All chemical shifts are expressed in relative terms. The shift, , is expressed in parts per million (ppm) versus some reference compound. For 1H and 13C, this reference compound is normally tetramethylsilane (TMS), (CH3)4Si. A compound having a chemical shift of 1.00 ppm would show a peak 60 Hz downfield of TMS if run on a 60 MHz spectrometer, or 400 Hz downfield of TMS on a 400 MHz spectrometer. Chemical shift is thus independent of the instrument's field strength.
For 1H spectra of most organic compounds, chemical shifts can range from 0-10 ppm. The spectrum is divided into several general regions, although there is a large amount of overlap.
0-0.8 ppm Rarely seen; typical of cyclopropanes, silanes, other organometallic species.
0.8-1.5 ppm Alkane region. Hydrogen attached to aliphatic carbon with no other electronegative substituent. Here and elsewhere, the more substituted the carbon, the further downfield the proton will be (CH>CH2>CH3 in chemical shift).
1.5-2.5 ppm Carbonyl region. Protons attached to carbons next to a C=O, C=C or phenyl ring come here. Each of these groups induces a slight polarization of the C-H bond, decreasing electron density and deshielding the proton.
3.0-4.5 ppm Ether region. (Also alcohols and esters have the CH-O group.) Protons on carbon next to a highly electronegative element such as oxygen (or halogens) come here. If there are more than one electronegative substituents, the proton may come even further downfield (note CHCl3 at 7.27 ppm).
5.0-7.0 ppm Double bond region. Hydrogens directly attached to C=C double bonds come here.
7.0-8.0 ppm Aromatic region. The electronic properties of an aromatic ring lead to a phenomenon called anisotropy (AAHN-eye-SOH-tro-pee) that deshields aromatic protons relative to other protons on sp2 carbon.
9-10 ppm Aldehyde region. Carbonyls also exhibit anisotropy, and on top of significant electronegativity of the C=O group, this leads to large deshielding.
-OH Alcohols can literally appear at any region of the spectrum. The nature of this peak is strongly affected by factors which affect H+ exchange: pH, concentration, temperature, solvent. Generally phenols and more acidic alcohols are further downfield, but this is not necessarily a reliable predictor.
-NHR, -NH2Like alcohols, these have exchangeable protons. However, there is more uniformity in that most amines and amides show broad resonances in the 2-3 ppm range.
-CO2H Also exchangeable, like alcohols, but normally highly deshielded (>11 ppm)
Caution! These files are quite large, and require the Chime 2.0 plug-in. This section is under test (2/23/98).
Thanks to the Pacific Lutheran University FTNMR FID library for these spectra.
Good tables of NMR chemical shifts for other functional groups may be found in:
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Last updated: 8/5/97
Comments to K. Gable