The interpretation of quasi-kinematical single-crystal electron diffraction intensity data from paraffins

Dorset, Douglas L.

doi:10.1107/s0567739476000491

Cited by 55 publications

(21 citation statements)

References 24 publications

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“…Thin organic crystals are not cntircly flat within the sampling diamctcr of the selected-area experiment. Therefore, a phenomenological Lorentz correction was not applied to the raw intensity data for extraction of relative structurc-factor magnitudes (Dorset, 1976). Normally, intensities on rcciprocal-latticc rows common to two projections are used to scale one pattern to the other in order to build up a three-dimensional set.…”

Section: Intensity Measurementmentioning

confidence: 99%

Electron Crystallography of Small Organic Molecules: Criteria for Data Collection and Strategies for Structure Solution

Dorset¹,

McCourt²,

Li³

et al. 1998

J Appl Cryst

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By crystallization onto an organic substrate such as naphthalene, thin microcrystals of a small organic molecule, such as triphenylene, can be grown. These crystals are less perturbed by shear and erratic bend disorder than the samples produced by rapid growth from dilute solutions. Selected-area electron diffraction intensities are consistent from specimen to specimen, show the symmetry expected for the crystalline projection and, furthermore, correspond to the Fourier transform of the entire unit cell. However, undersampling of the three-dimensional reciprocal lattice by goniometry can frustrate structure determination if conventional direct methods are used. Nevertheless, the crystal structure may still be solved quite accurately by energy minimization.

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Section: Intensity Measurementmentioning

confidence: 99%

Electron Crystallography of Small Organic Molecules: Criteria for Data Collection and Strategies for Structure Solution

Dorset¹,

McCourt²,

Li³

et al. 1998

J Appl Cryst

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“…For example, the isotropic temperature factor used for the atoms, B = 0.0 ,~2, is not physically meaningful for such a structure, especially when thermal motion was a refined parameter in the X-ray crystal structure (Degeilh & Marsh, 1959). From our experience, such an unrealistically low temperature factor is a sign of multiple-scattering effects (Dorset, 1976) which, on average, tend to suppress the relative scattering amplitudes of lowangle reflections. This multiple scattering can be coherent in the form of n-beam dynamical scattering (Cowley & Moodie, 1959) or incoherent in the form of so-called 'secondary' scattering (Cowley, Rees & Spink, 1951).…”

Section: Discussionmentioning

confidence: 89%

Electron diffraction structure analysis of diketopiperazine – a direct phase determination

Dorset¹

1991

Acta Cryst Sect A

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Three-dimensional electron-diffraction intensity data from crystalline textures of diketopiperazine (2,5-0108-7673/91/050510-06503.00 piperazinedione) published by B. K. Vainshtein in 1955 [Zh. Fiz. Kim. (1955 are found to be suitable for ab initio structure analysis via conventional direct phase determination based on the © 1991 International Union of Crystallography DOUGLAS L. DORSET 511 estimates of three-and four-phase structure invariants. Of the 289 non-zero reflections found in this data set, 196 with [E I ->0.5 were used to generate triples and quartets to phase 133 reflections correctly. The resultant [001] and [100] zonal electrostatic potential maps reveal an easily identifiable molecular structure and the atomic coordinates are close to the values found in an X-ray crystal structure.

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“…The success o f this endeavor has been less than consummate. Although the close correlation of single crystal electron diffraction data to earlier published texture data [1 ] has been established for orthorhombic paraffins [2 ], the naive use of kinem atical diffraction theory necessarily must yield to a consideration of n-beam dynamical effects -even for monomolecular layers [2,3]. Howrever, a kinem atical procedure may succeed in ab initio phase determination [4].…”

Section: Introductionmentioning

confidence: 99%

“…In our first attem pt to characterize quantitatively the electron diffraction from these materials [2 ], it was also noted th a t m ultilayer crystals give diffraction intensities as if each layer wrere scattering independently from the others. This wras tentatively interpreted in term s of bending of crystal layers by us [13] and as an eclipsing o f chain axes in multilamellar crystals by other workers [14][15][16][17],…”

Section: Introductionmentioning

confidence: 99%

Transmission Electron Diffraction Intensities from Real Organic Crystals: Thin Plate Microcrystals of Paraffinic Compounds

Dorset

1978

Zeitschrift Für Naturforschung A

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Solvent-grown microcrystals of long polymethylene compounds packing in the 0 ± methylene subcell are found to have some of the defect structures seen for crystalline polyethylene. Crystals composed either of rectangular layers or of oblique layers both exhibit the growth around a screw dislocation characterized earlier by other workers. In addition, rectangular layer crystals o f orthorhombic n-hexatriacontane give evidence of mfralamellar edge dislocations. When rectangular layer crystals of cetyl palm itate are grown from hot ethanol, the same surface corrugations are found as seen earlier for polyethylene and orthorhombic paraffins. Oblique layer crystals have not demonstrated these latter two defect structures. All crystals show evidence o f appreciable bending.As was found to be true for rectangular layers of n-paraffins, bending has some influence on the electron diffraction intensities from an oblique layer. Another important factor is a perturbation due to defects along long chain axes which disrupts the ordered packing of polar regions, and causes incoherent scatter from these regions. In addition, observed screw dislocations give a nonuniform end plane, causing isolation of layers from one another. Diffraction models accounting for bending or isolation o f chain layers in an oblique layer both explain the observed near in variance of certain intensities which are due to the coherent scattering from the line grating formed by rows o f carbon atoms in the tilted chain aggregate. Forbidden reflections predicted by a bending model are less important than those anticipated by the multiple scattering from a laminated layer model. As these forbidden reflections are mostly identified for multilayer crystals and since crystal structure analysis indicates most of the long chain in a oblique monolayer to be diffracting coherently, the concentration of Reneker-type defects in the centers of chains is probably not great enough to isolate small chain length laminae within a monolayer.

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The interpretation of quasi-kinematical single-crystal electron diffraction intensity data from paraffins

Cited by 55 publications

References 24 publications

Electron Crystallography of Small Organic Molecules: Criteria for Data Collection and Strategies for Structure Solution

Electron Crystallography of Small Organic Molecules: Criteria for Data Collection and Strategies for Structure Solution

Electron diffraction structure analysis of diketopiperazine – a direct phase determination

Transmission Electron Diffraction Intensities from Real Organic Crystals: Thin Plate Microcrystals of Paraffinic Compounds

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