| Nitrogen is an important element in all areas of chemistry, biology, and materials science. However, it is challenging to probe directly with solid-state nuclear magnetic resonance (SSNMR). The commonly studied isotope, 15N (nuclear spin, I, = ½), has a low natural abundance (0.36%); therefore, 15N NMR experiments often require isotopically enriched samples. 14N (I = 1) has a high natural abundance (99.64%) but is a quadrupolar nucleus. 14N SSNMR spectra are generally very broad due to the moderate quadrupole moment of 14N. Ultra-wideline (UW) 14N SSNMR spectra have very low signal-to-noise, which complicates their acquisition; however, they are useful probes of molecular-level structure and dynamics, and do not require isotopic enrichment for their acquisition.;This dissertation focuses on developing and applying direct-detection techniques for the acquisition of UW 14N SSNMR powder patterns. These techniques have been studied from both empirical and theoretical points of view and are facile to implement on modern NMR consoles.;First, we present a 14N SSNMR study of sp 3-hybridized nitrogen moieties in amino acids and related derivatives. 14N NMR powder patterns were acquired in relatively short experimental times using moderate field strengths (e.g., 9.4 T), and from these, the 14N quadrupolar parameters were determined. The 14N electric field gradient (EFG) tensors are extremely sensitive to intermolecular hydrogen-bonding, and in conjunction with plane-wave density functional theory calculations, correlations are found between these tensors and nitrogen structural environments. Second, 14N SSNMR was applied for the differentiation of polymorphs of glycine and its HCl salt. We also investigated the rotational dynamics of NH3 groups, finding that the 14N transverse relaxation is dependent upon motional variation with temperature and can be exploited to reduce the experimental times of CPMG-type 14N NMR experiments. Third, we describe improvements in the efficiency of 14N UW SSNMR experiments by utilizing broadband cross-polarization methods, improving the parameterization and execution of WURST pulses, and strategically considering the appearances of 14N powder patterns. Finally, we present a preliminary investigation of the tandem use of directly-detected 14N and indirectly-detected 15N SSNMR for the characterization of active pharmaceutical ingredients and their polymorphs, without isotopic enrichment. |
