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400 MHz Fouriertransform-Festkörper-NMR-Spektrometer

Subject Area Molecular Chemistry
Term Funded in 2012
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 224896728
 
Final Report Year 2017

Final Report Abstract

The 600 and 400 NMR spectrometers, purchased simultaneously as part of a solid-state NMR key lab within the BZKG, were used conjointly for most of the works as they complement each other. A higher field and faster MAS (600 spectrometer) greatly increase the resolution for 1H spectra while a lower field (400 spectrometer) is preferred for nuclei with larger chemical shift anisotropy like 13C. Finally, some nuclei, and in particular quadrupolar nuclei, like 27Al will benefit of the comparison of spectra acquired on both fields in order to correctly assess the effect of the quadrupolar interaction. Many NMR experiments in this report were carried out by members of our group as part of cooperation with other groups, since these experiments often require a relatively advanced practical and theoretical knowledge of solid-state NMR. Our research group is interested in establishing and characterizing hierarchically structured materials formed by self-assembly of chemical building units on nano- and mesoscopic length scales. We strive for a detailed understanding of the underlying mechanism controlling self-organization and of relevant interactions at surfaces and interfaces as well as to develop analytical strategies based on a combination of solid-state NMR spectroscopy, diffraction and computational modelling. This enables us to determine structural and dynamical aspects even for highly disordered, semi-, nano- and microcrystalline as well as inhomogeneous systems. This approach is usually called NMR crystallography and some of our strategies are meanwhile established in its repertoire. In this way we derived structure-property relations for low-bandgap semiconductors based on graphitic carbon nitrides, which are studied for the photocatalytic decomposition of water into hydrogen and oxygen. Furthermore, NMR crystallographic strategies allowed us to study the self-aggregation mechanism of supramolecular additives in polymer/additive composites. The formation of nano-objects from these additives within the polymer melt upon cooling, alters the morphology of the crystalline polymer domains by epitaxial growth and thus controls the mechanical, electrical and optical polymer properties. An important part of our research focuses on introducing selective adsorption of small molecules in microporous materials to improve their potential for applications ranging from gas storage and separation up to drug delivery. In cooperation with B. Lotsch and N. Stock, we exploit the supramolecular key lock concept for postsynthetically functionalised MOFs and analyse the efficiency of multiple hydrogen bond systems for the adsorption at elevated temperatures. In the course of these activities, we investigated several MOFs. Exploiting 1D and 2D HETCOR high-resolution MAS spectra for 1H, 13C, 15N, 27Al and 14N on the model framework MIL53-X with the anchors X = H, NH2 and NHCHO, we were able to analyse the distribution of the functional groups within the framework, to follow the adsorption of guests like water, ethanol and acetone, to determine preferred binding sites as well as the dynamical properties of the guests and to study the interconnectivity of the pore structure during the breathing transformation of the framework. Indeed, in spite of a pronounced dynamical disorder of the guests, we could identify a preferred binding of the acetone via a NH…OC hydrogen bond for the NH2 and the NHCHO anchor groups by analyzing trends in the 13C isotropic chemical shifts. At the same time 1H−1H through-space connectivities reveal a close vicinity of the acetone methyl groups to the benzene rings of the linkers. In contrast, for ethanol and water, the interaction with the anchor groups is too weak to compete with the thermal disorder at room temperature. To be able to introduce larger anchor groups with up to three hydrogen bond donor/acceptor functions and to adsorb medium sized organic molecules we used the mesoporous MOF Al-MIL-101-NH2 which was postsynthetically modified with methyl, ethyl, phenyl and pyridyl isocyanate. In this process the amino groups are converted into urea units in high yields around 80-90%. For phenyl urea anchors, additionally, a remarkable increase in the water resistance was observed. Based on 1H-27Al D-HMQC NMR data and water sorption isotherms we could demonstrate, that the inorganic building blocks are shielded against water attack by the phenyl rings. In cooperation with N. Stock and D. Volkmer, the NMR was also used to help the structure determination for several MOFs. A combination of 1H, 13C, 15N and 71Ga, helped to characterize a new type of MOF, CFA-6, with a structure closely related to MIL-53 one. The structure of another MOF, Al-MIL-53-ADP, based on the flexible aliphatic single-chain linker molecule adipic acid and which exhibits a breathing behaviour solely upon dehydration/rehydration, was determined by a combination of force field based computations and Rietveld refinement, Density Functional Theory calculations and 1H, 13C and 27Al high-resolution NMR MAS 1D data for both the narrow pore and large pore conformations. A similar group of porous materials having a higher chemical stability than the MOFs, the porous polymers, are considered to be a promising material class for gas capture and sequestration, leading to the synthesis of a substantial number of individual networks with noteworthy sorption properties. The successful introduction of different functional groups and/or complete cross-linking was confirmed by 13C and 15N NMR. Additionally, 15N NMR measurements revealed protonation of the imine unit most likely due to the decomposition of DMSO with the produced acid leading to an ionic structure of the polymers with a sulfonic counter ion. The surface of materials was also investigated. For example, in cooperation with D. Andreeva, the effect of high power ultrasound on AlNi particles was studied using 27Al NMR. A quantitative analysis of the 27Al NMR spectra indicates a slight enrichment of Al and Al3Ni at the surfaces of the alloy particles compared to the bulk composition and also supported the hypothesis that Al3Ni transforms slowly in Al3Ni2 during sonication. Finally, many more materials have been characterized using the 400 spectrometer such as glass materials (27Al and 29Si NMR), iron spin crossover complexes (1H T1 measurement) and graphene oxide (13C NMR to quantify the oxidation).

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