Separating an overlapped 1H peak and identifying its 1H-1H correlations with the use of single-channel 1H solid-state NMR at fast MAS

Fast magic-angle spinning (> 60 kHz) technique has enabled the acquisition of high-resolution H-1 NMR spectra of solid materials. However, the spectral interpretation is still difficult because the H-1 peaks are overlapped due to the narrow chemical shift range and broad linewidths. An additional C-13 or N-14 or H-1 dimension possibly addresses the issues of overlapped proton resonances, but it leads to the elongated experimental time. Herein, we introduce a single-channel H-1 experiment to separate the overlapped H-1 peak and identify its spatially proximal H-1-H-1 correlations. This sequence combines selective excitation, selective H-1-H-1 polarization transfer by selective recoupling of protons (SERP), and broadband H-1 recoupling by back-to-back (BABA) recoupling sequences. The concept for H-1 separation is based on (i) the selective excitation of a well-resolved H-1 peak and (ii) the selective dipolar polarization transfer from this isolated H-1 peak to one of the H-1 peaks in the overlapped/ poor resolution region by SERP and (iii) the detection of H-1-H-1 correlations from these two H-1 peaks to other neighboring (1)Hs by BABA. We demonstrated the applicability of this approach to identify overlapped peaks on two molecules, beta-L-aspartyl-L-alanine and Pioglitazone.HCl. The sequence allows the clear observation of H-1-H-1 correlations from an overlapped H-1 peak without an additional heteronuclear dimension and ensures efficient polarization transfers that leads to twelve fold reduction in experimental time compared to N-14 edited experiments. The limitation and the conditions of applicability for this approach are discussed in detail.