How to Fit an MQMAS spectrum

 Example :  87Rb MQMAS spectrum of RbNO3  
 Example :  27Al Syncrhonized MQMAS  

 Example :  27Al STMAS Spectrum of A9B2 at 830MHz  

Quadrupolar tensor definition :
  • principal values are sorted |Vyy|<=|Vxx|<=|Vzz| note that Vxx and Vyy have signs opposite to that of Vzz - usually NMR is not sign sensitive for EFG
  • Vzz+Vxx+Vyy=0
  • etaQ is the anisotropy of the Quadrupolar tensor etaQ=Vyy-Vxx/Vzz  with 0<=EtaQ<=1
  • and thus Vxx=1/2 (-EtaQ-1)Vzz  and Vyy=1/2 (EtaQ-1)Vzz

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Step 1 : Calibrate the MQ-MAS experiment

The fit of the MQ-MAS (or STMAS) experiment assumes that you sheared the acquired spectrum (with Bruker AU program, with RMN Grandinetti's program, or any other way...).

The F1 axis is assumed to be scaled similarly with the F2 axis so that the diagonal (slope 1) 1/1 line relates identical isotropic chemical shifts.

For datasets acquired with Bruker, using "in0" as the increment you can use the calibration algorithm :

  • Call the SpecParam dialog [Menu/File/View Param]

  • Set the proper nucleus (??? list box second line) which has to be quadrupolar !!

  • Click the "Calibrate MQ..." button at the bottom of the dialog.

  • Accept the values proposed in the "Calibrate MQ experiment" dialog (in which you can specify the shift that you applied or the folding)

Remark : depending on your pulse sequence and on the considered nucleus, it can happen that the slope of the chemical shift diagonal is negative, in that case you have to reverse the F1 dimension : [Menu/2D/Reverse F1]

Step 2 : if possible begin by Fitting the 1D spectrum.

This step is not necessary but can simplify the process of fitting the 2D dataset.

If you need you can learn more about fitting a 1D Quadrupolar broadened spectrum.

Step 3 : Switch to the 2D dataset.

Having the 1D spectrum and fit loaded, just go the the [Menu/File/Open] and open the 2D dataset (*.rr files for Bruker).


Load in the 2D dataset [Menu/File/Open] and open the 2D dataset (*.rr files for Bruker) and load the 1D fit parameters [Menu/Decomposition/Load Other]

Step 4 : Switch the decomposition mode from 1D to 2D

In the lef panel, change the fitting mode from "Fit 1D" to "Fit MQ" (if you do not see this option, enlarge the dmfit window to make it appear).

Click the and you should get the first guess

Step 5 :