12-12-11William H. Brown • Beloit College

William H. BrownChristopher S. FooteBrent L. IversonEric Anslynhttp://academic.cengage.com/chemistry/brown

Chapter 12Infrared Spectroscopy

12-12-22

Molecular SpectroscopyMolecular Spectroscopy

Molecular spectroscopyMolecular spectroscopy The study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure.• we study three types of molecular spectroscopy

Nuclear spin states

Electronic energy levelsInfrared

Radio frequency

Ultraviolet-visible

Absorption ofElectromagneticRadiation Results in Transition Between

Region of theElectromagneticSpectrum

Nuclear magneticresonanceInfraredUltraviolet-visible

Type of Spectroscopy

Frequency(hetz)

2.5 x1014-1.5x1015

3 x107-9x108

1 x1013-1x1014 Vibrational energy levels

12-12-33

Molecular vibrationsMolecular vibrations Fundamental stretching and bending vibrations

for a methylene group.

12-12-44

Molecular VibrationsMolecular Vibrations For a molecule to absorb IR radiation

• the bond undergoing vibration must be polar andits vibration must cause a periodic change in the bond dipole moment.

Covalent bonds which do not meet these criteria are said to be IR inactive. • The C-C double and triple bonds of symmetrically

substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds.

H3CC C

CH3

H3C CH3

H3C-C C-CH3

2,3-Dimethyl-2-butene2-Butyne

12-12-55

Infrared SpectroscopyInfrared Spectroscopy Infrared spectrum of 3-methyl-2-butanone.

Fingerprint Region:Highly Complex and Unique for Every Molecule

C-H Stretching

12-12-66

Correlation TablesCorrelation Tables Infrared stretching frequencies of selected

functional groups.

C-HN-HO-H

C=OC=C

C-O 1000-1250 strong

weak to medium1600-16801630-1820 strong

2700-3300 weak to medium

weak to strong3200-3650medium3100-3550

IntensityStretchingFrequency (cm-1)Bond

Potentially difficult to distinguish between one-another, but good indication of heteroatom-hydrogen bond

C=C often too weak or encroaching on fingerprint region

More ValuableLess Valuable

Buried in fingerprint region

Almost all organic molecules have C-H bonds

Signature Stretch. Very strong, very identifiable

12-12-77

Carboxylic acidsCarboxylic acids Infrared spectrum of pentanoic acid.

12-12-88

AminesAmines Infrared spectrum of 1-butanamine, a 1° amine.

12-12-99

Hydrocarbons-Table 12.5Hydrocarbons-Table 12.5

Stretching 3300 Medium to strongStretching2100-2250 Weak

Weak to medium1600 - 1680StretchingWeak to medium3000 - 3100Stretching

Alkyne

Alkene

AlkaneVibration

Stretching

Hydro-carbon

Frequency(cm-1) Intensity

2850 - 3000 MediumBending 1450-1475 MediumBending 1375 and 1450Weak to medium

C C

C-H

CH3

C=CC-H

C-H

C-C (Not useful for interpretation - too many bands

AreneStretching 3030 Weak to mediumC-H

Medium1450-1600StretchingC=CBending 690-900 StrongC-H

CH2

Difficult to distinguish between one-another

12-12-1010

AromaticsAromatics Infrared spectrum of toluene.

12-12-1111

Hydrocarbons-Table 12.5Hydrocarbons-Table 12.5

Stretching 3300 Medium to strongStretching2100-2250 Weak

Weak to medium1600 - 1680StretchingWeak to medium3000 - 3100Stretching

Alkyne

Alkene

AlkaneVibration

Stretching

Hydro-carbon

Frequency(cm-1) Intensity

2850 - 3000 MediumBending 1450-1475 MediumBending 1375 and 1450Weak to medium

C C

C-H

CH3

C=CC-H

C-H

C-C (Not useful for interpretation - too many bands

AreneStretching 3030 Weak to mediumC-H

Medium1450-1600StretchingC=CBending 690-900 StrongC-H

CH2

Can be useful. In fairly distinct regions

12-12-1212

AlkynesAlkynes Infrared spectrum of 1-octyne.