Spinor double-quantum excitation in the solution NMR of near-equivalent spin-1/2 pairs.

A family of double-quantum excitation schemes is described for the solution nuclear magnetic resonance (NMR) of near-equivalent spin-1/2 pairs. These new methods exploit the spinor behavior of two-level systems, whose signature is the change of sign of a quantum state upon a 2π rotation. The spinor behavior is used to manipulate the phases of single-quantum coherences to prepare a double-quantum precursor state, which is rapidly converted into double-quantum coherence by a straightforward π/2 rotation. One set of spinor-based methods exploits symmetry-based pulse sequences, while the other set exploits SLIC (spin-lock-induced crossing), in which the nutation frequency under a resonant radiofrequency field is matched to the spin-spin coupling. A variant of SLIC is introduced that is well-compensated for deviations in the radiofrequency field amplitude. The methods are demonstrated by performing double-quantum-filtered F19 NMR on a molecular system containing a pair of diastereotopic F19 nuclei. The new methods are compared with existing techniques.