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International Union of Crystallography

The IUCr is an International Scientific Union. Its objectives are to promote international cooperation in crystallography and to contribute to all aspects of crystallography, to promote international publication of crystallographic research, to facilitate standardization of methods, units, nomenclatures and symbols, and to form a focus for the relations of crystallography to other sciences.

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Solving difficult structures with electron diffraction

hi0137Electrons diffract in the same way as X-rays and neutrons, except that the electron wavelength is very small and the electron scattering cross-section is correspondingly much larger, about a million times that of X-rays. Since electron diffraction was discovered, transmission electron diffraction and the related electron imaging have developed into powerful tools for the analysis of defects, microstructure, surfaces and interfaces in a broad range of materials, so why haven’t more structures been solved with high-energy electrons?

The short answer lies in electron dynamic diffraction: the same strong interaction between electrons and matter that gives rise to large electron scattering cross sections also gives rise to strong multiple scattering. A general method for solving unknown crystal structures using dynamic diffraction intensities has yet to be developed.

In a topical review two researchers [Midgley, P. & Eggeman, A. (2015). IUCrJ, 2, 126-136; doi:10.1107/S2052252514022283] describe progress made in the technique of precession electron diffraction (PED).

Using PED intensities, crystal structures can be solved by a combination of phasing and structure refinement.

The quality of electron diffraction data, as well as speed of acquisition, is increasing rapidly; thus in the not too distant future we can expect more identifications of new structures and their solutions.

[Zuo, J. M. & Rouviere, J. L. (2015). IUCrJ2, 7-8; doi:10.1107/S2052252514026797]

Posted 22 Jan 2015 

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hi0138Photocrystallography is a rapidly developing technique that involves the determination of the full three-dimensional structure of a molecule or array material, using single-crystal X-ray diffraction techniques, while the molecular components are in a photoactivated metastable or short-lived high-energy state. The photoactivation is usually achieved by irradiating the single crystal with a laser or a set of LED lights in the ultra-violet or visible wavelength range.

To date photocrystallography has been used to study the structures of metastable linkage isomers of transition metal coordination complexes, and to investigate the changes in coordination geometry in mononuclear complexes.

In a complex that undergoes linkage isomerism, the compound contains one or more ligands that are capable of bonding to the metal centre in more than one way. At present, investigations at short lifetimes remain the domain of fast spectroscopy. Thus a combination of photocrystallography and time-resolved spectroscopy allows the evolution of structure with time to be evaluated.

The paper by Casaretto et al. [(2015). IUCrJ, 2, 35-44; doi:10.1107/S2052252514023598] provides new insights into the photactivated linkage isomerisation process and sets the benchmark for further studies that will lead to the development of electronic devices based on these materials.

[Raithby, P.R. (2015). IUCrJ, 2, 5-6; doi:10.1107/S2052252514026980]

Posted 21 Jan 2015 

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Serial crystallography for the masses?

pattersonmapThere are many important materials which do not form nice single crystals for X-ray diffraction experiments, such as proteins and other biomaterials. When the crystal size is too small then powder diffraction is normally the preferred method of analysis. Until now the minimum size of crystal needed to produce three-dimensional single-crystal data had been dependent on the instrument in operation; now, however, using serial micro crystallography, Ayyer et al. [(2015). IUCrJ, 2, 29-34; doi:10.1107/S2052252514022313] have shown that improvements can still be made from changes to the software and algorithms used.

When a sample diffracts well as a powder but does not grow larger crystals, serial micro crystallography will be the method of choice. This new way to obtain three-dimensional data could bring far more complex structures into reach for powder diffraction.

[Wright, J. P. (2015). IUCrJ, 2, 3-4; doi:10.1107/S2052252514026803]

Posted 20 Jan 2015 

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SHELXT – Integrated space-group and crystal-structure determination

sc5086Although crystal structure determination by means of X-ray diffraction has had a huge scientific impact over the last 100 years, it still requires the solution of the crystallographic phase problem. This problem arises because although the intensities of the diffracted X-rays can be measured, direct measurement of their relative phases is still only rarely practicable. Small-molecule crystal structures are usually solved by the use of probability relationships involving the phases of the stronger reflections, i.e. direct methods, or more recently by Fourier transform methods such as charge flipping.

SHELX is a system of nine programs for the solution and refinement of crystal structures against X-ray and neutron diffraction data.

