Analysis of IR Spectra

• identification of functional groups present (analysis of absorption bands)

• identification of unknown compounds (spectrum matching–fingerprint region)

• account for the wavenumber, intensity, and shape of the absorption bands

• usually not practical (or possible) to assign every observed band in the spectrum

Characteristic IR Absorptions

4000–3200 cm–1: 3000–2700 cm–1:

• O–H stretch (alcohols)

• N–H stretch (amines)

• C–H stretch (alkynes)

3200–3000 cm–1:

• =C–H stretch (alkenes)

• =C–H stretch (aromatics)

• O–H stretch (–CO2H)

• C–H stretch (alkanes)

• C–H stretch (aldehyde C–H)

2700–2000 cm–1:

• CΞN stretch (nitriles)

• CΞC stretch (alkynes)

Characteristic IR Absorptions

2000–1600 cm–1: 1300–1000 cm–1:

• C–C–O stretch (alcohols)

• C–O–C stretch (ethers)

• C=O stretch (carbonyls)

• C=C stretch (alkenes)

• N–H bend (–NH2)

1600–1200 cm–1:

• C=C stretch (aromatics)

• C–H bend (alkanes)

1000–500 cm–1:

• C–H rock (alkanes)

• =C–H bend (alkenes)

• =C–H bend (aromatics)

functional group region

4,000–1,500 cm–1

fingerprint region

1,500–400 cm–1

• assign all bands to functional groups

• spectrum matching

• assign only intense and broad bands

–OH–CH

The 3,000 cm–1 Line

• divides the C–H stretching bands for saturated and unsaturated hydrocarbons

saturated

below 3,000 cm–1

unsaturated

above 3,000 cm–1

The 3,000 cm–1 Line

IR Spectrum of Hexane

CH3 CH2 CH2 CH2 CH2 CH3–C–H stretches

IR Spectrum of 1-Hexene

CH2 CH CH2 CH2 CH2 CH3

=C–H stretch C=C

stretch

=C–H bends –C–H stretches

Alkene Out-of-Plane =C–H Bends

IR Spectrum of Toluene

CH3

=C–H stretch

out-of-plane C–H bends

ring C=C stretches

Aromatic Out-of-Plane C–H Bends

out-of-plane C–H bends

745 cm–1

740–770 cm–1

770, 690, cm–1

760–810 cm–1

680–710 cm–1

800 cm–1

800–855 cm–1

IR Spectrum of Toluene

CH3

overtone bands

Aromatic Overtone Bands