Rotor modulations and recoupling strategies in 13C solid-state magic-angle-spinning NMR spectroscopy:: Probing molecular orientation and dynamics

Recoupling strategies for anisotropic interactions enable the investigation of molecular structure, order and dynamics in a sensitive and site-specific fashion by solid-state NMR spectroscopy. Whereas magic-angle spinning (MAS) efficiently averages anisotropic interactions and enhances spectral resolution, recoupling pulse sequences selectively restore certain parts of rotor-modulated dipole-dipole couplings or chemical shift anisotropies (CSA). More specifically, it is possible to recouple either the omega(R)- or the 2omega(R)-modulated terms of an interaction Hamiltonian, which exhibit different orientation dependencies and, in this way, provide a means of distinguishing whether the observed NMR spectra are affected by molecular motion or by molecular orientation. Side-band patterns generated by reconversion rotor encoding allow for a precise and selective determination of coupling constants and anisotropies, which contain site-specific information on structure, orientation and/or dynamics of individual molecular segments. Corresponding recoupling schemes are presented in a common context, and the possibilities of exploiting these effects for the determination of order parameters of oriented materials, such as oriented polymer chains or extruded fibres of a discotic mesogen, are discussed. The obtained orientational order parameters are compared to results from two-dimensional wide-angle X-ray scattering (WAXS).