Infrared Absorption Spectroscopy

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Infrared Absorption

Spectroscopy

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Infrared Spectroscopy

** Provides information about the vibrations of

functional groups in a molecule, so organic chemists use IR spectroscopy to identify functional groups in a molecule

**Functional groups vibrate in a characteristic way, So when 2 atoms are bonded to each other,

covalent Bonds are static, they are like spring with weights at the ends

** IR cause changes in the vibrational motions of the molecule

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Molecular Vibrations Modes

Covalent bonds vibrate at only certain allowable frequencies.

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1) Stretching Vibrations of a CH₂ Group

Symmetric Antisymmetric

Stretch & contracts At the same time

One C-H stretches while the other contracts

Absorption between 2 atoms increases or decreases but the atoms remain On the same axis. Needs high energy

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Symmetrical

Stretching Asymmetrical Stretching

Stretching Vibrations

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2) Bending Vibrations of a CH₂ Group

Out of plane Out of plane

Needs less energy, the position of the atoms change relative to the original axis

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Wagging Twisting

Bending Vibrations

out-of-plane

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Bending Vibrations of a CH₂ Group

In plane In plane

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Scissoring Rocking

Bending Vibrations

In plane

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Stretching and Bending

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Two important parameters in all IR spectra:

1-The frequency of the signal,  which is represented using wavenumber ^- = 1/λ cm^-1 ^- α  α E

IR absorption range occurs from 4000 cm^-1 -600cm^-1 2-The intensity of the signal, I

Theory : when a molecule absorbs energy in the IR region,

the E absorbed (Δ E) causes the covalent bonds to change from a lower vibrational energy level (E₁) to a higher one (E₂)

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Electromagnetic spectrum

X-RAY

0,2 nm ULTRA-VIOLET

2 nm VISIBLE 400-800 nm

INFRARED

MICROVAWE

3 mm-20 cm RADIO 10 m-30 Km

FAR MID

NEAR

ʎ, cm (wavelength)

ʎ, cm^-1 (wavenumber) 12820 to 4000 4000 to 400

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The Spectrum and Molecular Effects

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IR Spectrum

Finger print region

-600-1400 cm^-1

-Unique for each compound -bending & stretching vibration

Functional group region -1400-4000 cm^-1

-funtional groups -stretching only

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Notes

-Not all bonds are available to absorb IR energy.

-Only bonds with dipole moments (difference in E.N)

--IR inactive e.g. H

₂

, H

₃

C- C≡C-CH

₃

No dipole moment, nonpolar compound, no IR absorption.

--IR active e.g. H-F, difference in E.N, dipole

moment, IR absorption.

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Wave number, cm^-1

% Transmittance

Strong peak

Weak peak

Peaks are inverted Intensity of IR “band”

100%

0% 4000 600

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IR Spectrum Record

– Wave number (frequency) location of IR “band”

(position of max %T) – Intensity of IR “band”

s = strong (low %T), m = medium, w = weak (high %T) Follows change in dipole caused by vibrating atoms -Polar bonds (strong bond dipoles) gives strong peaks – O–H

– C=O, C≡N – C–O

Nonpolar bonds gives weak peaks – C≡C, C=C

Wave number, cm^-1 for Stretching > Bending > Wagging/Twisting

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Factors affecting the absorption Frequencies (Wave number ) 1-bond strength

2-mass of the bonded atom 3-hydrogen bond

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1-bond strength

– C≡C-H › C=C H › C-C H

Increase bond strength

Increase frequency

The stronger the bond the higher the  & ^-

sp shorter

^- = 3300 cm^-1

sp2, longer

^- = 3100 cm^-1

sp3, longest

^- = 2900 cm^-1

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2-As the mass of one of the bonded atom decrease the frequency increases

C-H › C-C › C-O

Increase frequency Decrease mass

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3-hydrogen bond

The presence of hydrogen bond

decrease frequency (broad peak)

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Identifying a Molecule Each peak represent a bond of a functional group

IR spectra tell you whether a group is present, or not, it will NOT tell you how many groups or how large the molecule is.

