Interpret chemical shift
A fixed spectrum has signals δ 1.2 ppm (3H triplet), δ 3.6 ppm (2H quartet) and δ 9.8 ppm (¹H singlet).
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The core idea
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Learning objectives
- Interpret chemical shift
Read position before area
A fixed spectrum has signals δ 1.2 ppm (3H triplet), δ 3.6 ppm (2H quartet) and δ 9.8 ppm (¹H singlet).
Chemical shift reports environment; integration reports proton count; multiplicity reports neighbours.
Read a spectrum from three independent clues: horizontal position for environment, integrated area for relative proton number and splitting for neighbouring protons. Combining them is more reliable than treating any one clue as a structure proof.
Chemical shift δ
Chemical shift is the relative resonance-frequency displacement from TMS, reported in ppm.
Upfield means lower δ/more shielded; downfield means higher δ/more deshielded.
The ppm scale makes shifts comparable between instruments with different operating frequencies. TMS is assigned δ = 0, and ordinary proton spectra are conventionally drawn with larger δ values to the left.
Environment changes shielding
Electron density and anisotropic local fields alter effective field at a proton, shifting resonance relative to TMS.
Use supplied δ ranges as evidence; one shift alone rarely proves a whole structure.
Electronegative atoms, π systems and hydrogen bonding can move signals downfield, but chemical-shift ranges overlap. Use a range as supporting evidence and test the assignment against formula, integration and multiplicity.
Identify an aldehydic signal
A ¹H signal at δ 9.8 ppm lies in the aldehydic region; its integration of 1 confirms one contributing proton.
The shift, not the peak height, identifies the environment.
The 9.8 ppm assignment becomes persuasive because the position is aldehydic and the ¹H integral fits one –CHO proton. A singlet alone would not establish that conclusion.
Interpret δ 9.8 ppm, integral ¹H.
Check your answer
A strong answer should include aldehydic environment from shift and one proton from integration.
Separate three spectrum features
Supplied signal: δ 1.2 ppm, integral 3H, triplet.
State environment evidence cautiously until all features are combined.
Write three separate notes rather than one blended label: one for the position, one for the area and one for the splitting.
Label what each of δ, 3H and triplet tells you.
Check your answer
δ 1.2 is an alkyl-like, shielded environment; 3H means three equivalent protons, such as a CH₃; the triplet means two equivalent neighbouring protons (n + 1 = 3).
Compare ethyl-group signals
Supplied data from an oxygen-containing compound: δ 1.2 ppm (3H, triplet) and δ 3.6 ppm (2H, quartet).
Use the splitting to pair the two signals, the integrals to count protons, then δ to decide which group is nearer the electronegative atom.
Keep the three lines of evidence separate before combining them into an assignment.
Use δ 1.2 and 3.6 ppm with 3:2 integrals to identify the oxygen-adjacent group.
Check your answer
Assign δ 3.6 (2H quartet) to O–CH₂: oxygen withdraws electron density and deshields it, and three CH₃ neighbours give the quartet. Assign δ 1.2 (3H triplet) to the terminal CH₃, split by the two CH₂ protons.
Peak height is not chemical shift
Chemical shift is the x-axis position in ppm; height depends on line shape and acquisition.
Integration area, not height alone, is proportional to proton count.
Do not describe a taller line as ‘further downfield’. Downfield is a position on the δ axis; signal height and integrated area are different measurements.
Better reasoning: “The tallest peak has the greatest chemical shift.”
Check your answer
Refer to horizontal δ position and keep height/area separate.
Combine position with other evidence
After the check questions, interpret the anisole spectrum without notes. Return later for a different ester question.
Try this next: use deuterated solvents and labile-proton exchange.
For every assignment, write observation → inference → structural check. For example: δ 9.8 ppm → aldehydic environment → confirm one proton and a formula capable of containing –CHO.
Annotate the fixed three-signal spectrum by environment/count/neighbours.
Check your answer
A strong answer should include independent use of shift, integration and splitting without substituting one for another.
Interpret chemical shift scientific representation
All signals are listed in reading order with numeric δ, integral and multiplicity; no assignment depends on colour.
About 5 minutes
| δ / ppm | Integral | Multiplicity | Supported environment | Evidence limitation |
|---|---|---|---|---|
| 9.8 | 1 | singlet | aldehydic H | confirm with other evidence |
| 3.6 | 2 | quartet | OCH2 | shift is a range, not identity alone |
| 1.2 | 3 | triplet | CH3 next to CH2 | neighbour count assumes equivalent adjacent H |
Text alternative: All signals are listed in reading order with numeric δ, integral and multiplicity; no assignment depends on colour.