Wimperis and coworkers inserted a mixing time in MQMAS experiment to investigate the relative orientation of quadrupole tensors. The shapes of the cross-peaks are characteristic of the three Euler angles that describe the relative orientation of the two quadrupole tensors.
The sequence is similar to the z-filter sequence except that the interpulse delay between the last two pulses, normally of few microseconds, becomes the mixing time during which magnetization transfer occurs. This transfer occurs when only population states are retained by the phase cycling. High power 1H decoupling is normally applied during t1 and t2 periods but may be turned off during the mixing time to enhance the spin-diffusion rate.
For a spin I = 3/2 and 3QMAS experiment,
0Q -> +3Q -> -1Q is the anti-echo pathway,
0Q -> -3Q -> -1Q is the echo pathway.
For the other half-integer spins and 3QMAS experiment,
0Q -> +3Q -> -1Q is the echo pathway,
0Q -> -3Q -> -1Q is the anti-echo pathway.
Since the two coherence transfer pathways are symmetrical, amplitude-modulated experiment such as z-filter should generate pure absorption 2D lineshape.
ACQUISITION: Hyper-complex or TPPI
Shearing transformation is not necessary for correlation.