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Infrared SpectroscopyInfrared Spectroscopy
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IntroductionIntroduction
Spectroscopy is an analytical technique Spectroscopy is an analytical technique
which helps determine structurewhich helps determine structure
It destroys little or no sampleIt destroys little or no sample
The amount of light absorbed by the The amount of light absorbed by the
sample is measured as wavelength is sample is measured as wavelength is
variedvaried
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Types of SpectroscopyTypes of Spectroscopy
Infrared (IR) spectroscopyInfrared (IR) spectroscopy
– measures the bond vibration frequencies in a molecule and is measures the bond vibration frequencies in a molecule and is
used to determine the functional groupused to determine the functional group
Mass spectrometry (MS)Mass spectrometry (MS)
– fragments the molecule and measures the massesfragments the molecule and measures the masses
Nuclear magnetic resonance (NMR) spectroscopyNuclear magnetic resonance (NMR) spectroscopy
– detects signals from hydrogen atoms and can be used to detects signals from hydrogen atoms and can be used to
distinguish isomers distinguish isomers
Ultraviolet (UV) spectroscopyUltraviolet (UV) spectroscopy
– uses electron transitions to determine bonding patternsuses electron transitions to determine bonding patterns
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Electromagnetic SpectrumElectromagnetic Spectrum
Frequency and wavelength are inversely proportionalFrequency and wavelength are inversely proportional
cc = = , where , where cc is the speed of light is the speed of light
Energy per photon = Energy per photon = hh, where , where hh is Planck’s constant is Planck’s constant
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The Spectrum and Molecular EffectsThe Spectrum and Molecular Effects
=>
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The IR RegionThe IR Region
Just below red in the visible regionJust below red in the visible region
Wavelengths usually 2.5-25 Wavelengths usually 2.5-25 mm
More common units are wavenumbers, or More common units are wavenumbers, or
cmcm-1-1, the reciprocal of the wavelength in , the reciprocal of the wavelength in
centimeters (4000-400 cmcentimeters (4000-400 cm-1-1))
Wavenumbers are proportional to Wavenumbers are proportional to
frequency and energyfrequency and energy
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Molecular VibrationsMolecular VibrationsLight is absorbed when radiation frequency = Light is absorbed when radiation frequency = frequency of vibration in moleculefrequency of vibration in molecule
Covalent bonds vibrate at only certain Covalent bonds vibrate at only certain allowable frequenciesallowable frequencies– Associated with types of bonds and movement of Associated with types of bonds and movement of
atomsatoms
Vibrations include stretching and bendingVibrations include stretching and bending
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IR SpectrumIR Spectrum
No two molecules will give exactly the same IR spectrum No two molecules will give exactly the same IR spectrum (except enantiomers)(except enantiomers)
Simple stretching: 1600-3500 cmSimple stretching: 1600-3500 cm-1-1
Complex vibrations: 400-1400 cmComplex vibrations: 400-1400 cm-1-1, called the , called the “fingerprint region”“fingerprint region”
Baseline
Absorbance/Peak
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InterpretationInterpretation
Looking for presence/absence of functional Looking for presence/absence of functional
groupsgroups
Correlation tablesCorrelation tables
– Wade: Ch. 12 and Appendices 2A and 2BWade: Ch. 12 and Appendices 2A and 2B
– BTC: Chapter 11BTC: Chapter 11
A polar bond is usually IR-activeA polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule A nonpolar bond in a symmetrical molecule
will absorb weakly or not at allwill absorb weakly or not at all
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Carbon-Carbon Bond StretchingCarbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:Stronger bonds absorb at higher frequencies:
– C-C 1200 cmC-C 1200 cm-1-1
– C=C 1660 cmC=C 1660 cm-1-1
– CCC 2200 cmC 2200 cm-1 -1 (weak or absent if internal)(weak or absent if internal)
Conjugation lowers the frequency:Conjugation lowers the frequency:
– isolated C=C isolated C=C 1640-1680 cm1640-1680 cm-1-1
– conjugated C=C conjugated C=C 1620-1640 cm1620-1640 cm-1-1
– aromatic C=C aromatic C=C approx. 1600 cmapprox. 1600 cm-1 -1
