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DOSY processing in Topspin

You should always perform initial processing in Topspin to verify that experiments worked. DOSY processing must initially be done on spectrometer workstations after experiments are completed. The initial processing uses some AU programs that are NOT available on local Topspin installations. However, once processed on the NMR workstation, the dataset may be re-processed on a local user computer.

1. Phase the first fid:
   1. issue rser 1
   2. process
   3. phase
   4. press [nD] button to transfer phase values back to DOSY
   5. close Temp window
      
      
2. Edit ProcPars: ABS1 and ABS2 limits to 1000,1000 and -1000, -1000
   
   
3. Set SI for F1 to 2N 
   (Important: must be the next power of 2 greater than the number of gradient steps)
   
   
4. xf2; abs2
   
   
5. setdiffparm (this command needs to be issued only once in the experiment)
   
   
6. Issue eddosy
   Set:
   1. PC to minimum S/N peaks you would like to include
   2. F1mode to Peaks
   3. Imode to Intensity)
      
      
7. dosy2d setup
   (issue this command if you want to automatically setup D max and min limits)
   
   
8. Edit DISPmin and max limits to show necessary diffusion coefficient range.
   
   
9. issue dosy2d  to process to DOSY representation. Click on Spectrum tab to see the result:
   
   
   
   
10. Go to Spectrum, check if you have enough contour levels (Right click, Edit contour levels, set Level increment to 1.4, Number of levels to 64, click Fill and Apply)
    
    
11. NOTE If you nee to rerun DOSY with new range of D or other parameters, issue:
    1. xf2; abs2
    2. make necessary adjustments
    3. dosy2d

Read log(D) from the Y axis for each compound and correct it by a linear shift computed from log(D) of a reference compound like water. D of residual H2O in pure D2O at 298K is 1.902e-9 m2/s (in Claridge, p317 in 2nd and p397 in 3rd) => log(D) = -8.72.

For more presentable fitting results, use MNova DOSY protocol.

 

 

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Important notes:

• Computed values of a diffusion coefficient will be always off from real numbers by a fixed amount because calibration of NMR probe gradient coil is not perfect. To make a correction, find a literature value for the diffusion coefficient for water or other compound in your tube at your temperature in the solvent with identical visocity.

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• To correct all diffusion coefficients using a reference value:
  
  
  • Log10(D, corrected) = Log10(D, observed)  + [ log10(D, reference compound, from literature) - log10(D, reference compound, observed)]
    
    or
    
    
  • D(corrected) = D(observed)  * [D(reference compound, from literature)/D(reference compound, observed)]
    
    

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• In the first approximation, you may use this reference value: D of residual H2O in pure D2O at 298K is 1.902e-9 m2/s (in Claridge, p317 in 2nd and p397 in 3rd editions), which corresponds to log10(D) = -8.72

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• You can ONLY expect your measurement give you accurate diffusion coefficient if your molecule is present in only ONE form with one size/aggregation state. If you have monomers, dimers, trimers, etc. and chemical shifts are the same that is peaks are exactly overlapped, the D value will be some sort of weighted average. It will still reflect on the aggregation states but will not be interpretable directly. Fitting to obtain of more than one diffusion coefficient from a single peak (decay curve) will ONLY be reliable if your diffusion constants differ by at least of factor of ten. However, amount of signal from the heavy species will be minimal therefore this case is not practical either.
  
  
  Literature:
  High-Resolution NMR Techniques in Organic Chemistry, 3rd Edition, by Timothy D.W. Claridge. Elsevier Science (May 27, 2016), ISBN-10 ‏ : ‎ 0080999867, ISBN-13 ‏ : ‎ 978-0080999869