Infrared radiation is absorbed when its frequency matches a molecular vibration that changes the molecule’s . Different bond types absorb in useful wavenumber ranges.
Connect each physical stage to the downward trough that appears in a transmittance spectrum.
Move the slider. The marker sweeps across the wavenumber scale and the simplified transmittance spectrum is drawn as the instrument scans.
Every model now shows all main bonds so you can see exactly which bond is being matched to the band.
Above about 1500 cm⁻¹, several highly useful absorptions occur. Examples include broad O–H, strong C=O and C–H stretching bands.
Use ranges rather than expecting one exact number. Bond strength, neighbouring atoms, hydrogen bonding and sample conditions can shift a band.
The region below about 1500 cm⁻¹ contains many overlapping vibrations involving much of the molecule. It is shaded red in the interactive spectra because individual peaks in this region are usually less reliable for assigning one specific bond.
The overall jagged pattern can help confirm identity by comparison with a reference spectrum, but at VCE level use clearly diagnostic bands above 1500 cm⁻¹ first.
| Bond / group | Representative range | Typical appearance | What it suggests |
|---|---|---|---|
| O–H in an alcohol | 3200–3550 cm⁻¹ | Broad, strong | Alcohol or phenol; confirm with other evidence |
| O–H in a carboxylic acid | 2500–3300 cm⁻¹ | Very broad | Carboxylic acid, especially when C=O is also present |
| C=O | about 1680–1750 cm⁻¹ | Strong, sharp | A carbonyl-containing family such as aldehyde, ketone, acid, ester or amide |
| C–H, sp³ carbon | about 2850–3000 cm⁻¹ | Several medium peaks | Alkyl C–H bonds |
| C–O | about 1000–1300 cm⁻¹ | Often strong, but inside the fingerprint region | May support a C–O bond, but is less reliable as an isolated assignment because it lies below 1500 cm⁻¹ |
| N–H in a primary amine | about 3300–3500 cm⁻¹ | Often two relatively sharp N–H stretching bands | Primary amine; combine with other evidence |
The bank is split into four mixed sets of five questions. Each set combines instrument and interpretation content. Answer choices are shuffled once when the attempt begins, then remain fixed while you work.