IR spectroscopy: reading the functional-group region

An IR spectrum is a plot of transmittance (or absorbance) versus wavenumber, running from about 4000 cm⁻¹ on the left to 400 cm⁻¹ on the right. Each dip is a vibrational absorption — a bond that stretches or bends at exactly the wavenumber of the incoming photon. The trick to reading IR is to split the spectrum into two zones with different jobs: 4000-1500 cm⁻¹ names the functional groups; 1500-400 cm⁻¹ is a compound-specific fingerprint. This guide walks the diagnostic bands, the position/intensity/shape rules that name each one, and the four questions that lead from a spectrum to a functional-group list.

Prerequisites: functional-group vocabulary. If reading an NMR spectrum is comfortable, IR is easier — fewer axes, coarser resolution, faster reading.

Where the numbers come from

A bond stretches like a spring, and Hooke’s law plus the harmonic-oscillator quantisation gives the fundamental vibrational frequency:

$\tilde{\nu} = \frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}$

where k is the force constant (bond stiffness) and μ is the reduced mass of the two atoms. Two consequences:

1. Stiffer bonds vibrate at higher wavenumber. Triple bonds (2200-2100 cm⁻¹) > double bonds (1750-1600) > single bonds (1500 and below).
2. Lighter atoms vibrate at higher wavenumber. X-H stretches sit at 3600-2800 cm⁻¹ — highest wavenumber region — because μ is tiny when one atom is hydrogen.

The two effects combine cleanly: a C=O stretch is above a C=C stretch (bigger k) and both are above a C-C stretch. An O-H stretch is above a C-H stretch (smaller reduced mass with H, plus stiffer bond due to O’s electronegativity).

The two zones

Functional-group zone (4000-1500 cm⁻¹). Fewer bands. Each band position, intensity, and shape reliably diagnoses a specific bond type. Read this region first.

Fingerprint zone (1500-400 cm⁻¹). Many overlapping bands corresponding to C-C, C-O, and C-N single-bond stretches plus bending modes. Impossible to assign individually, but the pattern is unique to a specific molecule — the compound’s “fingerprint”. Match to a reference spectrum (NIST WebBook) to confirm identity.

Do not try to identify functional groups by looking in the fingerprint region. Do not try to match the whole spectrum without first extracting functional-group information from the top region.

The diagnostic band table

Committed to memory, this table converts any IR spectrum to a functional-group list in under a minute (Silverstein 8e Ch. 2, Coates 2000):

Reading the intensity codes: s = strong (over 60% absorption); m = medium (30-60%); w = weak (under 30%); v = variable.

The four-question workflow

Every IR spectrum answers four questions in order:

Question 1: Is there a C=O? Look for a strong, sharp band between 1830 and 1630 cm⁻¹. If yes, note the exact position; that position pins down the carbonyl subtype (acyl chloride high, amide low; see table).

Question 2: Is there an O-H or N-H? Look above 3200 cm⁻¹. O-H is broad and centred around 3400 (H-bonded alcohol) or 2500-3300 (H-bonded acid; overlaps with C-H). N-H is medium and sharp; primary amines/amides show two bands, secondary show one.

Question 3: Are there C-H bonds sp²/sp³/sp? Sp³ C-H at 2850-3000 (in every organic molecule with alkyl groups). Sp² C-H at 3000-3100 (alkene, aromatic — a small band just left of the alkane). Sp C-H at 3300 (terminal alkyne — sharp, distinctive).

Question 4: Are there aromatic C=C bands? Look for medium bands at 1600 and 1500 cm⁻¹ (sometimes 1450) and a set of overtones at 2000-1600 (weak). Aromatic rings show these consistently.

Answer these four in order and you have identified the functional-group class before touching the fingerprint region.

Worked example — identifying an unknown from IR

Overtones, combination bands, and Fermi resonance

Real spectra are messier than the table suggests. Three complications:

Overtones are absorptions at approximately 2× (or 3×) a fundamental. Weak, but visible at high concentration. The C=O overtone at ~3400 cm⁻¹ can be confused with an O-H stretch if you’re not careful.

Combination bands are absorptions at the sum of two fundamental frequencies. The 2000-1600 cm⁻¹ region for aromatic rings shows an “aromatic overtone/combination fingerprint” that identifies mono-, di-, and tri-substituted benzenes — a small but useful diagnostic.

Fermi resonance happens when an overtone or combination band lies near an allowed fundamental and mixes with it, giving two bands where one was expected. The aldehyde C-H stretch shows this: instead of one band, aldehydes give two bands at 2830 and 2720 cm⁻¹ (from Fermi resonance between the C-H fundamental and the overtone of the C-H bend). Seeing that specific pair is the strongest aldehyde diagnostic in the IR.

C=O position — a table within a table

The carbonyl stretch is the most reactivity-informative band in IR. Its exact position within the 1830-1630 cm⁻¹ window is set by three effects:

• Electron-withdrawing groups raise the frequency (stiffer C=O bond). Acyl chlorides (1815-1770) > anhydrides (1810 and 1750) > esters (1750-1720) > aldehydes/ketones (1735-1710) > amides (1690-1630).
• Conjugation lowers the frequency (partial delocalisation weakens the C=O). Aryl ketone (1685) is ~30 cm⁻¹ below saturated ketone (1715); α,β-unsaturated (1680) is similar.
• Ring strain raises the frequency. Cyclobutanone (1780) > cyclopentanone (1750) > cyclohexanone (1715). Ring strain compresses the C-C-C angle, stiffening the C=O.

Given a C=O near 1740 cm⁻¹, three candidates fit — an ester, a strained ketone, or an aldehyde. The aldehyde C-H Fermi doublet at 2830/2720 distinguishes the aldehyde from the others.

Common pitfalls

Confusing O-H with N-H. Both appear above 3200 cm⁻¹, but O-H (H-bonded) is broad and rounded while N-H is medium and sharp. Primary N-H shows two bands (symmetric and asymmetric stretch); primary O-H shows one. Acid O-H is uniquely broad and extends into the C-H region.

Ignoring the fingerprint region entirely. The four-question workflow tells you the functional-group class, but two isomers can share the same functional groups. To distinguish 1-pentanol from 2-pentanol, the fingerprint region (specifically C-O bending around 1080 vs 1120) is what makes the call. Compare with a reference spectrum from NIST WebBook or Aldrich to confirm.

Assigning “C-H stretches” as diagnostic. Everything organic has C-H stretches. Their exact position (sp³ below 3000 vs sp² above 3000) is diagnostic, but their presence is not.

Reading transmittance and absorbance backwards. Transmittance spectra: dips going DOWN are absorptions. Absorbance spectra: peaks going UP are absorptions. Most textbooks use transmittance; some journals use absorbance. Check the y-axis label.

Practice

Where this leads

IR is one leg of the structure-elucidation tripod:

• Multi-technique elucidation — IR (functional groups), NMR (connectivity), and MS (mass + formula) together identify almost any organic compound.
• Quantitative IR — Beer’s law works for a single sharp band; see Beer-Lambert for the failure modes.
• Reaction monitoring — in-situ IR (ReactIR) tracks C=O disappearance during ketone reduction or ester formation in real time.
• Solid-state IR — ATR (attenuated total reflectance) accessories let you measure powders and thin films directly without KBr pellets.

Every one of them starts with the four-question workflow: C=O, O-H/N-H, C-H hybridisation, aromatic C=C.