Dissipative dynamics of multiple-quantum NMR coherences in two-spin systems.

Multiple-quantum (MQ) NMR experiments were performed at a special orientation of a hambergite (Be2BO3OH) single crystal, which consists of alternating zigzag proton chains. At the chosen orientation, one of the dipolar coupling constants in the chain becomes zero and the system becomes a set of well-isolated dipolar coupled spin pairs. The relaxation of the spin pairs in the MQ NMR experiment was studied on the basis of the Lindblad equation. Fermi's golden rule was used to investigate the relaxation mechanism. The agreement of the calculated relaxation time with the experimental value (125 µs) suggests that the dipole-dipole interactions with protons surrounding the pair are responsible for the relaxation of MQ coherences.