IR Band Prediction Practice
Predicting what bands the structure below should show in its IR spectrum. The actual measured spectrum is revealed after you answer.
Common Questions
Why doesn't every bond in a molecule show an IR band?
A vibration only absorbs infrared light if it changes the molecule's dipole moment. Stretches of polar bonds (C=O, O–H, C≡N) change the dipole strongly and give intense bands. A symmetric internal alkyne or a symmetrically substituted alkene stretches without changing the dipole at all, so the mode is IR-inactive: the bond is real, the band is missing.
Which bands should I predict first from a structure?
Start from the groups whose bands are reliably strong: every carbonyl gives a strong C=O near 1700, every free O–H gives a broad band above 3150, N–H appears above 3250, C≡N near 2250, nitro near 1520. Then add the always-present C–H stretches near 2900 and, for aromatic compounds, ring stretches near 1600 and 1500. Weak modes (C=C, internal C≡C, S–H) are bonuses, never promises.
Do these problems use calculated spectra?
No. Every problem is checked against real data: a measured ATR-FTIR spectrum from the Chemotion open repository. When we say a band is absent, that is an observation about the real spectrum, not a model's opinion.