Chemical Shift (Delta)
Definition
The position of an NMR signal on the frequency axis, expressed in parts per million (ppm) relative to a reference (TMS = 0). It reports the electronic environment of the nucleus, with shielded nuclei at low ppm and deshielded nuclei at high ppm.
- Unit
- Parts per million (ppm)
- Reference
- TMS defined as 0 ppm
- Reports
- Electronic environment of the nucleus
- Low ppm
- Shielded nuclei
- High ppm
- Deshielded nuclei
Common questions
Why is chemical shift reported in ppm rather than absolute frequency?+
Absolute resonance frequency depends on the spectrometer's magnetic field strength, but expressing the shift relative to a reference compound as a ppm ratio gives a value that is the same regardless of which instrument or field strength was used, allowing spectra from different labs to be compared directly.
How does chemical shift help identify an unknown drug compound?+
Each type of proton or carbon in a molecule sits in a distinct electronic environment and produces a characteristic shift, so the overall pattern of shifts acts as a structural fingerprint that an examiner matches against reference spectra or interprets from first principles.
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