Steady-state free precession NMR in the presence of heteronuclear couplings and decoupling: More than meets the eye

Fourier Transform (FT) has been a mainstay of analytical 13C/15N NMR. On the other hand, it has been shown that Steady-State Free Precession (SSFP) experiments, which depart from this scheme, can, under certain conditions, endow 13C/15N small-molecule NMR with comparable sensitivity and resolution. SSFP is one of the earliest and most widely used NMR pulse sequences, yet its analyses have focused on isolated spin-1/2 ensembles such as water. The present study demonstrates that significant deviations from such isolated spin-1/2 behavior may occur when SSFP is applied in the presence of spin-spin couplings. Even in the simplest case supporting such couplings, a single 13C J-coupled to a 1H, departures from the isolated spin-1/2 behavior arise in the 13C SSFP response-both in the absence and in the presence of 1H spin decoupling. In the former case, deviations are produced by the differential relaxation of antiphase two-spin terms generated by the pulse train; in the latter case, magnified interferences may arise between the SSFP pulses and the coherent perturbation arising upon 1H decoupling. Although both phenomena are also known in FT NMR, the spectral distortions that they will originate may be much larger in the SSFP case-particularly if interpulse delays in large flip-angle SSFP pulse trains "resonate" with the coupling perturbations. The origins of these effects are here analyzed for heteronuclear spin-1/2 systems and corroborated with 13C NMR SSFP experiments recorded under different conditions. Additional considerations aimed at magnifying or suppressing these effects, as well as extensions to more complex scenarios, are also briefly discussed.