IR Structure Builder and Spectrum Simulator
Build a molecule from functional-group and fragment blocks, connect them, and generate an educational IR spectrum with band assignments and simple corrections for conjugation and aromatic substitution.
Structure builder and IR spectrum
Click a block to select it.
What do the connection positions mean?
For most groups, connections are used to recognize the immediate environment, for example a carbonyl conjugated with a double bond or an aromatic ring. The Ph block has six attachment points: when two are used, the program distinguishes ortho (1,2), meta (1,3), or para (1,4) substitution and adds indicative aromatic out-of-plane bands.
Simulated IR spectrum
Educational spectrum generated from correlation tables.
Band assignments
Click a row to highlight the fragment that generates the band in the builder. You can also click near a band in the spectrum.
| Assignment | Range [cm⁻¹] | Simulated peak [cm⁻¹] | Intensity | Shape | Source |
|---|
How is the spectrum estimated?
A bond vibration can be interpreted, as a first approximation, with the harmonic-oscillator model:
\[\tilde\nu=\frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}\]
where \(k\) is the bond force constant and \(\mu\) is the reduced mass. Different bonds therefore occupy characteristic regions, while chemical environment can shift the absorption.
The tool starts from experimental correlation ranges and applies only a few simple educational corrections. For example, it recognizes conjugation of selected carbonyl groups and ortho/meta/para substitution of a phenyl ring.
How to read an IR spectrum
A useful student workflow is to start with the most diagnostic regions instead of trying to assign every peak immediately. Check O–H/N–H above 3000 cm⁻¹, sp²/sp³ C–H around 3000 cm⁻¹, triple bonds at about 2100–2260 cm⁻¹, and especially strong carbonyl bands in the 1650–1780 cm⁻¹ region. Only then move into the fingerprint region below about 1500 cm⁻¹.
The chart uses the traditional transmittance convention, with absorptions pointing downward, and the wavenumber axis running from 4000 to 400 cm⁻¹.
Table sources
- Chemistry LibreTexts, Infrared Spectroscopy Absorption Table: absorption ranges, indicative intensity and shape.
- University of Colorado Boulder, Table of Characteristic IR Absorptions: correlation table for common functional groups.
Values are educational correlation ranges, not guaranteed values for a specific molecule.