IUCr Newsletter (1998). 6(3), 14-15
Each of the four subject groups of the British Crystallographic
Assn invited a speaker for a "Plenary Session" with
the theme of "Disorder" at the Spring meeting. K. Prout
(Oxford, "Slow motion and disorder in molecular crystals")
discussed the use of nuclear magnetic resonance and X-ray crystallographic
data to study molecular disorder in crystals. The different timescales
probed by the two techniques offer insight into dynamic processes.
Examples included phase transition in penicillin derivatives,
clarification of disorder as "static" or "dynamic"
in ferrocene, camphor and deoxycholic acid derivatives, and analysis
of a complex series of phase changes undergone by crystals of
pyridinium nitrate thiourea clathrates. In his presentation, "Computer
simulations as a tool for the interpretation of diffuse scattering",
R. Welberry (Australian Nat'l U.) used optical transforms to illustrate
the effect on the diffraction pattern of various simple types
of disorder. Knowing the basic pattern shapes can assist in the
interpretation of diffuse scattering patterns. Both measurement
and pattern interpretation were illustrated with examples including
KLiSO4, urea inclusion compounds and stabilized zirconias. P.
Fairclough (Sheffield, "Scattering studies of polymer crystallization")
discussed larger scale structures and small angle scattering and
how the formation of crystallinity in different types of polymers
affects their properties. Experimental approaches to the study
of crystallization in situ were described and the use of both
the Bragg and diffuse scattering in small angle X-ray scattering
(SAXS) and wide angle X-ray scattering (WAXS) patterns was shown
to be important in understanding how a polymer crystallizes and
organizes itself. K. Wilson (York, "Proteins: selected disorder")
spoke about the disorder in protein structures and how it is to
be treated depending on the type of information one is attempting
to extract from the structural analysis. He discussed disorder
of parts of the protein molecule itself, solvent disorder near
the protein and the presence of alternate solvent networks (illustrated
for example in work on vitamin B12).
Question and Answer Session
A question and answer session at the meeting produced the following interesting exchanges.
Q: What is the smallest size of a single crystal from which diffraction data can be collected nowadays?
A: M. Harding suggested that for simple inorganic material, such as CaF2, the answer might be around 0.5 microns. With increasing complexity larger crystals would be needed since the mean reflection intensity decreases as the number of atoms increases.
Q: Acta C makes its acceptance criteria more and more stringent. Is this because they only want boring and routine structures to be published there? Does the journal discriminate against chemically interesting results which have been extracted with skill and patience from poor quality crystals?
A: (from three Acta C Co-Editors in the audience) While high standards have been used to establish the criteria for publication of the average structure in Acta C, extenuating circumstances are taken into account. For example, where a crystal diffracted weakly the resolution limit might be relaxed. The editorial in the January 1998 issue of Acta C contains examples of "valid extenuating circumstances" and "unacceptable excuses".
Other questions concerned achiral molecules in enantiomorphous
space groups; the importance of polarity in space groups like
Pnc21; the interpretation of Flack parameter values; the importance
or correct weighting of reflections whether you are refining on
F or F2; when a structure can be considered as refined to convergence;
and the reasons for the prevalence of the space group P21/c-is
this entirely due to its inherently efficient translational symmetry,
or because many of us like it so much.
A.J. Blake, U. of Nottingham
The Chemical Crystallography group (CCG) organized a session and a workshop. The session included presentations entitled: "Adventures in Molecular Recognition" (J. Sanders), "Synthesis of Inorganic Supramolecules Networks" (N. Champness), "Intermolecular Interactions Involving Metal Complexes" (G. Orpen), "Designing Non-linear Optical Materials: Supramolecular Requirements" (J. Cole), and "Charge-Transfer Complexes: Molecular Flexibility and Packing" (A. Batsanov).
The CCG workshop was on twinning. R. Herbst-Irmer (U. of Gottingen)
began by defining twins and giving simple examples of twinning,
twin laws and fractional contribution, before detailing the four
different types of twinning: merohedral, pseudo-merohedral, reticular
merohedral and non-merohedral. She presented detailed examples
of the different types, their diagnosis and their treatment in
refinement. She compared the use of twin laws to disorder modeling
in twinned crystals and showed the former not only gave superior
results but did so at the expense of only one additional refinement
variable. E. Hovestreydt of Bruker AXS contributed a short presentation
on software being developed by B. Sparks for automatic treatment
of non-merohedral twins on both area detector and four-circle
diffractometers. The warning signs for twinning include metric
symmetry higher than Laue symmetry, similar Rint values for higher
and lower Laue groups, values of |E2-1|<<0.736 and unusually
long cell axes.
A.J. Blake, U. of Nottingham
BCA Industrial Group
The topics of the Industrial Group (IG) sessions were "Data Quality - Fit for Purpose" and "Hardware and Software Developments". The highlight of the IG program was the presentation of the Alun Bowen Industrial Lecture "The Development of X-Ray Analysis" by awardee B. Snyder of Ohio St. U. Snyder painted a broad tapestry of X-ray utilization, starting with Roentgen in 1895 and finishing with the latest methods such as grazing incidence and EXAFS. Pointing out that, within 15 years of Roentgen's discovery most of the tools available today had been defined, each specific discovery was exemplified with slides of the original work followed by its development and the latest implementation of that idea. As examples of the modern scene, the arrival of synchrotron radiation with its wavelength tunability has led to EXAFS and XANES analysis of poorly crystalline materials. The precision of high resolution instruments has revealed fundamentals of superconductivity, and grazing incidence has opened the door to thin film analysis. Time dependent studies are now readily achievable, allowing reactions to be followed together with, for example, phase transitions, kinetics, etc. A thoroughly fascinating review of our field, and a superb re-statement of the utility of X-rays was presented.
The Data Quality session included presentations on production and identification of samples and the developement of the Powder Diffraction File (R. Jenkins, ICDD), techniques for line broadening analysis of new materials (R. Todd, U. Manchester), cation site orderng in Olivine and the structure of fast ion conductors in fuel cells (R. Knight, RAL), and the challenges of analyzing pharmacutical materials (J. Anwar, Kings College, London). Anwar noted that the existence of polymorphs, hydrates, phase instability, low symmetry, weak scattering, grinding and texture variations make pharmacuticals expecially challenging samples.
Representatives of Bruker AXS, Nonius BV, Philips, Bede Scientific,
Stoe and Eie and Oxford Cryosystems all contributed to the session
on Hardware and Software Developments.