Infrared Spectroscopy - University of Texas at Austincolapret.cm.utexas.edu/courses/Chapter 12.pdf• Electromagnetic radiation: light and other forms of radiant energy ... Organic

Organic Lecture Series

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InfraredInfrared

SpectroscopySpectroscopy(Chapter 12)(Chapter 12)


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This reaction from Ochem 1

How do we know if it worked? The reactant is cyclohexene; the product is cyclohexanol. How can we tell the difference?

Infrared Spectroscopy (IR) is Infrared Spectroscopy (IR) is an easy way to do it.an easy way to do it.


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•• Electromagnetic radiation:Electromagnetic radiation: light and other forms of radiant energy

•• Wavelength (Wavelength (λλ):): the distance between consecutive peaks on a wave

•• Frequency (Frequency (νν):): the number of full cycles of a wave that pass a given point in a second

•• Hertz (Hz):Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”)

Electromagnetic Radiation


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Electromagnetic Radiation

• Common units used to express wavelength λ

Angstrom (Å) 1 Å = 10-10 m

Relationto MeterUnit

1 mm = 10-3 m

1 nm = 10-9 m1 μm = 10-6 m

Nanometer (nm)Micrometer (μm)

Millimeter (mm)Meter (m) ----

E = hνE is kJ/mol

h= 3.99 X 10-13 kJ s mol-1

ν = frequency in Hz

c = λν


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E=hν


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Infrared Spectroscopy• The vibrational IR extends from 2.5 x 10-6 m (2.5

μm) to 2.5 x 10-5 m (25 μm)– the frequency of IR radiation is commonly expressed in

wavenumbers

–– Wavenumber Wavenumber νν:: the number of waves per centimeter, with units cm-1 (read reciprocal centimeters)

– expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1

ν = = 400 cm-1

= 4000 cm-1

ν = 10-2 m•cm -1

2.5 x 10-6 m

10-2 m•cm -1

2.5 x 10-5 m

25 μm to 2.5 μm


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Sections of an IR Spectrum

This is the most common scale.


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• IR spectrum of 3-methyl-2-butanone

Strong absorption


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• IR spectrum of 3-methyl-2-butanone

C=OStretch

C-HStretch


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Molecular Vibrations–atoms joined by covalent bonds undergo continual

vibrations relative to each other

–the energies associated with these vibrations are

quantized; within a molecule, only specific

vibrational energy levels are allowed

–the energies associated with transitions between

vibrational energy levels correspond to

frequencies in the infrared region:

4000 to 400 cm-1


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• For a molecule to absorb IR radiation– the bond undergoing vibration must be polar and

– its 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 polarized bonds

H3 C

C C

CH3

H3 C CH3

H3 C-C C-CH3

2,3-Dimethyl-2-butene 2-Butyne


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Molecular Vibrations

• Consider two covalently bonded atoms as two vibrating masses connected by a spring– the total energy is proportional to the frequency of vibration

– the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring

K = a force constant, which is a measure of the bonds’strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cmμ = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams

Kν = 4.12 μ


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The simplest vibrational motions are bending and stretching.

Here are the fundamental stretching and bending vibrations for a methylene group:

http://en.wikipedia.org/wiki/Infrared_spectroscopy


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Symmetric Asymmetric Scissoring

Rotation Wagging Twisting


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Molecular Vibrations

• From this equation, we see that the positionposition(i.e. wavenumber) of a stretching vibration:– is proportional to the strength of the vibrating bond– is inversely proportional the masses of the atoms

connected by the bond

• The intensity intensity (i.e. weak, s, m) of absorption depends primarily on the polarity of the vibrating bond

Kν = 4.12 μ


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Correlation Tables

Table 12.4Table 12.4 Characteristic IR absorptions for the types of bonds and functional groups encountered most often:

