Infrared Spectroscopy. 2 Introduction Spectroscopy is an analytical technique which helps determine structure It destroys little or no sample The amount.

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