Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy

The electrical conductivity of aluminium doped zinc oxide (AZO, ZnO:Al) materials depends on doping induced defects and grain structure. This study aims at relating macroscopic electrical conductivity of AZO nanoparticles with their atomic structure, which is non-trivial because the derived materials are heavily disordered and heterogeneous in nature. For this purpose we synthesized AZO nanoparticles with different doping levels and narrow size distribution by a microwave assisted polyol method followed by drying and a reductive treatment with forming gas. From these particles electrically conductive, optically transparent films were obtained by spin-coating. Characterization involved energy-dispersive X-ray analysis, wet chemical analysis, X-ray diffraction, electron microscopy and dynamic light scattering, which provided a basis for a detailed structural solid-state NMR study. A multinuclear (Al-27, C-13, H-1) spectroscopic investigation required a number of 1D MAS NMR and 2D MAS NMR techniques (T-1-measurements, Al-27-MQMAS, Al-27-H-1 2D-PRESTO-III heteronuclear correlation spectroscopy), which were corroborated by quantum chemical calculations with an embedded cluster method (EEIM) at the DFT level. From the combined data we conclude that only a small part of the provided Al is incorporated into the ZnO structure by substitution of Zn. The related Al-27 NMR signal undergoes a Knight shift when the material is subjected to a reductive treatment with forming gas. At higher (formal) doping levels Al forms insulating (Al, H and C containing) side-phases, which cover the surface of the ZnO:Al particles and increase the sheet resistivity of spin-coated material. Moreover, calculated Al-27 quadrupole coupling constants serve as a spectroscopic fingerprint by which previously suggested point-defects can be identified and in their great majority be ruled out

N-o-Vanillylidene-L-histidine: Experimental Charge Density Analysis of a Double Zwitterionic Amino Acid Schiff-Base Compound

A double zwitterionic Schiff base was synthesized using the amino acid L-histidine and o-vanillin (2-hydroxy-3-methoxy benzaldehyde). Both the phenol and carboxyl groups are deprotonated, and the imine nitrogen atom and histidine-imidazole ring are protonated to give the double zwitterion with an intramolecular (imine)N-H(+)center dot center dot center dot(-)O(phenol) hydrogen bond (ketoamine form). Such a ketoamine form in a double zwitterion is assumed in the catalytic cycle of enzymatic transformations of amino acids with the cofactor (vitamin B6) pyridoxal-5-phosphate (PLP). A high-resolution, low-temperature, single-crystal X-ray diffraction data set on N-o-vanillylidene-l-histidine (also named 3-methoxysalicylidene-l-histidine or N-(2-oxy-3-methoxy-benzylidene)-L-histidine, OVHIS) is used in the analysis of molecular electrostatic properties and intermolecular interactions. All oxygen atoms in the molecule (four in total) are mutually almost coplanar and located (externally) on the same side of molecule. These four O atoms carry significant negative charge and form a large area of strong negative electrostatic potential (appropriate for bonding to a metal atom). The protonated and, thus, positively charged imidazole ring is situated on the opposite side of the molecule from the area of the O atoms. Consequently, the OVHIS molecule is highly polarized and has a very high molecular dipole moment of 42.4 D in the solid state (calculated from experimental X-ray data). Two strong intermolecular charge-assisted N-H(+)center dot center dot center dot(-)O hydrogen bonds (with H center dot center dot center dot O distances of 1.61 angstrom) together with other D-H center dot center dot center dot O interactions (D = N, C) contribute to a large molecular dipole enhancement which occurs upon crystallization. The topologies of the bonding within the molecule as well as its hydrogen bonds have been investigated according to Baders quantum theory of atoms in molecules (QTAIM). (1)H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) was used to confirm the zwitterionic structure in the solid state

Constant Volume Gate-Opening by Freezing Rotational Dynamics in Microporous Organically Pillared Layered Silicates

Microporous organically pillared layered silicates (MOPS) are a class of microporous hybrid materials that, by varying pillar density, allows for optimization of guest recognition without the need to explore different framework topologies. MOPS are found to be capable of discriminating two very similar gases, carbon dioxide and acetylene, by selective gate-opening solely through quenching pillar dynamics. Contrary to conventional gate-opening in metal organic frameworks, the additional adsorption capacity is realized without macroscopic volume changes, thus avoiding mechanical stress on the framework. Of the two gases studied, only CO₂ can accomplish freezing of pillar dynamics. Moreover, the shape of the slit-type micropores in MOPS can easily be fine-tuned by reducing the charge density of the silicate layers. This concomitantly reduces the Coulomb attraction of cationic interlayer space and anionic host layers. Surprisingly, we found that reducing the charge density then alters the gate-opening mechanism to a conventional structural gate-opening involving an increase in volume