What is it about?
Analysis of the δX data obtained for a 1% solution of sodium 3-(trimethylsilyl)propane-1-sulfonate (DSS) in D2O, extended by the results of other closely related NMR experiments reported in the literature, provided new guidelines for performing routine 1H/13C NMR spectra in aqueous solvents externally referenced to neat liquid tetramethylsilane (TMS) contained in a coaxial capillary. Therefore, the previously proposed correction to the δX values determined in this way, which is mainly due to the difference in volume magnetic susceptibility, χv, between the NMR sample and the external standard used, usually called the bulk (isotropic) magnetic susceptibility (BMS) correction, was increased by +0.05 ppm (7%). The new value of this correction, +0.73 ppm, based on NMR measurements, was confirmed in a classical approach using critically reviewed χm, χM, and ρ data for TMS, D2O, and H2O. The analogous BMS correction for H2O solutions is +0.75 ppm.
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Why is it important?
Some crucial issues concerning magnetic susceptibility measurements for D2O and H2O, coaxial bulb-ended inserts, and the geometry of two-tube NMR cells (average shape factor αav) are critically discussed here, partly from a historical perspective. In addition, the high utility of the δDSS value of the residual HOD signal from the D2O of the analyte sample as an accurate "NMR thermometer" was confirmed. The obtained related correlation equations are very close to those found earlier by Gottlieb et al. (1997). On the other hand, it was a great surprise to discover that the inconsistent results of magnetic susceptibility measurements for heavy water from the 1930s (especially concerning temperature dependence) were not later verified with modern apparatus. The second surprise for me, as someone who has been using NMR spectroscopy for 50 years, was the realization that the commonly used 5-mm NMR tubes, contrary to popular belief, do not meet the criterion of infinitely long cylinders (and therefore have a nonzero focus factor), which affects the chemical shift values measured using an external reference located in a coaxial capillary insert placed in such a typical NMR tube.
Perspectives
During the writing of this article, I reviewed all available literature on the subject. Google's search engine proved invaluable in this task. In addition to the title BMS corrections, several other aspects were also addressed here. In the text above, I wrote more extensively about two unexpected but important findings made in this work. So, I believe that there is a need to make new magnetic susceptibility measurements for D2O and to make the scientific community more widely aware of the need to account for the nonzero shape factor of typical two-tube NMR cells used in the external referencing method.
Associate Professor Ryszard Bolesław Nazarski
University of Lodz
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This page is a summary of: Ambient temperature 1H/13C NMR spectra of sodium 3‐(trimethylsilyl)propane‐1‐sulfonate (DSS) in D2O referenced to external TMS: A discussion of these and closely related results. Corrections for the bulk magnetic suscep..., Magnetic Resonance in Chemistry, February 2024, Wiley,
DOI: 10.1002/mrc.5440.
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