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Carbon-Hydrogen StretchingCarbon-Hydrogen Stretching
Bonds with more Bonds with more ss character absorb at a character absorb at a
higher frequencyhigher frequency
– spsp33 C-H, just below 3000 cm C-H, just below 3000 cm-1-1 (to the right) (to the right)
– spsp22 C-H, just above 3000 cm C-H, just above 3000 cm-1-1 (to the left) (to the left)
– spsp C-H, at 3300 cm C-H, at 3300 cm-1 -1
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An Alkane IR SpectrumAn Alkane IR Spectrum
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An Alkene IR SpectrumAn Alkene IR Spectrum
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An Alkyne IR SpectrumAn Alkyne IR Spectrum
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O-H and N-H StretchingO-H and N-H Stretching
Both of these occur around 3300 cmBoth of these occur around 3300 cm-1-1, but , but
they look differentthey look different
– Alcohol O-H, broad with rounded tipAlcohol O-H, broad with rounded tip
– Secondary amine (RSecondary amine (R22NH), broad with one NH), broad with one
sharp spikesharp spike
– Primary amine (RNHPrimary amine (RNH22), broad with two sharp ), broad with two sharp
spikesspikes
– No signal for a tertiary amine (RNo signal for a tertiary amine (R33N)N)
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An Alcohol IR SpectrumAn Alcohol IR Spectrum
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An Amine IR SpectrumAn Amine IR Spectrum
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Carbonyl StretchingCarbonyl Stretching
The C=O bond of simple ketones, aldehydes, The C=O bond of simple ketones, aldehydes,
and carboxylic acids absorb around 1710 cmand carboxylic acids absorb around 1710 cm -1-1
Usually, it’s the strongest IR signalUsually, it’s the strongest IR signal
Carboxylic acids will have O-H alsoCarboxylic acids will have O-H also
Aldehydes have two C-H signals around 2700 Aldehydes have two C-H signals around 2700
and 2800 cmand 2800 cm-1-1
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A Ketone IR SpectrumA Ketone IR Spectrum
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An Aldehyde IR SpectrumAn Aldehyde IR Spectrum
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O-H Stretch of a Carboxylic AcidO-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cmThis O-H absorbs broadly, 2500-3500 cm-1-1, due , due to strong hydrogen bondingto strong hydrogen bonding
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Variations in C=O AbsorptionVariations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching Conjugation of C=O with C=C lowers the stretching
frequency to ~1680 cmfrequency to ~1680 cm-1-1
The C=O group of an amide absorbs at an even lower The C=O group of an amide absorbs at an even lower
frequency, 1640-1680 cmfrequency, 1640-1680 cm-1-1
The C=O of an ester absorbs at a higher frequency, The C=O of an ester absorbs at a higher frequency,
~1730-1740 cm~1730-1740 cm-1-1
Carbonyl groups in small rings (5 C’s or less) absorb at an Carbonyl groups in small rings (5 C’s or less) absorb at an
even higher frequencyeven higher frequency
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An Amide IR SpectrumAn Amide IR Spectrum
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Carbon - Nitrogen StretchingCarbon - Nitrogen Stretching
C - N absorbs around 1200 cmC - N absorbs around 1200 cm-1-1
C = N absorbs around 1660 cmC = N absorbs around 1660 cm-1-1 and is much stronger and is much stronger
than the C = C absorption in the same regionthan the C = C absorption in the same region
C C N absorbs strongly just N absorbs strongly just aboveabove 2200 cm 2200 cm-1-1. The . The
alkyne C alkyne C C signal is much weaker and is just C signal is much weaker and is just belowbelow
2200 cm2200 cm-1 -1
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A Nitrile IR SpectrumA Nitrile IR Spectrum
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Summary of IR AbsorptionsSummary of IR Absorptions
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Strengths and LimitationsStrengths and Limitations
IR alone cannot determine a structureIR alone cannot determine a structure
Some signals may be ambiguousSome signals may be ambiguous
The functional group is usually indicatedThe functional group is usually indicated
The The absenceabsence of a signal is definite proof that of a signal is definite proof that
the functional group is absentthe functional group is absent
Correspondence with a known sample’s IR Correspondence with a known sample’s IR
spectrum confirms the identity of the compoundspectrum confirms the identity of the compound