MolDraw › Chemistry tools › IR Spectrum Predictor
IR Spectrum Predictor
In short: Draw a molecule or paste its SMILES. The tool finds its functional groups and lists the IR bands you should expect, with wavenumber range, intensity and shape, on a numbered sketch of the spectrum.
This is a correlation-chart prediction: the bands come from standard IR tables for each group, adjusted for conjugation, ring size and substitution. It does not calculate vibrations, so it is not a simulated spectrum. Use it to know which peaks to look for.
CCOC(C)=O, should show a strong ester C=O at 1735–1750 cm⁻¹, strong C–O bands between 1300 and 1000 cm⁻¹ and sp³ C–H at 2850–2960 cm⁻¹, with no O–H or N–H band.Loading editor…
Loading the structure engine…
How to use the IR spectrum predictor
- Draw the molecule, or paste a SMILES string and press Enter.
- The functional groups found are shaded on the drawing and named above it.
- Each expected band is numbered on the sketch and listed in the table with its range, intensity, shape and the group that causes it.
- Compare the list with your measured spectrum. A missing strong band (for example no C=O near 1700 cm⁻¹) is as informative as a present one.
- Copy the list, or download it as CSV or the sketch as SVG.
Key bands the predictor uses
| Group | Band (cm⁻¹) | Look |
|---|---|---|
| Alcohol O–H | 3200–3550 | strong, broad |
| Carboxylic acid O–H | 2500–3300 | strong, very broad, runs under C–H |
| Primary amine / amide N–H | 3300–3500 (amine), 3150–3370 (amide) | two medium bands |
| Terminal alkyne ≡C–H + C≡C | 3260–3330 and 2100–2140 | strong narrow, then weak |
| Nitrile C≡N | 2240–2260 (conjugated 2215–2240) | medium, sharp |
| Aldehyde C–H | ≈2820 and ≈2720 | two weak bands |
| Anhydride C=O | 1800–1830 and 1740–1775 | two strong bands |
| Acid chloride C=O | 1785–1815 | strong |
| Ester C=O | 1735–1750 (aryl ester 1750–1770, conjugated 1715–1730) | strong |
| Aldehyde C=O | 1720–1740 (conjugated 1685–1710) | strong |
| Ketone C=O | 1705–1725 (conjugated 1666–1700, cyclopentanone ≈1745) | strong |
| Carboxylic acid C=O | 1700–1725 (conjugated 1680–1710) | strong |
| Amide C=O (amide I) | 1650–1690 | strong |
| Nitro N–O | 1500–1560 and 1330–1385 | two strong bands |
| C–O (alcohol, ether, ester) | 1000–1300 | strong |
Conjugation with a C=C or an aromatic ring lowers a C=O band by about 20–30 cm⁻¹. Ring strain raises it. For every band in one searchable table, see the IR spectroscopy chart.
Sources: D. L. Pavia, G. M. Lampman, G. S. Kriz & J. R. Vyvyan, Introduction to Spectroscopy, 5th ed. (Cengage, 2015); R. M. Silverstein, F. X. Webster, D. J. Kiemle & D. L. Bryce, Spectrometric Identification of Organic Compounds, 8th ed. (Wiley, 2014). Exact positions shift with phase (solid, liquid, solution), hydrogen bonding and neighbouring groups.
What this tool does not do
- It does not compute vibrational frequencies (no DFT or force field). The plot is a sketch, not a simulated spectrum. Real peak positions, widths and heights differ.
- It does not predict the fingerprint region (1500–400 cm⁻¹) band by band. Only the group bands that textbooks list are shown.
- It cannot tell cis from trans alkenes or count exact overtone patterns. Hydrogen bonding, concentration and sample state (KBr, ATR, solution) are ignored.
- Salts, zwitterions, metal complexes and unusual groups are covered only partly. Groups the detector does not know give no band.
Have a measured spectrum? Open your JCAMP-DX or CSV file in the free IR viewer to pick peaks and label them.
Open IR viewerFrequently asked questions
Is this a simulated IR spectrum?
No. The tool finds functional groups and lists the bands a textbook correlation chart gives for them. The plot is a sketch that places a dip in the middle of each range. It does not calculate vibrational frequencies.
Where does a C=O band appear in IR?
Between about 1650 and 1830 cm⁻¹. Saturated ketones absorb near 1715, aldehydes near 1725, esters at 1735–1750, carboxylic acids at 1700–1725, amides at 1650–1690 and anhydrides give two bands near 1820 and 1760 cm⁻¹. Conjugation lowers the band by about 20–30 cm⁻¹.
How do I tell an alcohol from a carboxylic acid by IR?
An alcohol gives a broad O–H band at 3200–3550 cm⁻¹ and no C=O. A carboxylic acid gives a very broad O–H band from about 2500 to 3300 cm⁻¹ that runs under the C–H peaks, plus a strong C=O near 1710 cm⁻¹.
Which bands show an aldehyde?
A C=O band near 1725 cm⁻¹ (about 1700 when conjugated) together with two weak C–H bands near 2820 and 2720 cm⁻¹. The 2720 band is the most useful one.
Last updated: