Separating an overlapped H peak and identifying its H-H correlations with the use of single-channel H solid-state NMR at fast MAS.

Fast magic-angle spinning (≥60 ​kHz) technique has enabled the acquisition of high-resolution H NMR spectra of solid materials. However, the spectral interpretation is still difficult because the H peaks are overlapped due to the narrow chemical shift range and broad linewidths. An additional C or N or H dimension possibly addresses the issues of overlapped proton resonances, but it leads to the elongated experimental time. Herein, we introduce a single-channel H experiment to separate the overlapped H peak and identify its spatially proximal H-H correlations. This sequence combines selective excitation, selective H-H polarization transfer by selective recoupling of protons (SERP), and broadband H recoupling by back-to-back (BABA) recoupling sequences. The concept for H separation is based on (i) the selective excitation of a well-resolved H peak and (ii) the selective dipolar polarization transfer from this isolated H peak to one of the H peaks in the overlapped/poor resolution region by SERP and (iii) the detection of H-H correlations from these two H peaks to other neighboring Hs by BABA. We demonstrated the applicability of this approach to identify overlapped peaks on two molecules, β-L-aspartyl-l-alanine and Pioglitazone.HCl. The sequence allows the clear observation of H-H correlations from an overlapped H peak without an additional heteronuclear dimension and ensures efficient polarization transfers that leads to twelve fold reduction in experimental time compared to N edited experiments. The limitation and the conditions of applicability for this approach are discussed in detail.