Mixed MQ coherence MQ-MAS
Jerschow and coworkers introduced 5Q3Q mixed MQMAS experiment, in which the 3Q coherence instead of the 1Q coherence is used for refocusing the second-order quadrupole broadening.
The constants k and k' are defined by
C2(p1)*k + C2(p2)*k' = 0 , with
C2(p) = p[18I(I + 1) - 17p2/2 - 5] .
The constants p1 and p2 are the coherence orders involved during the first period (between the first and the second RF pulses) and the second period (between the second and the third RF pulses), such that C2(p1) and C2(p2) have opposite sign.
For 5QMAS experiment with a spin I = 5/2, 7/2, or 9/2,
0Q -> +5Q -> +3Q -> +1Q -> -1Q is the anti-echo pathway. Therefore, the anti-echo is recorded.
The first half part of this RF pulse sequence uses the split-t1 approach, whereas the second half part uses the shifted-echo approach. As a result, the position of the anti-echo signal remains fix.
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.
Acquiring the whole anti-echo generates pure absorption 2D lineshape without shearing.
This sequence achieves the best resolution for spin I = 5/2 systems, but with very low sensitivity. The two parameters are k = 19/44 and k' = 25/44.
The sensitivity of the experiment is enhanced when low-amplitude RF pulses are used, but the anisotropic lineshape along the F2 dimension is distorted.