What is it about?
The very common indirect reference of 1H and 13C NMR spectra of solutions of various organic or bioorganic species to the 1H residual and 13C signals of six selected deuterated TMS-free organic NMR solvents is discussed in detail (also from a historical perspective). A list of recommended values of these reference signals is given for CDCl3, (CD3)2CO, (CD3)2SO, C6D6, CD3CN and CD3OD. The title method is compared with four other NMR methods commonly used to reference 1H/13C spectra, namely the usual default method using 2H field frequency locking, the IUPAC-recommended method based on ksi values (only occasionally used for 1H/13C spectra), and an external reference requiring a correction for the difference in bulk (isotropic) magnetic susceptibility of the two liquids involved; see, e.g., Magn. Reson. Chem. 2024; DOI: 10.1002/mrc.5440.
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Why is it important?
This review discusses some of the benefits and disadvantages of indirect referencing to TMS for 1H and 13C NMR spectra taken in typical organic solvents without TMS as an internal NMR standard. The correct application of the title method, which is likely to become more widely used in the future, is considered in detail. Numerous examples of its misapplication are also shown and discussed, mainly resulting from the unreflective use of the very popular MestReNova postprocessing program.
Perspectives
An indirect reference method discussed in the paper, if carefully applied according to the recommendations presented, can find wide application in the structural 1H/13C NMR analysis of complex organic or bioorganic molecules, especially their samples, which are available in very small (microgram) quantities. Undoubtedly, parallel DFT NMR calculations will be very useful (due to the synergistic effect) in all such structural considerations, especially for diastereomeric systems; see, for example, Tetrahedron Lett. 2021, 71, 152548.
Associate Professor Ryszard Bolesław Nazarski
University of Lodz
Read the Original
This page is a summary of: On the Use of Deuterated Organic Solvents without TMS to Report 1H/13C NMR Spectral Data of Organic Compounds: Current State of the Method, Its Pitfalls and Benefits, and Related Issues, Molecules, May 2023, MDPI AG,
DOI: 10.3390/molecules28114369.
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