Reference module
Band atlas & normal-mode assignments
Characteristic group frequencies with explicit vibrational-mode assignments, symmetry where relevant, and relative intensities. Search, filter by region, and resolve any peak with the identifier below.
4000–2500
2500–2000
2000–1500
Assignment notation
ν stretch (νs symmetric, νas antisymmetric) · δ in-plane bend / scissor · γ out-of-plane bend (oop) · ρ rock · ω wag · τ twist. Amide modes (A/I/II/III) are mixed coordinates defined on the biomolecular page. Intensities: strong medium weak.
🔎 Peak identifier
Enter an observed wavenumber to list every catalogued mode that absorbs there (±15 cm⁻¹), ranked by proximity.
cm⁻¹Full band table
Search by bond, group, mode or wavenumber, or filter by region.
| Bond / group | Mode | Wavenumber (cm⁻¹) | Int. | Diagnostic notes |
|---|---|---|---|---|
| O–H · alcohol/phenol (free) | ν(O–H) | 3650–3580 | m | Sharp; gas phase / dilute aprotic only. |
| O–H · alcohol/phenol (H-bonded) | ν(O–H)··· | 3550–3200 | s | Broad; width tracks H-bond strength/distribution. |
| O–H · carboxylic acid (dimer) | ν(O–H) | 3300–2500 | s | Very broad cyclic-dimer band; pair with ν(C=O) ~1710. |
| N–H · 1° amine | νas,νs(NH₂) | 3500 & 3400 | m | Two bands (~70 cm⁻¹ split) = primary amine. |
| N–H · 2° amine | ν(N–H) | ~3300 | m | Single band. |
| N–H · amide (amide A) | ν(N–H) | 3350–3180 | m | Amide B (~3100) is a Fermi resonance with 2×amide II. |
| ≡C–H · terminal alkyne | ν(≡C–H) | ~3300 | s | Sharp spike; separates from broad O–H. |
| =C–H · alkene / aromatic | ν(=C–H) | 3100–3000 | m | Above 3000 ⇒ unsaturation. |
| C–H · sp³ alkane | νas,νs(CH₂/CH₃) | 3000–2840 | s | CH₂ ~2926/2853; CH₃ ~2962/2872. |
| C–H · aldehyde | ν(C–H) Fermi | 2850 & 2750 | w | Fermi doublet with δ(C–H) overtone; confirms –CHO. |
| S–H · thiol | ν(S–H) | 2600–2550 | w | Weak; little H-bonding so position is stable. |
| C≡N · nitrile | ν(C≡N) | 2260–2220 | m | Narrow; sensitive Stark probe (see Methods). |
| C≡C · internal alkyne | ν(C≡C) | 2260–2150 | w | Absent if centrosymmetric (mutual exclusion). |
| C≡C · terminal alkyne | ν(C≡C) | 2140–2100 | w | Pairs with ν(≡C–H) ~3300. |
| N=C=O · isocyanate | νas(NCO) | 2275–2250 | s | Very strong in the quiet triple-bond window. |
| C=O · anhydride | νas,νs(C=O) | 1830 & 1760 | s | Coupled doublet (~60 cm⁻¹ split). |
| C=O · acid chloride | ν(C=O) | 1815–1770 | s | Highest single carbonyl (electron-withdrawing Cl). |
| C=O · ester | ν(C=O) | 1750–1735 | s | With two strong ν(C–O) at 1300–1000. |
| C=O · aldehyde | ν(C=O) | 1740–1720 | s | With the 2850/2750 Fermi doublet. |
| C=O · ketone | ν(C=O) | 1725–1705 | s | Reference carbonyl; α,β-unsaturation lowers by 20–40. |
| C=O · carboxylic acid | ν(C=O) | 1725–1700 | s | With very broad acid ν(O–H). |
| C=O · amide (amide I) | amide I, ν(C=O) | 1690–1630 | s | ~80% C=O stretch; secondary-structure marker. |
