Infrared Spectroscopy Infrared Spectroscopy • Near Infrared: Near Infrared: 770 to 2500 nm 770 to 2500 nm 12,900 to 4000 cm 12,900 to 4000 cm -1 -1 • Mid Infrared: Mid Infrared: 2500 to 50,000 nm (2.5 2500 to 50,000 nm (2.5 to 50 to 50 m) m) 4000 to 200 cm 4000 to 200 cm -1 -1 • Far Infrared: Far Infrared: 50 to 1000 50 to 1000 m 200 to 10 cm 200 to 10 cm -1 -1
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Infrared Spectroscopy Near Infrared: 770 to 2500 nm Near Infrared: 770 to 2500 nm 12,900 to 4000 cm -1 Mid Infrared: 2500 to 50,000 nm (2.5 to 50 m)
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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
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