INFRARED SPECTROSCOPY

Electromagnetic Spectrum

Bond Stretching

High Frequency StretchingLow Frequency Bending

Frequency of Absorption Measured in cm-1 (Wavenumbers)

Wavelength units typically microns (m)

10,000 m = 1 cm or 1m = 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 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

AlcoholsO-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

1o Amines

2-Methylpentanediamine

2o Amine

Dipropylamine

Carbonyl Compounds

Acetone

Cycloheptanone

Acetophenone

Conjugation Lowers 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

CH3CH2OCCH3

O

CH2CH

O

C N

CH2=CHCH2CH2CH3

HC CCH2CH2OH

CH3CH2CH2NH2

CH2CH2CH2CH2CO2H