NMR Spectra of Secondary Reference Compounds

3-(Trimethylsilyl)-l-propanesulfonic acid sodium salt (sodium 4,4-dimethyl-4-sUapentane-l-sulfonate DSS) 

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Chemical shifts in NMR spectra are usually reported relative to the peak position of tetramethylsilane (TMS) added to the sample as an internal reference. If TMS is not sufficiently soluble, a capillary with TMS may be used as external reference. In this case, owing to the different volume susceptibilities, the local magnetic fields in solvent and reference differ, and the peak position of the reference must be corrected. For a D2O solution in a cylindrical sample and neat TMS in a capillary, the correction amounts to +0.68 and -0.34 ppm for superconducting and electromagnets, respectively. These values must be subtracted from the chemical shifts relative to external TMS if its position is set to 0.00 ppm. Alternatively, secondary references with (CH3)3SiCH2 groups may be used. The following spectra of two such secondary reference compounds in D2O were measured at 500 MHz with TMS as external reference. Chemical shifts are reported in ppm relative to TMS upon correction for the difference in the volume susceptibilities of D2O and TMS. As a result, the peak for the external TMS appears at 0.68 ppm. 

For solid-state Si NMR spectra, other referencing techniques are required. Usually a crystalline organosilicon compound is used (e.g. QgMg , octakis(tri-methylsiloxy)octasilsesquioxane), which gives sharp and intensive Si NMR signals under MAS conditions and H decoupling. Since the 8 Si values of this material are known (—109.3, +11.7) relative to SiMc4, it serves as a reliable secondary reference. 

As mentioned in Section 2.3.1, the almost universally adopted primary reference for chemical shifts in NMR spectra is the compound tetramethylsilane (TMS). Incidentally, TMS is also employed as the primary chemical shift reference in H and Si NMR [18]. Some old reports give chemical shifts relative to benzene or carbon disulfide, but the use of the TMS resonance as the zero chemical shift mark is now commonplace. Unfortunately, TMS is highly volatile, toxic, and flammable, which makes problematic its use as a direct reference sample. A simple alternative is to use another sample with a known spectrum as an external and secondary reference. In the case of solid-state NMR there are many known secondary standards, such as adamantane (higher-frequency peak at 38.6 ppm), hexamethylbenzene (higher-frequency peak at 132.2 ppm), and glycine (higher-frequency peak at 176 ppm) [4,15,98]. 

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