Chapter 16: Quadrupole Relaxation Enhancement for ¹H¹⁴N and ¹H²H Dipolar Couplings

Quadrupole relaxation enhancement is a phenomenon observed in field-cycling relaxometry applications to systems containing dipole nuclei dipolarly coupled to quadrupole nuclei. As typical examples, ¹⁴N and ²H nuclei interacting with protons in biological systems, including living leeches, are considered. Since the protons are in their high-field limit, crossings of the proton resonance with the low-field resonances of the quadrupole nuclei can be scanned using the field-cycling technique. Due to the strong coupling of quadrupole moments to the electric field gradients in the molecules, the spins of the quadrupole nuclei can be assumed to be permanently in equilibrium. Thus, proton spin–lattice relaxation is enhanced by spin energy exchange mediated by flip-flop transitions with the quadrupole nuclei at the resonance crossings.