FTIR Wavenumber Ranges
The mid-infrared range used in standard FTIR spectroscopy spans 4000–400 cm⁻¹. This range is divided into four diagnostic regions, each associated with specific bond types and vibration modes. Understanding what each region tells you is the first step in reading an FTIR spectrum — and in narrowing down which functional groups are present in your sample.
The Four Main Spectral Regions
When you look at an infrared spectrum, it helps to mentally divide it into these four zones. Each zone highlights a different class of molecular vibration, making systematic interpretation possible. The FTIR spectrum table lists individual peaks within each of these regions.
| Region | Wavenumber Range | Bond Types | What to Look For |
|---|---|---|---|
| X-H Stretching | 4000–2500 cm⁻¹ | O-H, N-H, C-H, S-H | Broad O-H (alcohol/acid), sharp N-H doublet (1° amine), C-H below/above 3000 (sp3/sp2) |
| Triple Bond | 2500–2000 cm⁻¹ | C≡C, C≡N, cumulated C=C=C | Nitrile (sharp, 2200–2260), alkyne (weak/variable, 2100–2260), allene |
| Double Bond | 2000–1500 cm⁻¹ | C=O, C=C, C=N, aromatic ring | Carbonyl (strongest, most diagnostic), alkene C=C, aromatic ring breathing |
| Fingerprint | 1500–400 cm⁻¹ | C-O, C-N, C-C, bending modes | Complex pattern unique to each molecule; best for confirmation, not initial ID |
X-H Stretching Region (4000–2500 cm⁻¹)
This is the first region you encounter scanning from high to low wavenumber. It contains stretching vibrations of bonds between hydrogen and heavier atoms — O-H, N-H, C-H, and S-H. Because these bonds are light and stiff, they vibrate at the highest frequencies in the mid-IR range.
O-H Stretches
- Free O-H — sharp peak, 3610–3670 cm⁻¹. Observed in dilute solution or gas phase where hydrogen bonding is absent.
- Hydrogen-bonded O-H (alcohols) — broad absorption, 3200–3550 cm⁻¹. The breadth reflects the range of hydrogen-bond strengths in the sample.
- Carboxylic acid O-H — very broad, 2500–3300 cm⁻¹. Often overlaps with C-H stretches and can obscure other features in this region.
N-H Stretches
- Primary amine (–NH₂) — doublet, 3350–3500 cm⁻¹. Two peaks from symmetric and asymmetric stretching.
- Secondary amine (–NH–) — single peak, 3300–3500 cm⁻¹.
- Amide N-H — 3180–3360 cm⁻¹. Shifted lower due to conjugation with the carbonyl.
C-H Stretches
- sp3 C-H — below 3000 cm⁻¹ (typically 2850–2960 cm⁻¹). Alkanes, CH₂, and CH₃ groups.
- sp2 C-H — above 3000 cm⁻¹ (typically 3010–3100 cm⁻¹). Alkenes and aromatics.
- sp C-H — near 3300 cm⁻¹. Terminal alkynes (≡C-H).
Tip: The position of C-H stretches relative to 3000 cm⁻¹ is a quick diagnostic for saturation — peaks only below 3000 suggest a fully saturated (no double bonds) molecule.
Triple Bond Region (2500–2000 cm⁻¹)
This narrow region is relatively sparse in most spectra, which makes any absorption here particularly diagnostic. If you see a peak between 2500–2000 cm⁻¹, it is almost certainly a triple bond or cumulated double bond.
- C≡N (nitriles) — sharp, medium-strong peak, 2200–2260 cm⁻¹. One of the easiest functional groups to identify by IR.
- C≡C (alkynes) — often weak or absent in symmetrical alkynes, 2100–2260 cm⁻¹. Look for the accompanying ≡C-H stretch near 3300 cm⁻¹ to confirm.
- Cumulated double bonds (allenes, ketenes, isocyanates) — 2000–2200 cm⁻¹. Less common but distinctive when present.
Double Bond Region (2000–1500 cm⁻¹)
The double bond region is arguably the most information-rich part of the spectrum. It contains the carbonyl stretch — the single most diagnostic absorption in IR spectroscopy — along with C=C, C=N, and aromatic ring vibrations.
- C=O (carbonyls) — strong, sharp absorption between 1650–1800 cm⁻¹. The exact position distinguishes ketones, aldehydes, esters, acids, and amides. See the carbonyl peaks reference for a detailed breakdown.
- C=C (alkenes) — 1620–1680 cm⁻¹. Medium intensity; can be weak or absent in symmetrical alkenes.
- C=N (imines) — 1620–1690 cm⁻¹. Overlaps with C=C; look for nitrogen-containing starting material context.
- Aromatic ring modes — characteristic pattern at 1600, 1580, 1500, and 1450 cm⁻¹. Two or more of these bands together strongly suggest an aromatic ring.
Fingerprint Region (1500–400 cm⁻¹)
The fingerprint region contains a dense, complex pattern of absorptions from single-bond stretches (C-O, C-N, C-C) and various bending modes. This region is unique to each compound — like a molecular fingerprint — but it is generally too complex for assigning individual peaks by hand. Key features to note:
- C-O stretches — 1000–1260 cm⁻¹. Strong; useful for identifying alcohols, ethers, and esters.
- C-N stretches — variable position and intensity depending on the amine type.
- Bending modes — C-H bending, N-H bending, and skeletal vibrations produce many overlapping peaks.
The fingerprint region is best used for confirmation rather than initial identification — compare your spectrum against a reference library rather than trying to assign each peak. For more detail, see the fingerprint region guide.
Putting It Together
A practical workflow for interpreting any FTIR spectrum:
- Scan left to right (high to low wavenumber). Note every significant peak and the region it falls in.
- Assign the X-H region first — determine whether O-H, N-H, or both are present. Check C-H position relative to 3000 cm⁻¹.
- Check the triple bond region — any peaks here are usually easy to assign.
- Focus on the double bond region — the carbonyl peak is often the most diagnostic single feature in the spectrum.
- Use the fingerprint region for confirmation — cross-reference C-O stretches and the overall pattern.
- Cross-reference with the FTIR spectrum table to match your observed peaks to known functional group absorptions.
For a complete walkthrough of this process with annotated examples, see How to Read FTIR Spectra. You can also explore the FTIR chart for a visual overview of where common functional groups absorb.