Chemical Shift Values

How are proton chemical shifts measured on the δ scale?  

Tetramethylsilane (TMS)

The reference from which chemical shifts are measured in ¹H spectroscopy is tetramethylsilane, Si(CH3)4.  Tetramethylsilane (TMS) is a convenient standard because the central silicon atom has low electronegativity, the compound is symmetric, and the shielding of protons is greater than for nearly all other organic molecules.  Consequently, the NMR signals of ¹H-containing biologic molecules are shifted downfield from TMS.  On the commonly used delta (δ) scale, TMS is assigned a value of 0.0 ppm, and most other organic molecules will have chemical shifts between 0 and 12.  For example, water has a chemical shift of about 4.7 ppm at physiologic temperatures. Most aliphatic protons have chemical shifts less than water but higher than TMS. Aromatic compounds and molecules with nearby carbonyl groups typically have chemical shifts greater than that of water. 

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¹H NMR spectrum of a complex molecule (from Kumar PLoSOne. 2013;8:e71805 by CC open license)

The reasons why these various molecular environments produce different chemical shifts is a little complicated and the subject of a later Q&A. ​

Advanced Discussion (show/hide)»

TMS is not very soluble in water, so for NMR calibration in aqueous solutions, a common alternative reference standard is DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid). This molecule has the structural formula:

(CH₃)₃Si−CH₂−CH₂−CH₂−SO₃⁻Na⁺

which is essentially TMS with one of the silane methyl groups replaced by a propyl chain terminated with a sulfonic acid group. Although the spectrum of DSS contains minor peaks from these propyl hydrogens, the dominant peak for calibration still comes from the 9 methyl hydrogens with δ = 0.

Heavier nuclei have relatively larger chemical shifts than ¹H. This is primarily due to paramagnetic effects resulting from restriction of free circulation of p and d electrons. Here the (de-)shielding is anisotropic, but averages out for molecules tumbling freely in solution. The relative range of chemical shifts for nuclei commonly used in biomedical NMR are given in the table below along with their typical reference standards:

Nucleus	Chemical Shift Range	External Reference Standard
1H	15	TMS or DSS
13C	250	TMS
15N	1000	CH3NO2
17O	600	D2O
19F	500	CCl3F
23Na	75	NaBr or NaCl
31P	300	H3PO4

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References
     Meyer LH, Saika A, Gutowsky HS. Electron distribution in molecules. III. The proton magnetic spectra of simple organic groups. J Am Chem Soc 1953; 75:4567-73. (First large published table of NMR chemical shifts of organic molecules) 
     Reich HJ. Structure determination using spectroscopic methods. Syllabus for Course 605. University of Wisconsin, Madison, 2014. (Contains extensive web-based measurements of hydrogen and carbon chemical shifts). Excellent on-line resource for structural NMR, available at this link.

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Related Questions
     What is meant by a chemical shift? 
     How can you predict the chemical shift of hydrogen with a molecule?  

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