H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain anisotropic effects on chemical shift
H3 Chemistry: Explain anisotropic effects on chemical shift
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain anisotropic effects on chemical shift
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Map local fields
Benzene protons lie in the deshielding region of the induced π-electron ring current and appear near δ7.2.
The applied field creates a directional local field, so position in space matters.
Define anisotropy
Magnetic anisotropy is direction-dependent shielding produced by circulating electrons in an applied field.
It is distinct from a simple electronegativity trend.
Aldehyde protons are strongly deshielded near δ9–10, while an ethyne proton can be relatively shielded near δ2–3.
The alkyne proton lies along a shielding region of the C≡C induced field.
Text alternative: Text names B0, induced-field direction, proton position and predicted shift for every map.
Compare π systems
Aldehyde protons are strongly deshielded near δ9–10, while an ethyne proton can be relatively shielded near δ2–3.
The alkyne proton lies along a shielding region of the C≡C induced field.
Explain benzene
The ring current reinforces B0 outside the ring where peripheral H atoms lie.
Their larger effective field accounts for δ about 7.2.
- Explain benzene's aromatic shift.
Open the feedback checkpoint after attempting
- Credit induced ring current, proton position and deshielding.
Explain ethyne
The C≡C induced field opposes B0 along the bond axis where terminal H lies.
This places ethyne H upfield of alkene H despite sp carbon.
- Compare ethyne and ethene H.
Open the feedback checkpoint after attempting
- Credit directional shielding, not a hybridisation slogan.
Use an aldehyde map
A fixed C=O field map puts CHO H in a deshielding zone and predicts δ9.8.
Combine that position with a 1H integral before assignment.
- Assign the fixed δ9.8 signal.
Open the feedback checkpoint after attempting
- Credit aldehydic anisotropy and bounded evidence.
Repair electronegativity-only reasoning
Electron density alone cannot explain why an alkyne H is relatively upfield.
Add the direction of the induced π field at the nucleus.
- Repair the one-factor explanation.
Open the feedback checkpoint after attempting
- Credit spatial induced-field analysis.
Read fixed maps
Complete twelve map checks and two different π-system cases.
Next: hydrogen bonding at /learning/h3-nmr-hydrogen-bonding-lesson.html.
- Annotate shielding and deshielding zones.
Open the feedback checkpoint after attempting
- Credit B0 direction, induced field and proton location.