| C=C · alkene | ν(C=C) | 1680–1620 | w | Weak/absent if symmetric; conjugation lowers it. |
| C=C · aromatic ring | ν(C=C) ring | 1600–1450 | m | Quartet near 1600/1580/1500/1450. |
| C=N · imine / oxime | ν(C=N) | 1690–1640 | m | Overlaps carbonyl region; no O on carbon. |
| N–H bend · amide (amide II) | amide II, δ(N–H)+ν(C–N) | 1570–1510 | m | Shifts to ~1450 (amide II′) on N-deuteration. |
| N=O · nitro | νas,νs(NO₂) | 1550 & 1350 | s | Two strong bands diagnostic of –NO₂. |
| C–H bend · CH₂ / CH₃ | δ(CH₂), δs(CH₃) | 1465 & 1375 | m | 1375 doublet ⇒ gem-dimethyl (iPr/tBu). |
| S=O · sulfone / sulfonamide | νas,νs(SO₂) | 1350 & 1160 | s | Two strong bands. |
| C–O · alcohol/ether/ester/acid | ν(C–O) | 1300–1000 | s | Strong, position-variable; often the most intense band. |
| P=O · phosphate / phosphine oxide | ν(P=O) | 1300–1250 | s | Common in nucleotides, phospholipids, organophosphates. |
| C–N · amine | ν(C–N) | 1250–1020 | m | Supports amine assignment. |
| C–O–C · ether | νas(C–O–C) | 1150–1070 | s | Strong ν(C–O) with no O–H and no C=O ⇒ ether. |
| S=O · sulfoxide | ν(S=O) | 1070–1030 | s | Single strong band; shifts with coordination. |
| =C–H oop · vinyl | γ(=C–H) | 990 & 910 | s | Two bands ⇒ monosubstituted (vinyl) alkene. |
| =C–H oop · trans-alkene | γ(=C–H) | 980–960 | s | One band ~965 ⇒ trans double bond. |
| Aromatic C–H oop · mono | γ(C–H) | 770 & 700 | s | Two bands ⇒ monosubstituted benzene. |
| Aromatic C–H oop · para | γ(C–H) | 850–800 | s | Single band ~820 ⇒ 1,4-disubstituted ring. |
| C–F · fluoride | ν(C–F) | 1400–1000 | s | Strong; overlaps ν(C–O). |
| C–Cl · chloride | ν(C–Cl) | 850–550 | s | Heavier halogen ⇒ lower ν (reduced mass). |
| C–Br · bromide | ν(C–Br) | 690–515 | s | Low fingerprint band. |
| C–I · iodide | ν(C–I) | 600–500 | s | Lowest of the C–X stretches. |
The carbonyl ladder (high → low cm⁻¹)
Acid chloride ~1800 → anhydride 1830 & 1760 → ester ~1740 → aldehyde ~1725 → ketone ~1715 → acid ~1710 → amide ~1650. Drivers: inductive electron withdrawal raises ν(C=O); conjugation and amide N→C=O resonance lower it; ring strain in small lactones/lactams raises it (β-lactam ~1745–1780).
By compound class
Diagnostic band sets when the class is known or suspected.
Alcohols / phenols
3550–3200 ν(O–H) · 1300–1000 ν(C–O). No carbonyl.
Carboxylic acids
3300–2500 ν(O–H) dimer · 1725–1700 ν(C=O) · ~1410/1290 coupled C–O.
Esters
1750–1735 ν(C=O) · two strong 1300–1000 ν(C–O).
Amides
1690–1630 amide I · 1570–1510 amide II · 3350–3180 amide A.
Amines
3500–3300 ν(N–H) (1–2 bands) · 1650–1580 δ(N–H) · 1250–1020 ν(C–N).
Aromatics
3100–3000 ν(=C–H) · 1600–1450 ring · 900–675 γ(C–H) substitution pattern.
Reconstruct any of these band sets as a spectrum in the on-site simulator — or see the computation notes on harmonic/anharmonic accuracy.