Coherence transfer between spy nuclei and nitrogen-14 in solids

Coherence transfer from 'spy nuclei' such as 1H or 13C (S = 1/2) was used to excite single- or double-quantum coherences of 14N nuclei (I = 1) while the S spins were aligned along the static field, in the manner of heteronuclear single-quantum correlation (HSQC) spectroscopy. For samples spinning at the magic angle, coherence transfer can be achieved through a combination of scalar couplings J(I, S) and second-order quadrupole-dipole cross-terms, also known as residual dipolar splittings (RDS). The second-order quadrupolar powder patterns in the two-dimensional spectra allow one to determine the quadrupolar parameters of 14N in amino acids.