Dynamic hydrogen disorder in yttrium-dihydride phases as seen by variable-temperature 89Y CP MAS NMR.

Variable-temperature solid-state MAS NMR studies on some yttrium-dihydride phases YH2+x are reported and yield evidence that 89Y CP MAS NMR techniques are an experimentally feasible route to investigate order-disorder phenomena in such metal-hydride phases.

Supercontracted spider dragline silk: a solid-state NMR study of the local structure.

The local structure of supercontracted dragline silk from the spider Nephila madagascariensis was investigated by solid-state nuclear magnetic resonance. Two-dimensional (2D) spin-diffusion experiments did not show any significant conformational changes in short-range order (and the secondary structure of the protein) upon supercontraction. Our results are in accordance with the proposal by Vollrath et al. (Proc R Soc London B 1996;263:147-151) that urea-supercontraction does not alter the local structure of spider dragline silk fundamentally. However, significant differences in the dynamics of the polypeptide chain upon supercontraction are detected at room temperature. At low temperature, these dynamics are frozen out. In addition, the role of the solvent (water) in the silk is investigated in Nephila edulis. Mobile water is detected at temperatures significantly below the freezing point of bulk water.

Order-disorder phenomena determined by high-resolution powder diffraction: the structures of tetrakis(trimethylsilyl)methane C

The compounds tetrakis(trimethylsilyl)methane C[Si(CH(3))(3)](4) (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH(3))(3)](4) (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm{\bar 3}m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) Å(3) for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) Å(3) for TC. X-ray scattering data can be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at T(c) = 225 K, as is characterized by a jump of the lattice parameter of Deltaa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with DeltaH = 11.7 kJ mol(-1) and DeltaS = 50.0 J mol(-1) K(-1). The structure of the low-temperature phase is refined against X-ray powder data measured at 200 K. It has space group P2(1)3 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å(3). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as is shown by analyzing distances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at T(c1) = 268 K, with Deltaa(1) = 0.065 Å, DeltaH(1) = 3.63 kJ mol(-1) and DeltaS(1) = 13.0 J mol(-1) K(-1). A second first-order phase transition occurs at T(c2) = 225 K, characterized by Deltaa(2) = 0.073 Å, DeltaH(2) = 6.9 kJ mol(-1) and DeltaS(2) = 30.0 J mol(-1) K(-1). The phase transition at higher temperature has not been reported previously. New NMR experiments show a small anomaly in the temperature dependence of the peak positions in NMR to occur at T(c2). Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P2(1)3, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa{\bar 3}, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

Solid Me3Sn(O2CH): molecular and spin dynamics as viewed by 1H, 13C and 119Sn nuclear magnetic resonance spectroscopy.

Because of favourable dynamic properties in the solid state, crystalline trimethyltin formate, Me3Sn(O2CH), 1, offers quite unique possibilities to probe questions related to molecular dynamics (2 pi/3 Me3Sn reorientation) and to structural aspects (rigidity of the polymeric backbone, effectively isolated 1H-13C spin pair in the bidentate chain-building formate anion) by 1H, 13C and 119Sn high-resolution solid-state nuclear magnetic resonance (NMR) methods. The combined use of various one- (1D) and two-dimensional (2D) variable-temperature NMR experiments, including consideration of the short-time-cross-polarization (CP) behaviour of the 1H-13C spin pair in 13C-labelled Me3Sn(O2*CH), 1*, yields a fairly comprehensive description of the solid-state structure of 1, also in the absence of the single-crystal X-ray structure determination of 1. Taking advantage of the anisotropic polarization transfer within the formate 1H-13C spin pair allows to relate the orientation of the 1H-13C dipolar interaction, i.e. the C-H bond direction to the principal axes system of the 13C shielding tensor of the formate 13C from static 13C powder patterns of 1*.

A versatile method for rotation-synchronised 2D exchange spectroscopy of solids.

A versatile, reliable, easy-to-use method to achieve rotor-synchronisation during the mixing time tau m in 2D magnetisation transfer experiments on rotating solids is shown. The limits of accuracy of the method are discussed and some examples of application are given. A full description of all technical details is provided.