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

Abstract

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