Fitting Second order broadened Quadrupolar spectrum...

 Example :  23Na MAS spectrum of Nasicon - central transition fitted with a 2nd order quadrupolar lineshape  
 Example :  27Al MAS 400MHz spectrum of YAG fitted with 1st order quadrupolar for AlO6 and 2nd order quadrupolar for AlO4  

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

Most of the procedure is similar to that described for Gaussian/Lorentzian lines except for the computing Model and its parameters...

Step 1 : Setting up the starting solution (the different lines)

  1. Open the file in dmfit [Menu/File/Open] or drag it to dmfit window from file manager

  2. Initiate a decomposition [Menu/Decomposition/New Fit]

  3. Select a nucleus by Clicking the button or calling the [Spec Parameters] dialog. If the nucleus is a quadrupolar nuclei, the list of model will be extended with to Quadrupolar interaction models.

  4. Click the [Add] button to add a new line

  5. Select the "Q-MAS 1/2" to model the central transition under infinitely fast MAS spinning (or the "Q stat 1/2" for static conditions).

  6. Click the [Drag] button to drag this new line to a convenient position, Quadrupolar coupling and amplitude, click left to accept (click right toggles between Amp/Pos and Amp/quadrupolar coupling modes).

  7. Check or modify the lines parameters in the [Fit Pannel] (bottom of screen)

  8. Press the [Compute] button to recompute the model spectrum

  9. Repeat this procedure for all the lines that you expect.

remark : only the quadrupolar coupling can be dragged on screen, the asymmetry parameter is changed by modifying the value in the bottom window.

Line Parameters (Bold parameters are selected for automatic optimization):
    Amplitude :
height of the maximum of the line
    Position : Position of the line (in Hertz, ppm or point depending on the current unit)
    Em : Apodisation of the theoretical lineshape, this value is not calibrated in this window, its actual value can be checked in the Report window. Positive values are Lorentzian apodisation, negative are Gaussian. To make your mind start with 5 or -5.
    nuQ : is the quadrupolar frequency expressed in kHz, the display can also be set in quadrupolar coupling (CQ) by changing a switch in the Fit Parameters dialog box .
    etaQ : is the asymmetry parameter [0..1]

You can always click use the Fit Paramters Pannel buttons

Step 2 : Optimizing the fit

  1. Check the different lines and their parameters (only Bold face parameters will be optimized)

  2. If necessary click the [Parameters] button to call the [Model Parameters full list] dialog and modify the list of variable paramters

  3. Click the [Optimize] button to optimize the current solution

Step 3 : Reporting and Saving

Fit parameters can be saved in machine readable binary format [Menu/Decomposition/Save Fit] or [Menu/Decomposition/Save as Fit] and thus retrieved later exactly in same state.

Parameters can be reported in human readable format by clicking the [Report] button to report the current line paramters in a dialog box from which you can Select/Copy and further Paste word processing or SpreadSheet programs.