Chemical Shift Referencing - IUPAC recommendations 2001
How to calibrate chemical shift
- Start with a calibrated 1H spectrum of your sample of interest.
You may calibrate it on: added TMS (organic solvents), added DSS (aqueous solvents), residual solvent signals, or any other assigned peak of defined chemical shift in the spectrum.
If all you want to calibrate is your 1H spectrum, then you are finished.
If you want to calibrate an X NMR spectrum (a non-1H experiment), then, the sample should now be in the magnet, and, if desired, locked and shimmed. IN ADDITION, continue on with the following instructions.
- Click on ProcPar (or type: edp) and copy & paste the SF value from your 1H spectrum into a spreadsheet, SF(1H) = _______.
- Without changing the field (don't change the lock or the shim), acquire spectra for all the other nuclei of interest, one after the other.
- To calibrate an X NMR spectrum you will need the frequency ratio (Xi value or Ξ) from the IUPAC article (Tables 1 and 2 - notice that the column heading is Ξ/% so you will need to divide by 100 to get just Ξ).
For instance, for 31P, Ξ = 0.40480742.
- Read in the experiment number of the spectrum to be calibrated.
Ignore any error messages.
In ProcPar (or type: edp), set sr to zero.
- Set SF of the X spectrum to be equal to the product of Xi and the calibrated (1H) SF
SF(X) = Ξ*SF(1H).
Here's an example:
1H calibrated spectrum, SF(1H) was 200.1300000.
Your X spectrum is 31P, the IUPAC Ξ value is 0.40480742.
Set SF(X) to be 81.01410896 - and yes, you MUST put all those numbers after the decimal.
NOTE: remember that BF = SF - SR. So, after you set SF, SR (signal reference) will also change. You can calibrate the spectrum by changing either of the values. BF (base frequency) is a constant.
Summary and highlights of the IUPAC article
(1) Standard automatic calibration
The Bruker Avance spectrometers use the parameter called "solvent" (type: solvent in xwin-nmr to display the value; if you type lock and choose a solvent, it will update the parameter in that expno) in order to determine the 1H chemical shift, based on the external field needed for the 2H signal to be on resonance. If you use the automatic lock with the correct solvent parameter, you will usually get an accurate chemical shift calibration - except for D2O as solvent.
The chemical shift of water (or heavy water) depends on the hydrogen (or even stronger deuterium) bonding. Water will be calibrated to 4.7 ppm with the automatic lock system; it is common to see spectra taken at two different concentrations (or temperature, or pH, or ionic strength), where the water has the same chemical shift, but EVERY peak of the material shifted 1.5 ppm. In reality, only the water peak shifted.
(2) Internal calibration
In XWIN-NMR (both 1D and 2D menus), you can use the button on the left called "calibrate" to define the chemical shift according to any reference peak you choose in the spectrum. Common choices would be TMS (you can buy solvent with TMS added, or add your own directly to your solvent bottle), the residual proton signal of the deuterated solvent, a known peak of the material (e.g. CH3 group of a membrane bilayer in water), or DSS (similar to TMS, but water soluble).
(3) Universal referencing
For X-nuclei (non-hydrogen), the
IUPAC guidelines explain how to convert between an internal calibration of the
1H signals (#2 above) and ANY other NMR sensitive nucleus
(based on the ratio of the magnetogyric ratios).
In this system ALL nuclei are calibrated according to the 1H signal of TMS
(1% TMS in CDCl3, to be exact), which is the universal reference.
To convert from the 1H calibration to an X calibration (you must be using the exact same field),
use the tables given in the
IUPAC (2001) article.
(4) External referencing
This method is popular for solid-state NMR.
It is also an older method for solution NMR. For 31 P NMR, be aware of the problems with H3 PO4 : It is very sensitive to temperature variations, sample purity, and the concentration of the acid.
The three listings given above are preferable for solution NMR (more exact, less error prone, and less time consuming). The tables in the IUPAC article give standard external references for all NMR sensitive nuclei.