Interpretation of electron diffraction data from organic microcrystals: a theoretical perspective.
The current framework for the quantitative analysis of transmission electron diffraction data from organic microcrystals is discussed. The appropriate formulation depends upon the crystal perfection: solution-grown material tends to be nearly perfect with regular elastic bends whilst the epitaxial samples are usually considerably less perfect with highly irregular bends. It is possible to obtain Bragg intensity data which are near-kinematical and thus amenable to ab initio structure analysis. Thereafter, the further refinement of the model proceeds with a consideration of dynamical and crystal bending effects. If both of these are small it is a good approximation to analyse them independently rather than use the correct description of dynamical scattering from a deformed foil. The analysis of three-dimensional data sets from tilt series is considered. Although the technique has been successfully demonstrated in several instances there are still details of the analysis that need further clarification.
Radiation damage and structural studies: Halogenated phthalocyanines
Abstract Radiation damage makes it more difficult to accomplish structural studies of beam-sensitive materials. Nevertheless, we report here a 500 kV high-resolution study of halogenated phthalocyanines. Epitaxially grown molecular crystals of fully brominated and fully chlorinated phthalocyanines were found to be isomorphous and had only the α-type polymorph structure. Despite the sensitivity of these crystals to beam damage, the characteristics of thin film growth and details of structural defects could be determined, provided that molecular image resolution was available. The effect of electron beam damage was a degradation of crystallinity which varied considerably according to local morphology and depended on the ease with which the displaced halogen could diffuse away from the parent molecule and thereby avoid recombination. Our findings demonstrate the feasibility of similar structural studies of other beam-sensitive materials.
Identification of Dihydroxy Aromatic Compounds in a Low-Temperature Pyrolysis Coal Tar by Gas Chromatography−Mass Spectrometry (GC−MS) and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS)
Dihydroxy aromatics in coal tar undergo rapid coupling reactions with each other and with some coal-derived species during coal pyrolysis and liquefaction. However, only limited information on the structure of dihydroxy aromatics is known. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and gas chromatography−mass spectrometry (GC−MS) were used to characterize the dihydroxy aromatics in a low-temperature pyrolyzed coal tar and its distillate and residue fractions. Negative-ion electrospray ionization (ESI) FT-ICR MS spectra of coal-tar-derived samples had low molecular masses and narrow mass ranges. Negative-ion ESI is extremely selective for the dihydroxy aromatics present in coal tar. The O2 class species had the highest ion intensity among the species in the negative-ion FT-ICR mass spectrum for each sample. Dihydroxy compounds with an aromatic core structure of benzene, indan, biphenyl (and/or acenaphthene), and naphthalene were identified in the distillate fract...