Infrared SpectroscopyInfrared Spectroscopy

• Near Infrared: Near Infrared: 770 to 2500 nm770 to 2500 nm

12,900 to 4000 cm12,900 to 4000 cm-1-1

• Mid Infrared: Mid Infrared: 2500 to 50,000 nm (2.5 to 50 2500 to 50,000 nm (2.5 to 50 m)m)

4000 to 200 cm4000 to 200 cm-1-1

• Far Infrared: Far Infrared: 50 to 1000 50 to 1000 mm

200 to 10 cm200 to 10 cm-1-1

Infrared Spectroscopy: Vibrational ModesInfrared Spectroscopy: Vibrational Modes

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Pretsch/Buhlmann/Affolter/Pretsch/Buhlmann/Affolter/Badertscher, Badertscher, Structure Structure Determination of Organic Determination of Organic CompoundsCompounds

Group FrequenciesGroup Frequencies

2/1k

2

1

c

Estimate band location:

Are you getting the concept?Are you getting the concept?

Estimate the stretching vibrational frequency for a carbonyl Estimate the stretching vibrational frequency for a carbonyl group with a force constant, k, of 12 N/cm. If a C=S bondgroup with a force constant, k, of 12 N/cm. If a C=S bondhad the same force constant, where would its stretchinghad the same force constant, where would its stretchingband appear in the infrared absorption spectrum?band appear in the infrared absorption spectrum?

Recall:1 amu = 1.6605 x 10-27 kg1N = 1 kg*m*s-2

Atomic masses

C → 12.000 amuO → 15.995 amuS → 31.972 amu

Infrared SpectroscopyInfrared Spectroscopy

• Near Infrared: Near Infrared: 770 to 2500 nm770 to 2500 nm

12,900 to 4000 cm12,900 to 4000 cm-1-1

** OvertonesOvertones

* Combination tones* Combination tones

* Useful for quantitative measurements* Useful for quantitative measurements

• Mid Infrared: Mid Infrared: 2500 to 50,000 nm (2.5 to 50 um)2500 to 50,000 nm (2.5 to 50 um)

4000 to 200 cm4000 to 200 cm-1-1

** Fundamental vibrationsFundamental vibrations

* Fingerprint region 1300 to 400 cm* Fingerprint region 1300 to 400 cm-1-1

(characteristic for molecule as a whole)(characteristic for molecule as a whole)

• Far Infrared: Far Infrared: 2.5 to 1000 um2.5 to 1000 um

200 to 10 cm200 to 10 cm-1-1

** Fundamental vibrations of bonds with heavyFundamental vibrations of bonds with heavy

atoms (useful, e.g., for organometallics) atoms (useful, e.g., for organometallics)

Example of an OvertoneExample of an Overtone

• Wagging vibration at 920 cmWagging vibration at 920 cm-1-1..• Overtone at approximately 2 x 920 cmOvertone at approximately 2 x 920 cm-1 -1 = 1840 cm= 1840 cm-1-1.. H

H

H3C OH

H

Fermi ResonanceFermi Resonance

N.B. Colthup et al., N.B. Colthup et al., Introduction to Infrared and Raman Spectroscopy, Introduction to Infrared and Raman Spectroscopy, Academic Press, Academic Press, Boston, 1990.Boston, 1990.

Example of a Fermi ResonanceExample of a Fermi Resonance

• Stretching vibration of C-C=(O) at 875 cmStretching vibration of C-C=(O) at 875 cm-1-1..• Overtone at approximately 2 x 875 cmOvertone at approximately 2 x 875 cm-1 -1 = 1750 cm= 1750 cm-1-1

coincides with C=O stretchcoincides with C=O stretch

ClO

Cl

Light Source: GlobarLight Source: Globar

Silicon Carbide Rod (5mm diameter, 50 mm long)Silicon Carbide Rod (5mm diameter, 50 mm long)

Heated electrically to 1300 – 1500 KHeated electrically to 1300 – 1500 K

Positive temperature coefficient of resistancePositive temperature coefficient of resistance

Electrical contact must be water cooled to prevent arcingElectrical contact must be water cooled to prevent arcing

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Sample Preparation for IR SpectroscopySample Preparation for IR Spectroscopy

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Liquid Samples: Cell ThicknessLiquid Samples: Cell Thickness

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

2

nb

Window and Cell MaterialsWindow and Cell Materials

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

SolventsSolvents

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

Suspension Media for Solid SamplesSuspension Media for Solid Samples

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

InterferencesInterferences

Pretsch/Buhlmann/Affolter/Pretsch/Buhlmann/Affolter/Badertscher, Badertscher, Structure Structure Determination of Organic Determination of Organic CompoundsCompounds