C-H

N-H

O-H

C=OC=C

C-O 1000-1250 strong

weak to medium1600-16801630-1820 strong

2700-3300 weak to medium

weak to s trong3200-3650

medium3100-3550

Intens ity

Stretching

Frequency (cm-1)Bond


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Hydrocarbons-Table 12.5

C-H

CH3

C=CC-H

C-HC C

C-C

C-H

C=CC-H

Intensity

2850 - 3000 Medium

Stretching 3300 Medium to strongStretching 2100-2250 Weak

Weak to medium1600 - 1680StretchingWeak to medium3000 - 3100Stretching

Alkyne

Alkene

Alkane

Vibration

Stretching

Hydro-carbon

Frequency(cm-1)

Bending 1450-1475 Medium

Bending 1375 and 1450 Weak to medium

(Not useful for interpretation - too many bands

AreneStretching 3030 Weak to medium

Medium1450-1600StretchingBending 690-900 Strong


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Alkanes

• IR spectrum of decane (Fig 12.4)

3000


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Alkenes

• IR spectrum of cyclohexene (Fig 12.5)

3000


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Alkynes

• IR spectrum of 1-octyne (Fig 12.6)

3000


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Aromatics

• IR spectrum of toluene (Fig 12.7)

3000


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Alcohols

O-H (free)

O-H (H bonded)

C-O

Bond IntensityFrequency, cm-1

Medium1000 - 1250

Medium, broad3200 - 3500

3600-3650 Weak


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Effect of Concentration Upon Hydrogen Bonding


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Effect of Concentration Upon Hydrogen Bonding


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Ethers

• IR spectrum of dibutyl ether (Fig 12.9)

3000


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• IR spectrum of anisole (Fig 12.10)

Ethers

3000


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Amines

• IR spectrum of 1-butanamine (Fig 12.11)


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IR of Molecules with C=O Groups

C=O

C=O

C-H

O HRCOHO

RCHO

RCR'O

C=O

Strong1700-1725StretchingCarboxylic acids

Aldehydes

VibrationCarbonyl GroupFrequency

(cm-1) Intensity

Stretching 1630-1820 StrongStretching 2720 Weak

Stretching 2500-3300 Strong (broad)

KetonesStretching 1630-1820 Strong


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C=O

sp3 C O

sp2 C O

C=O

C=O

N H

C≡NRC N

RCOCRO O

RCOR'O

RCNH2

O

C O Stretching 900-1300 Strong

Strong1735-1800Stretching

Carboxylic esters

Stretching 1000-1100 Strong

Stretching 1200-1250 Strong

Acid anhydrides

Stretching 1740-1760 and1800-1850

Strong

Strong1630-1680StretchingAmides

Stretching 3200, 3400 Medium(1° amides have two N-H stretches)(2° amides have one N-H stretch)

NitrilesStretching 2200-2250 Medium

IR of Molecules with C=O Groups


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Aldehydes and KetonesAldehydes and Ketones

• IR spectrum of menthone (Fig 12.12)


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Carboxylic acidsCarboxylic acids

• IR spectrum of pentanoic acid (Fig 12.13)


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AmideAmide

• IR of N-methylpropanamide (Fig 12.14)


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EstersEsters

• IR of Ethyl butanoate (Fig 12.15)


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Strategies for IR Interpretation Strategies for IR Interpretation

Inspect the spectrum from left to right. If there is a strong, but broad band 3500 cm-1

then, OH is present. One or two weak peaks in this area are indicative of amines (N—H stretch).

Examine the 3000 cm-1 C—H aliphatic stretches are to the right and C—H from alkenes & aromatics are to the left.

Aldehyde C—H stretch will be ~ 2720 cm-1

Check the area from 1820 to 1630 cm-1. Strong peaks in this area indicate C=O and this is often the strongest peak in the spectrum.

The area from 1250 to 1000 cm-1 are the C—O stretches of ethers, esters, acids.

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Infrared Spectroscopy - University of Texas at Austincolapret.cm.utexas.edu/courses/Chapter 12.pdf• Electromagnetic radiation: light and other forms of radiant energy ... Organic