Today, I ask how accurate and consistent are actual experimental chemical shifts?
In many ways, this post probably should have preceded the one I linked above because in reality, before a discussion about prediction accuracy can begin, the topic of experimental accuracy needs to be addressed.
The issue of experimental accuracy can be important from two perspectives. For example, accuracy is important for a chemical shift database that is used for producing the predictions (Hence ACD/Labs' Purgatory Database). In addition, it is also important in identifying the accuracy of a predicted chemical shift when comparing it to an experimental one. How can we determine where the inaccuracy occurred?
In the process of producing an experimental NMR spectra, there are many variables that can affect a chemical shift that are not always carefully controlled. They include, but are certainly not limited to:
- Concentration of the sample
- Temperature of the probe
- Equilibration time in the probe
- Solvent type
- Residual water content of the solvent
- pH of the sample (if aqueous media)
- Digitization of the spectrum
- Shimming and phasing inaccuracies
- Choice of reference standard
Of course many of these factors, can significantly affect the chemical shift of the peaks in the spectrum. How much they are affected is sometimes hard to measure, but as an example, we can consider the range of database entries in our database for the shift of the methyl group protons in toluene. All of the following have been published in peer-reviewed journals.
Of course the deviations in these shifts are primarily based on the fact that each chemical shift was recorded in a different solvent. The reason for adding 8 sources of toluene in our database is so we can attempt to take the solvent into account when solvent-specific prediction is performed.
But as mentioned, solvent is not the only variable that can affect how an experimental chemical shift is recorded.
1. Prog. Nucl. Magn. Reson. Spectrosc.,1996,v.28,p.161 (Toluene-d8)
2. Zh. Org. Khim.,v.12,p.275 (Tetrahydrofuran-d8)
3. J. Org. Chem.,1997,v.62,p.7512 (Chloroform-d; 300 MHz; 24 C)
4. J. Org. Chem.,1997,v.62,p.7512 (Acetone-d6; 300 MHz; 24 C)
5. J. Org. Chem.,1997,v.62,p.7512 (Dimethylsulfoxide-d6; 300 MHz; 24 C)
6. J. Org. Chem.,1997,v.62,p.7512 (Benzene-d6; 300 MHz; 24 C)
7. J. Org. Chem.,1997,v.62,p.7512 (Acetonitrile-d3; 300 MHz; 24 C)
8. J. Org. Chem.,1997,v.62,p.7512 (Methanol-d4; 300 MHz; 24 C)