The first version of SHELX was written around 1970 and officially released in 1976. Using data compression written specially for the purpose, it proved possible to pack the 5000 lines of FORTRAN, five test data sets and a little FORTRAN program to unpack the rest, into one box of 2000 punched cards. This greatly facilitated its distribution by post (email, internet etc. had not yet been invented) and enabled the program to spread around the world. The current version of SHELX has about 9000 registered users in about 90 countries and may be obtained, together with documentation and tutorials etc., via the SHELX homepage.

In 2008 Sheldrick published a review paper [Sheldrick, Acta Cryst. (2008), A64, 112–122] about the system, noting that it might serve as a general literature citation whenever any of the SHELX programs were used. The paper has since received no fewer than 38,000 citations making it the highest-ranked paper published in the last two decades, according to a recent study that appeared in Nature [Van Noorden et al. Nature, (2014), 514, 505-553].

This latest addition to the SHELX program [Sheldrick, Acta Cryst. (2015). A71, 3-8; doi:10.1107/S2053273314026370] employs a novel dual-space algorithm to solve the phase problem for single-crystal reflection data expanded to the space group P1. Missing data are taken into account and the resolution extended if necessary. In testing, SHELXT has already solved many thousands of structures with a high success rate.

A recent upgrade to the SHELXL refinement program is also available [Sheldrick, Acta Cryst. (2015), C71, 3-8; doi:10.1107/S2053229614026540].

In a recent interview for the IUCr the author, commenting on why he thought the SHELX programs are so well received, said, “the programs are extremely robust and adopt a strict 'zero dependencies' philosophy, i.e. no third-party libraries, DLLs, environment variables etc. are required to run them”.

Posted 08 Jan 2015 


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Chemistry and structure in Acta Crystallographica Section C

Acta Crystallographica Section C is continuing its transition to a journal that publishes exciting[journal cover] science with structural content, in particular important results relating to the chemical sciences. Section C is the journal of choice for the rapid publication of articles that highlight interesting research facilitated by the determination, calculation or analysis of structures of any type, other than macromolecular structures. Articles that emphasize the science and the outcomes that were enabled by the study are particularly welcomed. Authors are encouraged to include mainstream science in their papers, thereby producing manuscripts that are substantial scientific well-rounded contributions that appeal to a broad community of readers and increase the profile of the authors.

As part of the transition, the journal aims to publish special issues on a regular basis. In February 2014, a special issue on Computational Materials Discovery was published and attracted considerable attention and downloads. A virtual issue on Coordination Polymers was also published in July 2014. Two more special issues are in the pipeline: one on The Structural Chemistry of Homogeneous and Heterogeneous Catalysts and one on NMR Crystallography. The latter is a rapidly emerging field and its importance to structural scientists was recently recognized by the establishment of the IUCr Commission on NMR Crystallography and Related Methods during the 2014 IUCr Congress in Montreal. It is hoped that Section C can become a leading journal for the publication of results relating to this field.

raithbyIn conjunction with the new outlook of the journal, it was decided to add a second Main Editor to the editorial board. I am delighted to welcome Paul Raithby from the University of Bath, UK, as a Main Editor. Paul's reputation and network in the chemistry and crystallography communities make him an invaluable addition to the team and will bolster the journal's interface with the chemistry community.

We are excited to present two must-read articles in the January 2015 issue, which are of particular interest for the structural chemistry community. The first describes the new features in the 2014 version of the highly popular structure refinement program SHELXL [Sheldrick (2015). Acta Cryst. C71, 3-8; doi:10.1107/S2053229614024218]. The new features include improved restraint options, easier disorder handling, advances in absolute structure determination and inclusion of more comprehensive information in the generated CIF, to mention just a few. The advances in the program make it highly worthwhile for users to upgrade to the newest version and this journal strongly recommends its use for future work. The second article describes in detail the current implementation of the SQUEEZE procedure in the program PLATON for dealing with hard-to-model disordered species, usually solvent, in a structure [Spek (2015). Acta Cryst. C71, 9-18; doi:10.1107/S2053229614024929]. This technique has been available for many years, but has recently been modified to allow the contribution to the structure factors of the omitted disordered component to be included in Fcalc, rather than being subtracted from Iobs, thereby retaining the integrity of the original intensity data file. This has been made possible by modifications incorporated into the new version of SHELXL. Both papers are highly recommended reading.

