Comments on `Hydrogen bonds in crystalline D-alanine: diffraction and spectroscopic evidence for differences between enantiomers'

The recent paper by Belo, Pereira, Freire, Argyriou, Eckert & Bordallo [(2018), IUCrJ, 5, 6–12] reports observations that may lead one to think of very strong and visible consequences of the parity-violation energy difference between enantiomers of a molecule, namely alanine. If proved, this claim would have an enormous impact for research in structural chemistry. However, alternative, more realistic, explanations of their experiments have not been ruled out by the authors. Moreover, the theoretical calculations carried out to support the hypothesis are unable to differentiate between enantiomers (molecules or crystals). Therefore, the conclusions drawn by Belo et al. (2018) are deemed inappropriate as the data presented do not contain sufficient information to reach such a conclusion

Stacking disorder: the hexagonal polymorph of tris(bicyclo[2.1.1]hexeno)benzene and related examples

X-ray diffractograms of tris(bicyclo[2.1.1]hexeno)benzene, crystallized at the interface between a benzene solution and a layer of acetonitrile, show hexagonal symmetry and streaks of diffuse scattering along c*. The heavily faulted layer stacking is analyzed qualitatively and quantitatively in terms of a systematic protocol. This protocol requires partitioning the crystal structure into layers in such a way that pairs of adjacent layers may be stacked in different, but geometrically equivalent ways, which are dictated by the layer group symmetry. This approach is shown to provide a consistent alternative for analysis of a number of related cases provided the layers are defined on the basis of geometrical criteria rather than chemical intuitio

Novel Host-Guest Structures of 2,4,6- Tris (4-Halophenoxy)-1,3,5-Triazines(XPOT): Inclusion of C60 and Pyridine

The crystal structures of two halophenoxytriazine host-guest compounds are reported and discussed. They feature inclusion of C60 into cages of 2,4,6-tris(4-iodophenoxy)-1,3,5-triazine [IPOT, hexagonal, P63/m, a=16.367(2)Å, c=20.661(4)Å, V=4793.1(13)Å3, Z=2] and of pyridine6-clusters into cages of 2,4,6-tris(4-bromophenoxy)-1,3,5-triazine (BrPOT, rhombohedral, R $$\bar{3}$$ , a=15.5186(8)Å, c=39.521(3)Å, V=8242.7(8)Å3, Z=6). The stackings of the threefold symmetric layers of XPOT host molecules are different from each other and from those of all previously reported XPOT inclusion compounds (X: Cl, Br, I). Graphical Abstract: The new compounds IPOT3·C60 and BrPOT2·py3, represent new packing types in the family of threefold symmetric XPOT inclusion compounds (XPOT=2,4,6-tris(4-halophenoxy)-1,3,5-triazine; X=Cl, Br, I

Glycyl-L-alanine: A multi-temperature neutron study

Neutron diffraction data have been collected at 12, 50, 150 and 295 K for the dipeptide glycyl-L-alanine, C5H10N2O3, in order to obtain accurate positional and anisotropic displacement parameters for the H atoms. The values of these parameters serve as a benchmark for assessing the equivalent parameters obtained from a so-called Hirshfeld-atom refinement of X-ray diffraction data described elsewhere [Capelli et al. (2014). IUCrJ, 1, 361-379]. The flexibility of the glycyl-L-alanine mol­ecule in the solid and the hydrogen-bonding inter­actions as a function of temperature are also considered

Distinction of disorder, classical and quantum vibrational contributions to atomic mean-square amplitudes in dielectric pentachloronitrobenzene

The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its role in causing anomalous dielectric properties are investigated. Normal coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs) is employed to distinguish disorder contributions from classical and quantum-mechanical vibrational contributions. The analysis relies on multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational frequencies extracted from the temperature dependence of the ADPs are in good agreement with THz spectroscopic data. Aspects of the static disorder revealed by this work, primarily tilting and displacement of the molecules, are compared with corresponding results from previous, much more in-depth and time-consuming Monte Carlo simulations; their salient findings are reproduced by this work, demonstrating that the faster NCA approach provides reliable constraints for the interpretation of diffuse scattering. The dielectric properties of PCNB can thus be rationalized by an interpretation of the temperature-dependent ADPs in terms of thermal motion and molecular disorder. The use of atomic displacement parameters in the NCA approach is nonetheless hostage to reliable neutron data. The success of this study demonstrates that state-of-the-art single-crystal Laue neutron diffraction affords sufficiently fast the accurate data for this type of study. In general terms, the validation of this work opens up the field for numerous studies of solid-state molecular disorder in organic materials.Comment: Now published in Physical Review

