TRAnsfer of Population in DOuble Resonance

Grey and coworkers designed TRAPDOR sequence which uses the heteronuclear dipole coupling between quadrupole and spin-1/2 nuclei to ascertain connectivity and spatial interaction information. It facilitates the recoupling of quadrupole spins in Zeeman states other than ¦±1/2>.

Both the ³¹P 180° pulse and the start of the acquisition are synchronized with the rotor echoes. The ²⁷Al spins are irradiated for one half of the echo period of Scheme (2), with Scheme (1) providing the blank period.

In Scheme (2), the continuous irradiation of the ²⁷Al nuclei during N rotor periods modifies the Zeeman population levels of the ²⁷Al spin, which in turn affects the magnetization of the ³¹P nuclei dipole coupled to it.

Lang and coworkers suggested that to minimize variations in experimental conditions and eliminate spectrometer timing artifacts, ²⁷Al decoupling pulse should be triggered in both schemes with minimal offset, from very high power attenuation in Scheme (1) for no ²⁷Al irradiation to low power attenuation for maximum ²⁷Al irradiation in Scheme (2). In other words, the sequences in Schemes (1) and (2) should be applied on alternate scans to avoid any instrument instabilities affecting the outcome of the experiments.

As in SEDOR experiment, a normalized TRAPDOR difference signal, defined as
(delta S)/S₀ = [signal from Scheme (1) - signal from Scheme (2)]/signal from Scheme (1),
is plotted versus the number of rotor periods of on-resonance ²⁷Al irradiation. The interatomic distance is extracted by fitting these data with a theoretical curve.

The curve representing the normalized TRAPDOR difference versus ²⁷Al offset frequency allows an estimation of the quadrupole coupling constant of ²⁷Al.