Two-Dimensional Nuclear Magnetic Resonance: Exploiting Spin Echoes To Maximize Information Content by Suppression of Diagonal Peaks in Homonuclear Experiments

Abstract

Two-dimensional nuclear magnetic resonance (2D NMR) correlation spectra help visualize inter- or intra-molecular spin connectivity through space or through bonds. This is accomplished by magnetization transfer between interacting (“connected”) spins located at different sites in molecules. In homonuclear 2D experiments, cross peaks which demonstrate spin connectivity and result from magnetization transfer between sites are unfortunately invariably accompanied by other peaks that result from magnetization that has not undergone any transfer, viz., diagonal peaks. The latter can often mask close-lying cross peaks. We report here the general principles that constitute a design strategy for diagonal suppression, relying on echo formation. Next, a novel experiment that effects diagonal suppression in the high-resolution mode is demonstrated. Pure phase capability is also introduced. Examples from both 2D exchange and high-resolution 2D correlation spectroscopy are included, and the proposed method is compared with other established as well as recent attempts to accomplish diagonal suppression.