The effect of high pressure on the crystal structures of -, -and -glycine has been investigated. A new polymorph, δ-glycine, is obtained from  glycine. δ-Glycine is monoclinic, P2 1 /a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) Å,  = 125.83(4)° at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of -glycine to a single crystal of δ-glycine, resulting in an equal number of NH…O hydrogen bonds, but an increase in the number and strength of CH…O hydrogen bonds, which act to close-up of 'holes' which are formed within the layers of -glycine in the centers of R-type hydrogen bonded motifs. Trigonal -glycine begins to undergo a transition to another high-pressure phase, -glycine, at 1.9 GPa, but the transformation is destructive; it is essentially complete at 4.3 GPa. The structure is monoclinic Pn, a = 4.8887(10), b = 5.7541(11), c = 5.4419(11) Å,  = 116.682(10)° at 4.3 GPa. The structure consists of layers similar those observed in -glycine with inter-layer separations of 2.38 and 3.38 Å and CH…O interactions formed between the layers. Monoclinic -glycine is known to be stable to 23 GPa, and we have obtained a single crystal structure of this polymorph at 6.2 GPa. Super-short NH…O hydrogen bonds are not formed up to 6.2 GPa, and they only shorten significantly if they are formed parallel to CH…O hydrogen bonds which strengthen, or vectors across holes which close-up, under pressure.

The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.

Neutron powder diffraction experiments on D(2)-loaded NOTT-112 reveal that the axial sites of exposed Cu(II) ions in the smallest cuboctahedral cages are the first, strongest binding sites for D(2) leading to an overall discrimination between the two types of exposed Cu(II) sites at the paddlewheel nodes. Thus, the Cu(II) centers within the cuboctahedral cage are the first sites of D(2) binding with a Cu-D(2) distance of 2.23(1) A.

Although CCD instruments are now widely used in single-crystal diffraction, they have not been employed so extensively in crystallographic studies at high pressure. This paper describes some practical experience in the application of one CCD instrument, the Bruker±Nonius SMART APEX (a ®xed-1 instrument). Centring a sample in a pressure cell is complicated by the restrictions on viewing the sample imposed by the body of the cell. The data collection strategy is de®ned by the requirements that (i) the incident beam must illuminate the sample and (ii) no more than 80% of the detector should be shaded by the body of the pressure cell. High-pressure diffraction images are contaminated by powder lines from the gasket and backing-disk materials, which form part of the pressure cell, and very intense spots from the diamond anvils. Procedures for the selection of spots for indexing are described. Integration routines attempt to harvest intensity data from regions of the detector that are shaded by the body of the pressure cell, and a procedure for generating dynamic masks is described. Shading also reduces the volume of reciprocal space that can be sampled, although this can be increased by performing data collections at more than one pressure-cell setting. Corrections for absorption are carried out in a two-stage procedure comprising an analytical correction for absorption by the cell, followed by a second multi-scan correction. Data sets collected at high pressure often contain some signi®cant outliers; these can be identi®ed during merging using a robust resistant weighting scheme, as described by Blessing [J.

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Grace under pressure: In the first high‐pressure crystallographic study on the metal–organic framework MOF‐5, increasing pressure initially results in the pressure‐transmitting fluid being squeezed into the pores. Further pressure increase causes a large reduction in pore content as solvent is evacuated from the pores, until a complete loss of crystallinity is observed at pressures above 3.2 GPa.

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The structural response of the nootropic drug piracetam (2-oxo-pyrrolidineacetamide) to both direct compression and high-pressure recrystallization from aqueous solution is reported. Crystals obtained by these methods have been characterized in situ by single-crystal X-ray diffraction. Compression of form II between pressures of 0.45-0.70 GPa caused a reversible, singlecrystal to single-crystal transition to give a new polymorph, form V. Crystallization from a dilute aqueous solution of piracetam at a pressure of 0.6 GPa via crystallization of high-pressure ice-VI resulted in the formation of a previously unreported dihydrate. The molecular packing arrangements of these new structures are compared with the known polymorphs and hydrates of piracetam. This study highlights how the systematic variation of pressure is a powerful method for the exploration of polymorphism and solvate formation and has the potential to add a further dimension to polymorph screening of pharmaceuticals.