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 omegaR- or the 2omegaR-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. Sideband 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).
Fischbach
Ingrid
I
Max-Planck-Institut für Polymerforschung, Postfach 3148 55021 Mainz, Germany.
Ebert
Florian
F
Spiess
Hans Wolfgang
HW
Schnell
Ingo
I
D000465:Algorithms; D016880:Anisotropy; D002247:Carbon Isotopes; D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D015394:Molecular Structure; D020959:Polyethylene / Q000737:chemistry; D011789:Quantum Theory; D013113:Spin Labels; D014961:X-Ray Diffraction
D002247:Carbon Isotopes; D013113:Spin Labels; D020959:Polyethylene