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