NATIONAL QUALIFICATIONS SYSTEM IN RUSSIA – AN EPISTEMOLOGICAL PERSPECTIVE

Introduction. The article offers an insight into an epistemological perspective of establishing the national qualifications system in Russia that is critical for creating a “market of qualifications in the country that would be adequate to the current context.The aim of the research was to examine the context and factors underpinning the formation and evolution of the structure of knowledge relating to the national qualification system in Russia.Methodology and research methods. The methods applied comprise a mixed-method approach that included: review and analysis of official documents, semi-structured background and individual interviews with key stakeholders (line ministries, experts from industry and qualifications awarding bodies etc.) and desk research.Results and scientific novelty. The performed analysis of documents and of research data has revealed that the process of generating and enhancing knowledge about the national qualifications system semantically and methodologically is in broad terms contingent on the needs and objectives of social and economic development, and in narrower terms – on the development of occupational standards that underpin the standards of vocational and higher education, as well as on the enhanced institutionalisation of employers’ involvement in the training of personnel and in the search of effective mechanisms and instruments of independent assessment of qualifications.It also offers an insight into current key paradigmatic gaps in the NQF-related epistemology in Russia that account for the specificity of the observed state of play and for the key conceptual epistemological contradiction that has been identified by the authors as the opposition of and conflict between occupational and education qualifications. The authors propose ways out of the situation based on international practices in this area starting from aligning the terminology and conceptual approaches with international ones. The performed epistemological research and the conclusions made by the authors contribute to the national qualifications system discourse. It is stressed that core principles underpinning the development of the national qualifications system should be adapted to the social and economic and local factors.Practical significance. The material of the article can be used by policy-makers dealing with the NQS development and implementation of the NQS.Введение. В статье рассматривается эпистемологическая перспектива становления в России национальной системы квалификаций, необходимой для появления в стране отвечающего современным реалиям «рынка квалификаций». Цель публикации – обсуждение содержания, факторов формирования и эволюции знаний об аспектах функционирования национальной системы квалификаций в российских условиях. Методология и методики. В ходе работы применялись комплексный подход к объекту изучения, включающий такие методы исследования, как обзор и анализ официальных документов, полуофициальные опросы и индивидуальные интервью с представителями наиболее заинтересованных сторон – руководителями и сотрудниками отраслевых министерств, отраслевыми экспертами, специалистами структур по присуждению квалификаций и т. д.Результаты и научная новизна. Проведенный анализ документальных и научных источников показал, что процесс генерирования и совершенствования знаний о системе национальных квалификаций семантически и методологически связан в широком понимании с потребностями и целями социально-экономического развития; в более узком – с разработкой профессиональных стандартов, которые должны служить основой стандартов профессионального и высшего образования, а также с усилением институционализации участия работодателей в подготовке кадров и поиске эффективных механизмов и инструментария независимой оценки квалификаций и компетенций. Вскрыты имеющиеся в настоящее время парадигматические пробелы в системе знаний о национальной рамке квалификаций, порождающие проблемы дальнейшего развития данного направления в России. В качестве узлового концептуального эпистемологического противоречия выделено неправомерное противопоставление профессиональных и образовательных квалификаций. С опорой на зарубежный практический опыт предложены пути выхода из сложившейся ситуации, в частности, рекомендуется прежде всего согласовать терминологию и концептуальные подходы с международными нормами. Предпринятое в контексте эпистемологии исследование и сделанные авторами выводы расширяют дискурс о национальной системе квалификаций; подчеркивается, что при соблюдении общих принципов разработки национальной системы квалификаций важно учитывать специфические внутригосударственные социально-экономические и территориальные факторы. Практическая значимость. Материалы статьи могут быть использованы в практике отечественных специалистов, ответственных за создание инфраструктуры и внедрение национальной системы квалификаций и формирующих политику в этой области

Disorder and Motion in Crystal Structures: Nuisance and Opportunities

Conventional structure analysis averages the structure of an entire crystal into a single unit cell. If there is disorder and thermal motion, this averaging obscures information on local structure, intermolecular interactions and molecular dynamics. Nonetheless structural detail at the molecular and supramolecular level can be retrieved even for heavily disordered molecular materials by analyzing their diffuse scattering. Molecular dynamics can be elucidated and distinguished from disorder on the basis of atomic displacement parameters (ADPs) determined over a range of temperatures from conventional structure analyses. Such studies are now becoming feasible for molecular crystals through improved experimental techniques, faster computers and new algorithms

Quantum crystallography

Approximate wavefunctions can be improved by constraining them to reproduce observations derived from diffraction and scattering experiments. Conversely, charge density models, incorporating electron-density distributions, atomic positions and atomic motion, can be improved by supplementing diffraction experiments with quantum chemically calculated, tailor-made electron densities (form factors). In both cases quantum chemistry and diffraction/scattering experiments are combined into a single, integrated tool. The development of quantum crystallographic research is reviewed. Some results obtained by quantum crystallography illustrate the potential and limitations of this field