What Nuclear Spins Teach us about Mesoscopic Structures and their Change

The characterization of the structure of molecules and their changes in solid phases is a very important spectroscopic task, and necessary for understanding their physicochemical and biological functions. In the talk several examples of our recent work are presented. The first example is a dipolar solid state NMR study of the structure of methoxy carbonyl urea (MCU). MCU was discussed recently as a potential long time nitrogen fertilizer. We succeeded to determine the full configuration of polycrystalline methoxy carbonyl urea and to elucidate details of the crystal packing, employing Rotational Echo Double Resonance (REDOR) spectroscopy, employing differently isotope labeled samples. In the second example solid state NMR spectroscopy is employed for a study of the molecular conformation and arrangement of a fifteen residue fragment of the Alzheimer b-amyloid peptides. Here pH dependent changes in the molecular arrangement are found. In the case of a sample lyophilized at pH 7.4 sample an anti-parallel b-sheet arrangement of the molecular strands with an one residue shift is found. Dynamical properties of guest molecules in mesoporous silica are investigated with 2H-solid state and 1H-MAS NMR spectroscopy. First the rotational dynamics of Benzene-d6 confined in the low-diameter hexagonal ordered cylindrical pores of mesoporous silica with different diameters is studied in the temperature range between 236 K and 19 K and compared to bulk benzene-d6. At all temperatures below the freezing point the spectra of benzene in the silica show the co-existence of several states with temperature dependent intensity ratios. From the pore volume and the filling factor the number of molecular layers on the inner glassy surfaces is determined as ca. 3-molecular layers of benzene. Next the adsorption of water in mesoporous silica materials with different diameters is studied by 1H-MAS and static solid state NMR spectroscopy. Depending on the preparation technique and pore diameter interesting differences in the spectra are observed and discussed. At higher filling levels interesting differences between low- and high pore diameter silica are observed. This result is attributed to a different pore filling mechanism of the two silica materials. Finally the micro-phase behavior of a binary liquid in these silica materials is studied by a combination of various NMR techniques. The phase separation temperature of the mixture in the pore is slightly lower than in the bulk mixture of the same composition and is broadened over a temperature range. A qualitative model of the phase separation process in the pores is developed, which assumes a temperature dependent domain-like structure of the liquid below the phase transition temperature and a breakdown of these domains upon reaching the transition temperature. (author)