Chemical Crystallography at the Advanced Light Source

High-pressure polymorphism in pyridine

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z' = 1 and phase III in P41212 with Z' = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

The development and exploitation of synchrotron single-crystal diffraction for chemistry and materials

A historical account is given of the 25-year development of dedicated synchrotron beamlines for single-crystal diffraction as applied to the so-called small-molecule fields of chemistry and materials science. Designs have drawn on previous successful models in macromolecular crystallography, with appropriate modifications in view of the different properties and behaviour of the respective sample types. Key factors in making these facilities attractive and productive for users include familiarity of operational procedures and the availability of experimental techniques and features normally found in local chemical crystallography laboratories, especially for the handling of samples and processing of diffraction data. Beamlines dedicated to single-crystal diffraction rather than shared with other techniques can be optimized for effective and efficient use. The experience gained from the original design, development and exploitation of stations 9.8 and 16.2SMX at the Daresbury Laboratory Synchrotron Radiation Source and beamline 11.3.1 at the Advanced Light Source have led to highly productive current facilities at ALS beamline 12.2.1 and Diamond Light Source beamline I19, including the recent introduction of remote-access operation. Such facilities have generated and continue to provide major impact in academic and commercial research that could otherwise not be achieved, including patents and applications in pharmaceuticals, energy and gas storage systems, and government policy. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

High-pressure polymorphism in l-threonine between ambient pressure and 22 GPa

The amino acid l-threonine undergoes three phase transitions between ambient pressure and 22.3 GPa which modify both hydrogen bonding and the molecular conformation.