NMR pulse sequence:
shifted-echo MQ-MAS
Shifted-echo MQ-MAS
Grandinetti and coworkers, who introduced the shifted-echo approach to DAS experiment, extended this approach to MQMAS.
Shifted-echo approach generates pure absorption 2D lineshape.
An additional selective pi pulse for the central transition is added at the end of the two-pulse sequence to shift the coherence transfer echo
The delay tau should be long enough so that the whole echo is acquired for t1 = 0. This delay is a multiple of the rotor period and about half the echo width.
PROCESSING: After the Fourier transform with respect to t2, a tau-dependent first-order phase correction is performed to remove the phase modulation due to the shift and a t1-dependent first-order phase correction to perform the shearing transformation. This results a 2D isotropic (in F1 dimension) anisotropic (in the F2 dimension) correlation spectrum.
The echo amplitude and the antiecho amplitude have opposite signs.
This figure represents the amplitude-modulated shifted-echo associated with 3Q-MAS experiment involving both pathways.
For a spin I = 3/2,
0Q -> +3Q -> +1Q -> -1Q is the anti-echo pathway,
0Q -> -3Q -> +1Q -> -1Q is the echo pathway.
For the other half-integer spins,
0Q -> +3Q -> +1Q -> -1Q is the echo pathway,
0Q -> -3Q -> +1Q -> -1Q is the anti-echo pathway.
ACQUISITION: Hyper-complex or TPPI
This figure represents the phase-modulated shifted-echo associated with 3QMAS experiment involving the echo pathway.
For a spin I = 3/2,
0Q -> -3Q -> +1Q -> -1Q is the echo pathway.
For the other half-integer spins,
0Q -> +3Q -> +1Q -> -1Q is the echo pathway.
ACQUISITION: F1-QF
This figure represents the phase-modulated shifted-echo associated with 3QMAS experiment involving the anti-echo pathway.
For a spin I = 3/2,
0Q -> +3Q -> +1Q -> -1Q is the anti-echo pathway.
For the other half-integer spins,
0Q -> -3Q -> +1Q -> -1Q is the anti-echo pathway.
ACQUISITION: F1-QF