adociasulfate-2 and toxicol A

A new stereocontrolled method for the formation of trans-anti cyclogeranyl-oxepene systems is described. The demanding stereochemistry is secured by stereoselective coupling of a cyclogeranyl tertiary alcohol with a 1,2-unsymmetrically substituted epoxide, while the formation of the highly strained oxepene is achieved employing ring-closing metathesis. Since the stereochemistry of the trans-fused 6,7-ring system is determined by the epoxide, the method also allows the construction of trans-syn 6,7-ring systems. This approach leads to the synthesis of the AB fragment of Adociasulfate-2 and Toxicol A, for the first time. The flexibility and efficiency of the presented strategy is demonstrated by the total asymmetric synthesis of (-)-Aplysistatin, (+)-Palisadin A, (+)-12-hydroxy-Palisadin B, and (+)-Palisadin B, employing two similar key intermediates.