What does wavenumber (cm⁻¹) mean in IR spectroscopy?

Wavenumber is the reciprocal of wavelength in centimeters. IR spectra are plotted from ~4000 cm⁻¹ (high energy, bond stretches) to ~400 cm⁻¹ (low energy, bending modes). Higher wavenumber = higher bond energy / lighter atoms.

What is the fingerprint region?

The region from 400–1500 cm⁻¹ is called the fingerprint region. It contains complex overlapping bending and skeletal vibrations unique to each molecule. It is used to confirm identity rather than identify specific functional groups.

Why does a carbonyl C=O stretch appear near 1700 cm⁻¹?

The C=O double bond is stiff and involves a moderately heavy oxygen atom. Conjugation with an aromatic ring or adjacent double bond lowers the stretch to ~1680–1650 cm⁻¹. Electron-withdrawing groups (like in acid chlorides) raise it to ~1800 cm⁻¹.

How do I distinguish an alcohol O-H from a carboxylic acid O-H?

Alcohol O-H gives a broad stretch at 3200–3550 cm⁻¹. Carboxylic acid O-H is broader, lower, and often appears as a very wide absorption from 2500–3300 cm⁻¹ that overlaps the C-H stretch region.

What intensity labels mean in this table?

Strong (s) means high absorbance — easy to spot. Medium (m) is moderate. Weak (w) is small peak, easy to miss. Broad (br) means the peak spans a wide wavenumber range. Variable (var) means intensity depends on the specific molecule or environment.

IR Spectroscopy Helper

Name: IR Spectroscopy Helper
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Author: Nham Vu

Look up characteristic IR absorption bands for functional groups by wavenumber range, intensity, and bond type.

Wavenumber lookup:

cm⁻¹

Intensity: s = strong m = medium w = weak br = broad var = variable

Functional Group	Bond Vibration	Wavenumber (cm⁻¹)	Intensity	Notes

Summary

Look up characteristic IR absorption bands for functional groups by wavenumber range, intensity, and bond type.

How it works

Use the search box to filter by functional group name or bond type.
Click a wavenumber range badge to highlight all bands in that region.
Select a functional group category from the dropdown to narrow results.
Read the wavenumber range, intensity, and notes for each band.
Use the "Wavenumber Lookup" tab to enter a specific wavenumber and find matching functional groups.

Use cases

Identify functional groups from an unknown compound's IR spectrum.
Study for organic chemistry exams by reviewing key absorption ranges.
Cross-reference FTIR output with known functional group signatures.
Quickly recall whether an O-H or N-H stretch appears in a given region.
Compare carbonyl (C=O) stretch positions across different functional groups.
Distinguish between primary, secondary, and tertiary amines by N-H count.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu