A zeolitic imidazolate framework with conformational variety: Conformational polymorphs versus frameworks with static conformational disorder

We show via structural considerations and DFT calculations that for a zeolitic imidazolate framework (ZIF) with sodalite (SOD) topology, [Zn(dcim)2]-SOD (dcim = 4,5-dichloroimidazolate), structural models of an infinite number of hypothetical conformational polymorphs with distinct linker orientations can be generated, which can be interconverted most likely only via reconstructive structural transitions. The relative total energies suggest that some of those polymorphs might be synthetically accessible. Efforts in that direction led to the synthesis of new trigonal 1 and previously known cubic 2 with improved crystallinity. According to structural analyses based on powder X-ray diffraction (PXRD) methods supported by NMR spectroscopy, 1 is the most stable of the theoretically predicted SOD-type framework conformers (isostructural to ZIF-7), whereas 2, at variance to a recent proposal, is a SOD-type material with a high degree of orientational disorder of the dcim linker units. The statistics of the linker orientations in 2 is close to that in 1, indicating that the disorder in 2 is not random. Rather crystals of 2 are likely twins consisting of nanoscopic domains of trigonal 1 that are deformed to a cubic metric, with linker disorder being located in the domain interfaces. As structural differences appear to be more related to characteristics of the real as opposed to the ideal crystal structures, we propose to not consider 1 and 2 as true conformational polymorphs. Systematic investigations of solvent mixtures led to the discovery of intermediate materials of 1 and 2. The PXRD patterns and SEM images indicate that they belong to a complete series of structural intermediates. Differences in the Ar adsorption/desorption behaviours reveal that 1, in contrast to 2, is a flexible ZIF framework

Description of the intermediate length scale structural motifs in sodium vanado-phosphate glasses by magnetic resonance spectroscopies

For the first time, the local and medium range orders in sodium vanado-phosphate glasses have been investigated by advanced magnetic resonance spectroscopy methods. One- and two-dimensional 31P/51V magic angle spinning nuclear magnetic resonance techniques (31P(51V) REAPDOR and 51V(31P) D-HMQC) have been used to monitor the formation of P-O-V5+ bonds and to provide the first accurate description of the intermediate length scale structural motifs in these glasses. The structural model has been completed by the investigation of the chemical environment of the V4+ ions (produced through the partial reduction of V5+ during the melting stage of the glass preparation) using standard continuous wave and advanced pulsed electron paramagnetic resonance techniques (HYSCORE). Finally, the combination of both sets of data leads to the first complete and precise structural model of the alkali vanado-phosphate glass system