INFRARED
SPECTROSCOPY
Electromagnetic Spectrum
Bond Stretching
High Frequency Stretching
Low Frequency Bending
Frequency of Absorption
Measured in cm-1 (Wavenumbers)
Wavelength units typically microns ( m)
10,000 m = 1 cm or 1m = 10 -4 cm
Frequency units typically measured in
cm-1
= # waves or cycles in 1 cm (wavenumbers)
Mid-Infrared frequency range is 4000 - 400 cm-1
(2.5 - 25 m)
High Frequency to Low Frequency
Functional Groups Region
4000 - 1400 cm-1
Fingerprint Region
1400-600 cm-1
General Trends:
i) Stretching frequencies are higher than
corresponding bending frequencies. (It is easier
to bend a bond than to stretch or compress it.)
ii) Bonds to hydrogen have higher stretching
frequencies than those to heavier atoms.
iii) Triple bonds have higher stretching
frequencies than corresponding double bonds,
which in turn have higher frequencies than single
bonds. (Except for bonds to hydrogen).
IR Spectrum of Octane
Decane
2,2,4-Trimethylpentane
Hybridization of Carbon
Affects n of Absorption
Hexane and Hexene
1-Pentene
1-Dodecene
2-Methyl-1,3-butadiene
(Isoprene)
Terminal Alkynes are Readily
Identifiable with IR
1-Heptyne
Aromatics
• C-H str (sp2) at 3050-3100 cm-1
• C-C str (breathing) 1600 & 1500 cm-1
H
Toluene
o-Xylene
Alcohols
O-H str at 3600-3200 cm-1
C-O str at 1050-1150 cm-1
Cyclohexanol
O-H Stretch
2-Methyl-1-butanol
Benzyl alcohol
o
1 Amines
2-Methylpentanediamine
2o Amine
Dipropylamine
Carbonyl Compounds
Acetone
Cycloheptanone
Acetophenone
Conjugation Lowers n of
Absorption
Ketone and Aldehyde
Octanal
Carboxylic Acid
Hexanoic Acid
Phenyl Acetate, an Ester
Ethyl Butanoate
Methyl Benzoate
Benzoic Anhydride, an
Acid Anhydride
Butanamide, an Amide
O
O
CH3CH2OCCH3
O
CH2CH
C N
CH2=CHCH2CH2CH3
HC
CCH2CH2OH
CH3CH2CH2NH2
CH2CH2CH2CH2CO2H