Surprising absence of strong homonuclear coupling at low magnetic field explored by two-field NMR spectroscopy

Strong coupling of nuclear spins, which is achieved when their scalar coupling 2πJ is greater than or comparable to the difference δω in their Larmor precession frequencies in an external magnetic field, gives rise to efficient coherent longitudinal polarization transfer. The strong-coupling regime can be achieved when the external magnetic field is sufficiently low, as δω is reduced proportional to the field strength. In the present work, however, we demonstrate that in heteronuclear spin systems these simple arguments may not hold, since heteronuclear spin-spin interactions alter the δω value. The experimental method that we use is two-field NMR (Nuclear Magnetic Resonance), exploiting sample shuttling between a high field, at which NMR spectra are acquired, and low field, where strong couplings are expected, at which NMR pulses can be applied to affect the spin dynamics. By using this technique, we generate zero-quantum spin coherences by means of non-adiabatic passage through a level anti-crossing and study their evolution at low field. Such zero-quantum coherences mediate the polarization transfer under strong coupling conditions. Experiments performed with an 13C labelled amino acid clearly show that the coherent polarization transfer at low field is pronounced in the 13C-spin subsystem under proton decoupling. However, in the absence of proton decoupling, polarization transfer by coherent processes is dramatically reduced, demonstrating that heteronuclear spin-spin interactions suppress the strong coupling regime even when the external field is low. A theoretical model is presented, which can model the reported experimental results.