Indirect detection of nitrogen-14 in solid-state NMR spectroscopy

NMR spectra of N-14 (spin 1 = 1) ore obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring "spy" nucleus with S = 1/2, such as C-13 or H-1, to single- or double-quantum transitions of IN nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter ore also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolor coupling constant Co (typically a few megahertz), on the asymmetry parameter il, of the N-14 nucleus, and on the orientation of the internuclear vector ris between the / (N-14) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional overoging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in C-13 or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.