Iron-catalyzed Doyle-Kirmse reaction of allyl sulfides with (trimethylsilyl)diazomethane

[formula: see text] Iron salts efficiently catalyze the Doyle-Kirmse reaction of allyl sulfides with (trimethylsilyl)diazomethane and ethyl diazoacetate in dichloroethane at 83 degrees C. Competitive dimerization is less of a problem with (trimethylsilyl)diazomethane than with ethyl diazoacetate. Good results are obtained using only 1.5 equiv of (trimethylsilyl)diazomethane, even without slow addition. Phosphine ligands affect the kinetics, but not the diastereoselectivity. Dppe and BINAP lead to higher yields than dppp, but no enantioselection was detected with R-(+)-BINAP.     

Density histograms in protein electron crystallography-direct phase determination of bacteriorhodopsin at 6A resolution

Direct methods for crystallographic phase determination have been used to solve the projected structure of native glucose-embedded bacteriorhodopsin (plane group p3, a=62.4A) to 6A, without accepting any information from electron micrographs. After defining the origin and enantiomorph and adding one algebraic phase term to the basis set, expansions into the complete set of 50 unique reflections via the Sayre equation, testing four possible values of the algebraic term, could be screened with figures of merit based on the protein density histogram, where the auto-correlation function of the ideal histogram is an endpoint for the cross-correlation of experimental histogram with this ideal distribution. Initial phase determination, followed by Fourier refinement and some additional phase permutation, yields a solution with an overall mean phase error of 60.8 or 35.4 degrees for the 18 most intense reflections.     

Electron crystallography of inorganic compounds. Direct determination of the basic copper chloride structure CuCl2·3Cu(OH)2

Experimental electron diffraction intensity data from basic copper chloride CuCl₂·3Cu(OH)₂ in its metastable form have been used for a direct structure analysis based on the evaluation of three and four phase structure invariants. The resultant electrostatic potential maps clearly show the correct atomic positions which correspond to those determined earlier by Voronova and Vainshtein (1958) from their interpretation of Patterson maps, and also to the X-ray analysis of the isostructural CuBr₂·3Cu(OH)₂ carried out by Aebi in 1948.     

Phase transformation and structure of n-C50H102/n-C60H122 solid solutions formed from the vapor phase

Binary solid solutions of n-paraffins (n-C₅₀H₁₀₂/n-C₆₀H₁₂₂) epitaxially prepared on potassium hydrogen phthalate substrate from the vapor phase have been studied by electron diffraction to characterize their phase transitions and structure. The continuity of solid solution in the n-C₅₀H₁₀₂/n-C₆₀H₁₂₂ system is demonstrated once lamellar ordering of the crystal packing is achieved. However, such ordering is achieved only by annealing and proceeds through a series of intermediate chain packings. At first, the electron diffraction patterns from all samples prepared at room temperature resemble those from polyethylene, in which no spots corresponding to the interlayer spacings appear. Longitudinal chain translations are induced by annealing to cause the lamellar reflections to appear, while the “polyethylene” subcell reflections remain unsplit until the crystal structure with well-defined methyl end planes is reached.     

Kink defects in linear chain molecules-structure analysis based on spot and continuous diffuse electron diffraction intensities

Calculated defect diffuse scattering patterns, using the kink defect model employed earlier to explain changes in Bragg electron diffraction intensities for heated n-paraffins, are shown to account best for observed diffuse data from solution-grown and epitaxially grown micro-crystals. The presence of significant diffuse scattering below 15°K, moreover, indicates that such chain kinks are stable in the crystal lattice.     

Epitaxial growth of long-chain molecules on potassium hydrogen phthalate and potassium chloride substrates from the vapor phase

Pentacontane, hexacontane, 15-ketohentriacontane (palmitone), and solid solutions of the two n-paraffins were deposited from the vapor phase onto surfaces of potassium hydrogen phthalate (PHP) crystals and cleavage faces of potassium chloride (KCl) crystals in vacuo. Epitaxial growth, with molecular chains parallel to the substrates, occurs within a temperature range (e.g., 25–50°C for palmitone), and the epitaxial relationship between molecules and substrates was broken at higher temperatures. The electron diffraction patterns, similar to that of polyethylene, were obtained from all samples prepared at room temperature. The [100] or [130] directions of palmitone crystals are parallel to the substrate surface of PHP, and the [110] direction is parallel to that of KCl; the crystals in both cases are the orthorhombic form with lattice parameters a = 7.56 Å, b = 4.93 Å, c = 82.8 Å and a space group of Pna2₁. The (100) planes of n-paraffin crystals are parallel to the surface of PHP, and the crystals are again orthorhombic. Also, solid solutions of n-paraffins can be formed from the vapor phase, with the nucleation process on the two substrates being the same as for pure paraffins.     

From Waxes to Polymers—Crystallography of Polydisperse Chain Assemblies

Exploiting available single-crystal diffraction patterns from individual microareas, electron crystallography has characterized rectangular layer chain packing for polydisperse extended chain polymethylene chain assemblies. From this work, one can appreciate how these chain assemblies form stable solid solutions and also how they may phase separate. It is clear that two types of stable solid solution are formed—one that is lamellar and one that is nematocrystalline. Respective examples are wax fractions with narrow chain length distributions and chain-extended, low molecular weight polyethylene.