When trying to identify an organophosphorus compound, one of the ways to proceed is to compare the phosphorus chemical shift of the compound of interest with those of a standard. However, this comparison is meaningful only when operating in the same conditions. When using deuterated water as a solvent, pH is one of the parameters that must be taken into account. So does pH really influences the 31P chemical shift ? and if so, how strong is this influence ?
It does, however it depends on the class of examinated organophosphorous compound. For most of 1-aminoalkylphosphonic acids (1-APAs) and hydroxyphosphonic acids (HAPAs) pH significantly influences the 31P NMR shift.
NMR-titration is not in my interest so I cannot give you exact pH values, however I have few examples just to give you overview:
1) 1-APAs 31P NMR chemical shift may change for about 5ppm.
My example is 1-(methyloctylamino)ethylphosphonic acid: 14ppm in acidic solutions (6M HCl); 10ppm in alkaline solutions (as disodium salt with NaOH).
Another example is 1-amino-1-methylethylphosphonic acid: 22ppm in acidic solutions (4M HCl); 17ppm in alkaline solutions (as disodium salt with NaOH).
2) APAs 31P NMR chemical shifts may change for about 5ppm. My example is 1-hydroxycyclohexylphosphonic acid: 27ppm in acidic solutions (4M HCl); 22ppm in alkaline solutions (3M NaOH).
For other classes of organophosphorous compounds pH-dependent changes in chemical shift may be as high as 10ppm. I may suggest few articles where this issue is discussed:
a) http://dx.doi.org/10.1016/j.molstruc.2012.08.033
b) J. Chem. Soc., Dalton Trans., 1999, 1603–1607
c) Fresenius Z Anal Chem (1984) 317:697-698
d) https://doi.org/10.1080/10426507.2018.1543304
e) dx.doi.org/10.1021/jm301866e
To identify compounds by 31P NMR I also compare multiplets (on 31P NMR spectra without decoupling) and coupling constants (as they are less sensitive to pH changes).
Best regards,
Anna Brol
As Anna Brol already said it realy does. I myself observed in a long time experiment a constant drift of the phosphate signal, probably by loss of NH3 from a buffer. In my case only ~2 ppm. It can be more than 10 if you look in the literature: Article pH dependence of phosphorus-31 chemical shifts and coupling ...
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