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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