ppm
Summary
In organic chemistry, ppm (parts per million) is the unit of chemical shift (δ) in NMR spectroscopy. It reports how far a signal lies from the reference (TMS, 0 ppm) as a fraction of the spectrometer frequency. Higher ppm means downfield and more deshielded.
Detail
Meaning: ppm is the unit of chemical shift (δ) on an NMR spectrum. The shift is the signal's frequency offset from a reference, usually tetramethylsilane (TMS, set at 0 ppm), divided by the spectrometer frequency. Dividing makes the value the same on a 300 MHz or a 600 MHz instrument. Nuclei with less electron density around them are deshielded and appear downfield (higher ppm, left side). Electronegative atoms, carbonyls and aromatic rings deshield nearby nuclei. Shielded nuclei appear upfield (lower ppm, right side).
Equation: δ = (νsample − νreference) / νspectrometer × 10⁶, where ν is frequency in Hz and δ is in ppm (a dimensionless ratio).
On the MCAT: You are given a ¹H NMR spectrum or a list of shifts and asked to pick the structure. Approximate ¹H ranges: alkyl C–H 0.9–1.5 ppm, H on carbon bonded to O or a halogen about 3–4, vinyl 5–6, aromatic 6.5–8, aldehyde 9–10, carboxylic acid 10–12. Predict the direction of the shift when an electronegative group is added nearby (it moves downfield).
Common confusion: Students read ppm as a frequency, or think a stronger magnet changes the shift. Peak separation in Hz grows with field strength, but the shift in ppm stays the same. The axis also runs from high to low, left to right.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.