| Summary: | <p>Strong coupling of nuclear spins, which is achieved when their scalar
coupling <span class="inline-formula">2<i>π</i><i>J</i></span> is greater than or comparable to the difference
<span class="inline-formula">Δ<i>ω</i></span> 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 <span class="inline-formula">Δ<i>ω</i></span> 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
<span class="inline-formula">Δ<i>ω</i></span> value. The experimental method that we use is two-field nuclear magnetic resonance (NMR), exploiting sample shuttling between the high field, at which NMR spectra are acquired, and the low field, where strong couplings are expected and 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 a nonadiabatic passage through a level
anticrossing and study their evolution at the low field. Such zero-quantum
coherences mediate the polarization transfer under strong coupling
conditions. Experiments performed with a <span class="inline-formula"><sup>13</sup>C</span>-labeled amino acid
clearly show that the coherent polarization transfer at the low field is
pronounced in the <span class="inline-formula"><sup>13</sup>C</span> 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.</p>
|
|---|