Thus a sharp peak at 1750cm will tell us that

there is a carbonyl group (ie ketone,ester,acid,

^etc

) but it will NOT tell us if there are two or three

ketone groups present.

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Applications of IR spectroscopy 1- Determination of mobile polluants:

-detect CO, at 2170,

-detect hydrocarbons at 3000- 2850 cm

^-1

2- Determination of ethanol in blood (OH at 3400 cm

^-1

)

diffusion of blood alcohol through lung lung

occurs into the blow of breath which is subject to IR Spectrophotometer in the OH region.

The test determines the % alcohol in blood

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Fingerprint of Molecule

• Whole-molecule vibrations and bending vibrations are also quantitized.

• No two molecules will give exactly the same IR spectrum (except enantiomers).

• Functional group region: 1600-3500 cm

^-1

.

• Complex vibrations: 600-1400 cm

^-1

, called the “fingerprint region.”

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The general 4 regions of the infrared spectrum in which various kinds of vibrational bands are observed are outlined in the following chart.

fingerprint region 1450 to 600 cm^-1 group frequency region.

4000 to 1450 cm^-1

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2000

3500 3000 2500 1500 1000 500

Wave number, cm^-1

Infrared Spectrum of Hexane

CH₃CH₂CH₂CH₂CH₂CH₃

C—H stretching

bending bending

bending

1460 1380 725

2850-2960 cm^-1 sat’d C-H

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Stretching Vibrations Functional

Class Range (cm^-1) Assignment Alkanes 2850-3000 CH₃, CH₂ & CH

2 or 3 bands Alkenes 3020-3100

1630-1680

=C-H & =CH₂ (sharp) C=C

Alkynes 3300

2100-2250

C-H (usually sharp) C≡C (symmetry

reduces intensity) Arenes 3030

1600 & 1500

C-H (may be several bands)

C=C (in ring) (2 bands) (3 if conjugated)

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Carbon-Carbon Bond Stretching

• Conjugation lowers the frequency:

– isolated C=C 1640-1680 cm^-1 – conjugated C=C 1620-1640 cm^-1

– aromatic C=C approx. 1600 cm-1

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Carbon-Oxygen Stretching

C—O 1025-1200 1050

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Differentiate 1-Hexene and Hexane using IR spectroscopy

2850-2960 cm^-1 sat’d C-H

2850-2980 cm^-1 sat’d C-H

3020-3080 cm

^-1

unsat’d C-H

---

1640-1680 C=C

---

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2000

3500 3000 2500 1500 1000 500

Wave number, cm^-1

Infrared Spectrum of Hexane

CH₃CH₂CH₂CH₂CH₂CH₃

C—H stretching

bending bending

bending

1460 1380 725

2850-2960 cm^-1 sat’d C-H

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2000

3500 3000 2500 1500 1000 500

Wave number, cm^-1

Infrared Spectrum of 1-Hexene

H₂C=CHCH₂CH₂CH₂CH₃ C=C—H H—C

C=C

H₂C=C

1640-1680 3020-

3080

cm^-1 910-920 &

990-1000

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n-Hexane: CH₃CH₂CH₂CH₂CH₂CH₃

1-Hexene: CH₂=CHCH₂CH₂CH₂CH₃

1650 cm^-1 3100 cm^-1

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Differentiate 1-Octyne and 4-Octyne using IR spectroscopy

≡C-H 3313 cm

^-1

C≡C 2119 cm-1

---

no dipole moment IR inactive

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An Alkyne IR Spectrum

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2000

3500 3000 2500 1500 1000 500

Wave number, cm^-1

Infrared Spectrum of tert-butylbenzene

H—C Ar—H

Monsubstituted benzene C₆H₅C(CH₃)₃

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