Advanced Topic 922 of 1102 Analytical, Lab & Spectroscopy
Chemical Shift
Position of an NMR signal reflecting the electronic environment of a nucleus.
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In-depth explanation of Chemical Shift
Chemical shift, symbol delta, is the position of an NMR signal in parts per million relative to a reference compound, usually tetramethylsilane (TMS), which is set at 0 ppm. Electron density around a nucleus shields it from the applied field, so electron-donating groups move peaks upfield to lower ppm, while electron-withdrawing groups deshield the nucleus and move peaks downfield to higher ppm. Typical proton shifts are about 0.9 to 1.5 ppm for alkyl hydrogens, 2 to 2.5 ppm for hydrogens next to a carbonyl, 3.3 to 4.5 ppm for hydrogens on carbon bonded to oxygen, 6.5 to 8 ppm for aromatic hydrogens and 9 to 10 ppm for aldehyde hydrogens. The chemical shift does not depend on the spectrometer frequency, which is why it is quoted in ppm.
Going deeper: Chemical Shift
Key ideas: delta (ppm) = (nu_sample minus nu_TMS)/spectrometer frequency in MHz. Anchors for 1H: alkyl 0.9 to 1.7, allylic 1.6 to 2.2, alpha to carbonyl 2.0 to 2.7, C-H next to N 2.2 to 3.0, C-H next to O 3.3 to 4.5, alkene 4.5 to 6.5, aromatic 6.5 to 8.5, aldehyde 9 to 10, carboxylic acid 10 to 13. Hydrogen bonding makes OH shifts variable.
Common mistakes and connections: Students mix up upfield and downfield, which refer to lower and higher ppm respectively. Electronegative atoms deshield, not shield. The shift scale in ppm is the same for any magnet, but the splitting in hertz is not. Concentration and solvent change O-H and N-H shifts. Related to ring current, inductive effects, TMS reference and 13C shifts.
Visualization of Chemical Shift
Proton chemical shifts increase with deshielding: alkyl hydrogens near 1 ppm, aromatic near 7.3 and aldehyde near 9.7 ppm relative to TMS at 0. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Benzene protons appear at about 7.3 ppm because the ring current deshields them.
Frequently asked questions about Chemical Shift
What is Chemical Shift?
Position of an NMR signal reflecting the electronic environment of a nucleus.
Is Chemical Shift a beginner, intermediate or advanced topic?
Chemical Shift is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Chemical Shift?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Fingerprint Region, Raman Spectroscopy, NMR Spectroscopy.
What comes after Chemical Shift?
The next topics in Analytical, Lab & Spectroscopy are Spin-Spin Splitting and Integration in NMR.
Can I practise Chemical Shift with quizzes?
Yes. The interactive lesson includes a self-check question on Chemical Shift, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Chemical Shift fits in Analytical, Lab & Spectroscopy
Chemical Shift is topic 46 of 99 in Analytical, Lab & Spectroscopy.