Principles of NMR Techniques to Detect Molecular Reorientations

In C NMR spectroscopy, deviations from a Lorentzian lineshape, which is usually obtained in liquids, can be caused by a chemical shift anisotropy (CSA). If a CSA is present, the position of the resonance line depends on the relative orientation of the molecule with respect to the direction of the magnetic field applied (27,22). The superposition of the individual resonance lines results in typical lineshape patterns that can be described by two parameters the chemical shift anisotropy, AS, and the asymmetry parameter, Tj, respectively. In the case of an axially symmetric CSA tensor, i.e., 17 = 0, the relation between the resonance frequency, w, and the orientation of the molecule is given by 

In contrast to n-paraffins, which exhibit no or only a slight C NMR CSA, aromatics or hydrocarbons with double or triple bonds show a much larger anisotropy. Therefore, benzene (57) and 2-butyne 14) were chosen as suitable probe molecules to study molecular motions by C NMR lineshape analysis. Theoretical lineshapes for different motional states of benzene and 2-butyne molecules are depicted in Figs. 3 and 4. The proton-decoupled C NMR spectra were recorded by means of the homemade NMR spectrometer UDRIS (University of Leipzig) and a BRUKER MSL 400 (Central Institute of Physical Chemistry, Berlin) at frequencies of 22.6 and 100.6 MHz (9,14,57). 

The NMR lineshape of solids is determined by the quadrupolar interaction, which can be described by two parameters the quadrupole frequency, o Q, and the asymmetry parameter, t/ (19,20). The parameter q)q is determined by the electric quadrupole moment of the deuteron and the zz component of the electric field gradient at the deuteron site. For deuterons bonded to carbon atoms, the asymmetry parameter is approximately zero and the z axis is along the C—D bond. In this case, the dependence of the resonance frequency, m, from the orientation of the molecule with respect to the magnetic field applied is given by a relation similar to Eq. (18) (19). 

As long as the correlation time, Tc, of a molecular motion is sufficiently large, i.e.. 

Analysis of Molecular Translations and Rotations by Combined H NMR Relaxation and NMR Self-Diffusion Studies 

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