Relaxation-Assisted Separation of Overlapping Patterns in Ultra-Wideline NMR Spectra

Efficient acquisition of high-quality ultra-wideline (UW) solid-state NMR powder patterns in short experimental time frames is challenging. UW NMR powder patterns often possess inherently low signal-to-noise (S/N) and usually overlap for samples containing two or more magnetically distinct nuclides, which obscures spectral features and drastically lowers the spectral resolution. Currently, there is no reliable method for resolving overlapping powder patterns originating from unreceptive nuclei affected by large anisotropic NMR interactions. Herein, we discuss new methods for resolving individual UW NMR spectra associated with magnetically distinct nuclei by exploiting their different relaxation characteristics using 2D relaxation-assisted separation (RAS) experiments. These experiments use a non-negative Tikhonov fitting (NNTF) routine to process high-quality T₁ and T₂^eff relaxation data sets to produce high-resolution, 2D spin-relaxation correlation spectra for both spin-1/2 and quadrupolar nuclei in organic and organometallic solids under static (i.e., stationary) conditions. It is found that (i) T₂^eff RAS data sets can be acquired in a fraction of the time required for analogous T₁ RAS data sets, because a time-incremented 2D data set is not required for the former, and (ii) Tikhonov regularization is superior to conventional non-negative least-squares fitting, as it more reliably and robustly results in cleaner separation of patterns based on relaxation time constants.