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In this paper, 2H nuclear magnetic resonance (NMR) was performed to study the motions of the embedded cations in the cation-doped lead bromide perovskite (MA0.6DMA0.4PbBr3). By selectively deuterating dimethylamine (DMA) and methylammonium (MA) cations, we achieved selective NMR observations of the different embedded cations in the materials. The results show that at low temperatures, both the DMA and MA cations in the material have molecular motions close to the double rotation model. As the temperature increases, the motion freedom of the DMA and MA cations increases, and the motions gradually transform into fast isotropic motions. It was observed that the DMA cations have higher mobility than the MA cations in the same materials at the same temperature, indicating that there is a significant inhomogeneity in the states of cations in the material. Based on the study of the cation motions, the molecular mechanism of the phase transition of the material was discussed.

In this work, the motions of methylammonium cations (MA) in different phase structures of MAPbCl3 perovskites, including orthorhombic, tetragonal and cubic phases, were studied by solid-state 2H nuclear magnetic resonance (NMR). By a combination of experimental 2H NMR and numerical simulations, the motion models in different phase structures of MAPbCl3 were established in this work. The NMR results disclosed the characteristics of the motions of MA in different phase structures. In the orthorhombic phase, the MA cations are not completely frozen but wobble locally while rapidly rotating around the C-N bond. As the temperature increases, MA cations show various modes of motion in the perovskite lattice. The NMR results also showed that the changes of the motions of MA in different temperatures are not accompanied by the changes of phase structures. The observations in this work will deepen our understanding of the molecular mechanism of the MAPbCl3 phase transitions.

Molecular rotors have earned substantial popularity in recent times, owing to the unique dependence of its crystalline properties on the rotational dynamics of the rotor. We have recently reported the synthesis and crystal structure of a phenylene-bridged macrocage as a gyroscope-like molecule in the crystalline state. The dynamics of the phenylene moiety was probed by solid-state 13C CP/MAS proton dipolar dephasing NMR spectroscopy. Herein, solid-state 2H NMR studies were performed to study the dynamics of the gyroscope-like molecule with a deuterated rotor in the crystalline state. A spectrum with a narrow line shape was obtained at 300 K. The facile exchange among three stationary states, which was observed by X-ray crystallography, was clearly confirmed. Additionally, a crystal-to-crystal phase transition that switches the motion of the rotor was observed in the DSC analysis of the powdered sample.

Abstract: Hydrophobic mismatch still represents a puzzle for transmembrane peptides, despite the apparent simplicity of this concept and its demonstrated validity in natural membranes. Using a wealth of available experimental 2 H NMR data, we provide here a comprehensive explanation of the orientation and dynamics of model peptides in lipid bilayers, which shows how they can adapt to membranes of different thickness. The orientational adjustment of transmembrane α-helices can be understood as the result of a competition between the thermodynamically unfavorable lipid repacking associated with peptide tilting and the optimization of peptide/membrane hydrophobic coupling. In the positive mismatch regime (long-peptide/thin-membrane) the helices adapt mainly via changing their tilt angle, as expected from simple geometrical predictions. However, the adaptation mechanism varies with the peptide sequence in the flanking regions, suggesting additional effects that modulate hydrophobic coupling. These originate from re-adjustments of the peptide hydrophobic length and they depend on the hydrophobicity of the flanking region, the strength of interfacial anchoring, the structural flexibility of anchoring side-chains and the presence of alternative anchoring residues. [Copyright &y& Elsevier]

Abstract: We investigated electron spin densities of pyrazolato-bridged complexes [Cu(pz)2] n (1) and [Cu2(pz)2(NO3)(H2O)(phen)2]NO3 (2) (Hpz=pyrazole, phen=1,10-phenanthroline) using solid-state high-resolution NMR to elucidate the magnetic interaction paths with the help of molecular orbital theory. We prepared deuterated analogue of these complexes, 1-d 6 and 2-d 6, to measure temperature dependence of 2H and 13C NMR shifts between 190 and 350K. The hyperfine coupling constants (HFCCs) and electron spin densities were determined from the slopes of the shifts as a function of the magnetic susceptibilities. The derived spin densities were all positive, which indicates the dominant magnetic interaction paths of these complexes are not π but σ orbitals of the pyrazolate ligand. The NMR results reasonably agreed with those of density functional theory (DFT) calculations for molecular models of 1 and 2. [Copyright &y& Elsevier]