The nuclear magnetic resonance (NMR) WEBLAB is described, providing an interactive tool for analyzing NMR spectra of solids and anisotropic liquids to elucidate their molecular dynamics. It is a collection of programs freely available over the Internet at http://www.mpip-mainz.mpg.de/weblab4O/ that permit the interpretation of one-dimensional NMR spectra in the case of motions occurring on a cone and of two-dimensional exchange NMR data for discrete jumps. The programs are described in detail and analytical formulae for the averaged interaction tensor components that govern the spectra in the fast motional limit are derived in an appendix. To facilitate the validation of the results obtained from the WEBLAB, we present examples for two-and three-site jumps and demonstrate how WEBLAB can be utilized for analyzing more complex motions around more than one axis as met, e.g., in poly(diethysiloxane), where experimental data are available.
Aromatic disk-shaped molecules tend to self-organize into a herringbone packing where the disks are inclined at angles ±θ with respect to the axis of the column. In discotic liquid crystals this can introduce defects between stacks of limited length. In a C(3)-symmetric hexa-peri-hexabenzocoronene, solid-state NMR, x-ray scattering, and rheology identifies such a packing with θ=43° and stacks of about seven disks. Disordered regions containing defects fill the space in between the ordered stacks. Biaxial intra- and intercolumnar dynamics differing by eight decades are identified.
Molecular motions in low molar mass (46200 g mol-') poly(diethylsi1oxane) (PDES) are studied by 2H NMR. From DSC measurements the existence of the crystalline phases a,, B, and a2, /I2, as well as a pronounced mesogenic phase a, , is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25". The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70" within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main chain axis.
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