Combining Fast 2D NMR Methods and Oriented Media

Abstract

NMR in oriented samples (anisotropic solvents) offers access to informative residual order-dependent NMR interactions, including chemical shift anisotropies (RCSAs), dipolar couplings (RDCs), and quadrupolar couplings (RQCs), while preserving high spectral resolution. The analysis of these spectra often requires two-dimensional (2D) NMR experiments due to these additional anisotropic interactions. In addition, accelerated 2D NMR experiments can be essential for particular investigations, including in situ monitoring of chemical reactions or the observation of dilute isotopes, e.g. 2H and 13C, since the stability of oriented phases is limited in time and the magnetic field, B0, can drift in the absence of 2H lock. We provide here an overview of these fast 2D NMR experiments in anisotropic media. These developments have benefited from those for isotropic solutions, such as fast pulsing techniques enhancing the polarization, sparse sampling, Hadamard spectroscopy or spatial encoding. Nevertheless, fast 2D NMR in mesophases and solids present particular challenges, since these 2D spectra usually display lower signal-to-noise ratios, non-Lorentzian lineshapes, lower spectral resolution and wider spectral widths than their counterparts for isotropic solutions. We discuss here the advantages and limitations of various approaches, which have been applied for the fast 2D NMR spectra of mesophases and solids.