Several Section C Co-editors retired at the IUCr Congress last August and it is my pleasure to record here my sincere appreciation of their dedication and efforts beyond the call of duty, their professionalism and their support of the journal. The retirees are: Ricardo Baggio, Mark Elsegood, George Ferguson, Ilia Guzei, Marcia Scudder and Amber Thompson.

I welcome Filipe Paz (University of Aveiro, Portugal), Chris Frampton (Brunel University, UK), Tong-Bu Lu (Sun Yat-Sen University, China), Noël Lugan (Laboratoire de Chimie de Coordination du CNRS, Toulouse, France) and Yoshiki Ohgo (Teikyo University, Japan) to the Co-editorial team and look forward to working closely with them over the coming years. All current Section C Co-editors and the Chester Editorial Office staff are warmly thanked for their ongoing dedicated support and contributions to the daily operations of the journal.

Anthony Linden
Editor, Acta Crystallographica Section C

This is an excerpt taken from the full editorial which can be found at http://journals.iucr.org/c/issues/2015/01/00/me0560/index.html

Posted 08 Jan 2015 


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Celebrating the past, looking to the future

[journal cover]It is with great excitement that we look forward to the New Year at Acta Crystallographica Section A: Foundations and Advances. As the International Year of Crystallography draws to a close, we were delighted to read in the Nature article The top 100 papers [Van Noorden, R., Maher, B. & Nuzzo, R. (2014). Nature, 514, 550-553] that crystallography was well represented. We were even more excited to find that a full seven of the top 100 articles were published in IUCr journals, and of these, four were in Acta A (in the order in which they appear in the list):

(1) A short history of SHELX by G. M. Sheldrick [Acta Cryst. (2008). A64, 112-122; doi:10.1107/S0108767307043930]

(2) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides by R. D. Shannon [Acta Cryst. (1976). A32, 751-767; doi:10.1107/S0567739476001551]

(3) Phase annealing in SHELX-90: direct methods for larger structures by G. M. Sheldrick [Acta Cryst. (1990). A46, 467-473; doi:10.1107/S0108767390000277] and

(4) Improved methods for building protein models in electron density maps and the location of errors in these models by T. A. Jones, J.-Y. Zou, S. W. Cowan and M. Kjeldgaard [Acta Cryst. (1991). A47, 110-119; doi:10.1107/S0108767390010224].

The 2008 paper by George Sheldrick is No. 13 on the top 100 list and is the highest-ranked paper published in the past two decades. As of the end of 2014, it has had over 39 000 citations. This speaks to the enduring scientific (and societal) impact of crystallography even into the 21st century, and to the important place that Acta A has in this rich and ongoing history. We are obviously delighted that in this January 2015 edition of Acta A we publish, as one of our three Advances papers, Professor Sheldrick's paper describing SHELXT, a new weapon in the crystallographer's arsenal and a new member of the SHELX suite of programs [Sheldrick, G. M. (2015). Acta Cryst. A71, 3-8; doi:10.1107/S2053273314026370]. We believe that, moving forward, the contributions of crystallographers (and nano-crystallographers and non-crystalline crystallographers) will continue to be pivotal and have broad scientific reach, and we believe that Acta A's wonderful tradition and position as the main foundational journal for advances in structure science will be maintained.

We would like to thank our editorial board, reviewers and authors for their hard work in 2014, the more so because striving for the highest standards, as we are, takes more care and application on everyone's part. We strongly believe that the hard work is worth it and that Acta A will go from strength to strength in 2015.

We are receiving increasing numbers of submissions, for which we thank you. The standards for being published as an Advance are high, and include the broad appeal and potential impact of the work as well as its scientific correctness, but we hope that these papers will be seen as the gold standard for exciting new developments in structure science, which is why we offer accelerated review and production for Advances articles along with promotion after they are published. We encourage you to submit your best work to us for consideration as an Advance.

As we strive to keep Acta A at the forefront of developments in crystallography and structure science, we are adopting higher standards for acceptance of articles across the board. For example, beyond just describing a development, each article should identify the rationale for making the development and how it would be used by the crystallographic community, and we look forward to collaborating with our authors to bring this about.

Finally, we would like to wish all of our authors, editors, reviewers and readers a wonderful, prosperous, productive and happy 2015.

S.J.L. Billinge and J. Miao
Editors, Acta Crystallographica Section A

This is an excerpt taken from the full editorial which can be found at http://journals.iucr.org/a/issues/2015/01/00/me0565/index.html

Posted 05 Jan 2015