Symmetry pathways in solid-state NMR

Two-dimensional NMR measurement and point dipole model prediction of paramagnetic shift tensors in solids

“…A problem with these two-pulse sequences [67] on a quadrupolar nucleus, however, is that it is not possible to acquire the signals associated with p I and d I symmetry and anti-symmetry pathways, a necessary requirement for obtaining pure 2D absorption mode lineshapes [33,[68][69][70].…”

“…To simplify analyses and discussions we adopt the symmetry pathway approach to describe the details of NMR signal phase evolution [33]. This framework extends the concept of coherence transfer pathways, defining two more fundamental pathways called the spatial pathway and the spin transition pathway to completely describe an NMR experiment.…”

“…Doing so requires the p I = −2 → −1 anti-path be collected in addition to the p I = +2 → −1 path in order to sample adjacent 2D signal quadrants in t 1 [33]. In practice, however, the resulting spectrum will still be distorted because of the significant loss of signal during receiver dead time.…”

“…There have been attempts to apply similar 2D sequences on rotating samples [27] to improve resolution while retaining anisotropy information, but these sequences fail to provide a separation of the shift and quadrupolar frequency contribution onto orthogonal dimensions. Recently, we have proposed a simplified picture [99] for sideband separation experiments and efforts are underway to use the symmetry pathway approach [33] outlined here to extend these MAS approaches to separating and correlating shift and quadrupolar anisotropies.…”

“…First, we describe two-pulse strategies for generating echoes in terms of the recently proposed symmetry pathway formalism [33], a framework that provides a simple picture for understanding many correlation experiments, a framework that provides a simple picture for understanding many correlation experiments. We then proceed to analyze the sequence of Antonijevic and Wimperis [32] in this context, from which we propose a modification that provides a two-fold sensitivity enhancement and eliminates a substantial spectral artifact.…”