Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces

Atomic chemisorption of hydrogen and oxygen on the Ni(100) surface has been studied using an Effective Core Potential (ECP) approach described in a previous paper. Clusters of up to 50 nickel atoms have been used to model the surface. The computed chemisorption energies are 62 kcal/mol (exp. 63 kcal/mol) for hydrogen and 106 kcal/mol (exp. 115–130 kcal/mol) for oxygen. Correlating the adsorbate and the cluster-adsorbate bonds is extremely important for obtaining accceptable results, particularly for oxygen. Reasonable convergence of chemisorption energies is obtained with 40–50 cluster atoms for both hydrogen and oxygen. For hydrogen the addition of a third cluster layer stabilizes the results considerably. Both hydrogen and oxygen are adsorbed at (or close to) the four-fold hollow site. The calculated barriers for surface migration are also in good agreement with the experimental estimates. The calculated equilibrium heights above the surface are on the other hand too high compared with experiments. This disagreement is believed to be due to core-valence correlation effects, which are not incorporated in the present ECP. The cluster convergence for the height above the surface is much slower than for the chemisorption energy.     

Practical Stannylation of Allyl Acetates Catalyzed by Nickel with Bu3SnOMe

A practical and scalable nickel‐catalyzed allylic stannylation of allyl acetates with Bu₃SnOMe is described. A variety of acyclic and cyclic allyl acetates, even with base‐sensitive moieties, undergoes the stannylation by using NiBr₂/4,4′‐di‐tert‐butylbipyridine (dtbpy)/Mn catalyst system to afford highly functionalized allyl stannanes with excellent regioselectivity and yields. Furthermore, the scope of protocol is also extended by the reaction of propargyl acetates, giving rise to propargyl or allenyl stannanes. Additionally, a unique diastereoselectivity using the nickel catalyst different from the palladium was demonstrated for the stannylation of cyclic allyl acetates. In the reaction, inexpensive and stable nickel complexes, abundant reductant (Mn), and atom‐economical stannyl source were used.     

Clarithromycin form I determined by synchrotron X‐ray powder diffraction

The structure of the metastable form I polymorph of the macrolide antibiotic clarithromycin, C₃₈H₆₉NO₁₃, was determined by a powder diffraction method using synchrotron radiation. The space group of form I is P2₁2₁2. The initial model was determined by a molecular replacement method using the structure of clarithromycin form 0 as a search model, and the final structure was obtained through Rietveld refinements. In the form I crystal structure, the clarithromycin molecules are aligned parallel along the a axis in a head‐to‐tail manner with intermolecular hydrogen bonds between the hydroxy O atoms. The dimethylamine groups of the clarithromycin molecule interdigitate between neighbouring head‐to‐tail clarithromycin alignments. The novel crystal packing found in form I provides a mechanism that describes the transformation of form 0 to form I.     

Asymptotic Behavior of Minimizers for the Ginzburg-Landau Functional with Weight. Part II

(Accepted July 23, 1996)     

Method of evaluation of the bitterness of clarithromycin dry syrup

The degree of bitterness of clarithromycin (CAM) dry syrup was evaluated using several methods. Using the inversion method, shaking method, and paddle method, a reasonable correlation between the bitter taste and the amount dissolved was not observed. A mini-column with inner diameter of 0.76 cm and height of 5 cm packed with CAM dry syrup was used for the release test. The release rate of CAM in test solution, which passed through the mini-column, was then measured to evaluate bitterness. The release rate of CAM in the release test using the mini-column correlated well with the results of a sensory test for the bitterness of CAM dry syrup. The dissolution rate constant, defined as the percentage of CAM dissolved from the unit void surface multiplied by the void volume, was inversely proportional to the linear velocity of the test solution. The critical factors affecting evaluation of bitterness were the void volume of the column and linear velocity of the test solution. The optimum linear velocity and void volume were 0.048-0.021 cm/min and 0.27-0.12 cm3, respectively. In addition, the threshold of bitterness of CAM dry syrup was defined as the concentration at which half of the volunteers recognized bitterness in the sensory test. This threshold was found to be 135 microg/ml using the mini-column.     

Toward a realistic description of NO(x) storage in BaO: the aspect of BaCO3

NO(x) storage over hexagonal BaCO3(110) is investigated using first-principles calculations. Special focus is put on the importance of surface decarbonation. Upon decarbonation, supported BaO quasi-molecules are formed and a small drive toward (BaO)n cluster formation is predicted. Introduction of NO2 makes the decarbonation energetically relevant, while forming NO2-BaO-NO2 units, on the decarbonated surface. With this configuration, it is possible to replace all surface carbonates with nitrites and nitrates, forming a BaCO3 supported BaNO3NO2 overlayer. Thermodynamic considerations are employed to elaborate on the thermal stability of the formed NO(x) overlayers.