H3 Chemistry 9813 · Study focus: H3 Chemistry: Interpret chemical shift
H3 Chemistry: Interpret chemical shift
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Interpret chemical shift
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
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.
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.
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.
| 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.
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.
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.
- Interpret δ 9.8 ppm, integral 1H.
Open the feedback checkpoint after attempting
- Credit aldehydic environment from shift and one proton from integration.
Separate three spectrum features
For δ 1.2 ppm, integral 3H, triplet: position, area and multiplicity carry different information.
State environment evidence cautiously until all features are combined.
- Label what each of δ, 3H and triplet tells you.
Open the feedback checkpoint after attempting
- δ concerns environment, 3H proton count, triplet two equivalent neighbours.
Compare ethyl-group signals
An ethyl fragment often gives a 3H triplet and 2H quartet at different δ values.
A CH2 bonded near oxygen is more downfield than its CH3 partner.
- Use δ 1.2 and 3.6 ppm with 3:2 integrals to identify the oxygen-adjacent group.
Open the feedback checkpoint after attempting
- Assign δ 3.6 to O–CH2 and δ 1.2 to terminal CH3.
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.
- Repair: “The tallest peak has the greatest chemical shift.”
Open the feedback checkpoint after attempting
- Refer to horizontal δ position and keep height/area separate.
Combine position with other evidence
Complete twelve fixed shift checks, then unseen anisole assessment and different ester re-test.
Next objective: use deuterated solvents and labile-proton exchange at /learning/h3-nmr-solvents-labile-protons-lesson.html.
- Annotate the fixed three-signal spectrum by environment/count/neighbours.
Open the feedback checkpoint after attempting
- Credit independent use of shift, integration and splitting without substituting one for another.