Infrared Spectroscopy

紅外線光譜儀

Chapter 12 2

Introduction

• Spectroscopy is an analytical technique

which helps determine structure.

• It destroys little or no sample (non-

destructive method).

• The amount of light absorbed by the

sample is measured as wavelength is

varied.

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Chapter 12 3

Types of Spectroscopy

• Infrared (IR) spectroscopy measures the bond

vibration frequencies in a molecule and is used

to determine the functional group.

• Mass spectrometry (MS) fragments the molecule

and measures the masses.

• Nuclear magnetic resonance (NMR)

spectroscopy detects signals from hydrogen

atoms and can be used to distinguish isomers.

• Ultraviolet (UV) spectroscopy uses electron

transitions to determine bonding patterns. =>

Chapter 12 4

Electromagnetic Spectrum• Examples: X rays, microwaves, radio waves, visible light, IR, and UV.

• Electromagnetic radiation has the characteristics of both waves and particles

• The wave nature of electromagnetic radiation is described by wavelength (l) or frequency (n)

• The relationship between wavelength (or frequency) and energy (E) is well defined

• Wavelength and frequency are inversely proportional (n= c/l)

• The higher the frequency, the greater the energy of the wave

• The shorter the wavelength, the greater the energy of the wave

Chapter 12 5

The Spectrum and

Molecular Effects

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Chapter 12 6

The IR Region

• Just below red in the visible region.

• Wavelengths usually 2.5-25 mm.

• More common units are wavenumbers, or

cm-1, the reciprocal of the wavelength in

centimeters.

• Wavenumbers are proportional to frequency

and energy. =>

Chapter 12 7

Molecular Vibrations

Covalent bonds vibrate at only certain

allowable frequencies.

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Chapter 128

Stretching Frequencies

• Frequency decreases with increasing atomic weight.

• Frequency increases with increasing

bond energy. =>

Chapter 12 9

Chapter 12 10

Vibrational Modes

Nonlinear molecule with n atoms usually has

3n - 6 fundamental vibrational modes.

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Chapter 12 11

Chapter 12 12

Fingerprint of Molecule• No two molecules will give exactly the

same IR spectrum (except enantiomers).

• Simple stretching: 1600-3500 cm-1has the most common vibrations, and we can use it to get information about specific functional groups in the molecule.

Complex vibrations (bending): 600-

1400 cm-1, called the “fingerprint region”

and has the most complex vibrations.

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Chapter 12 13

IR-Active and Inactive• A polar bond is usually IR-active.

• A nonpolar bond in a symmetrical

molecule will absorb weakly or not at all.

Chapter 12 14

Chapter 12 15

An Infrared Spectrometer

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FT–IR Spectrometer

• Has better sensitivity.

• Less energy is

needed from source.

• Completes a scan in

1 to 2 seconds.

• Takes several scans

and averages them.

• Has a laser beam that

keeps the instrument

accurately calibrated.

Chapter 12 17

Carbon-Carbon (C-C)

Bond Stretching

• Stronger bonds absorb at higher frequencies:

C-C 1200 cm-1

C=C 1660 cm-1

CC 2200 cm-1 (weak or absent if internal)

• 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 =>

Chapter 12 18

Chapter 12 19

Carbon-Hydrogen (C-H)

Stretching

Bonds with more s character absorb at a

higher frequency.

sp3 C-H, just below 3000 cm-1 (to the right)

sp2 C-H, just above 3000 cm-1 (to the left)

sp C-H, at 3300 cm-1

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Chapter 12 20

Examples

Chapter 12 21

An Alkane IR Spectrum

n >3000; d 1465, 1375

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Chapter 12 22

An Alkene IR Spectrum

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Chapter 12 23

An Alkyne IR Spectrum

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Chapter 12 24

O-H and N-H Stretching

• Both of these occur around 3300 cm-1,

but they look different.

Alcohol O-H, broad with rounded tip.

Secondary amine (R2NH), broad with one

sharp spike.

Primary amine (RNH2), broad with two

sharp spikes.

No signal for a tertiary amine (R3N) =>

Chapter 12 25

Chapter 12 26

Chapter 12 27

• The O-H stretching absorption is very characteristic

– In very dilute solutions, hydrogen bonding is absent and there is a

very sharp peak at 3590-3650 cm-1

– In more concentrated solutions, the hydroxyl groups hydrogen bond

to each other and a very broad and large peak occurs at 3200-

3550 cm-1

Chapter 12 28

An Alcohol IR Spectrum

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Chapter 12 29

An Amine

IR Spectrum

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Chapter 12 30

Carbonyl(n C=O) Stretching

• The C=O bond of simple ketones,

aldehydes, and carboxylic acids absorb

around n 1710 cm-1.• =>

Chapter 12 31

• Usually, it’s the strongest IR signal.

• Carboxylic acids will have O-H also.

• Aldehydes have two C-H signals around

2700 and 2800 cm-1.

Chapter 12 32

A Ketone

IR Spectrum

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Chapter 12 33

An Aldehyde (n HC=O)

IR Spectrum

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Chapter 12 34

O-H Stretch of a

Carboxylic AcidThis O-H absorbs broadly, 2500-3500 cm-1,

due to strong hydrogen bonding.

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Chapter 12 35

Variations in

C=O Absorption

• Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1.

• The C=O group of an amide absorbs at an even lower frequency, 1640-1680 cm-1.

• The C=O of an ester absorbs at a higher frequency, ~1730-1740 cm-1.

• Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency. =>

Chapter 12 36

Chapter 12 37

習題

• Portions of the infrared spectra of three cyclic ketones and

three exocyclic alkenes show the influence of ring strain on

the C=O and C=C stretching frequency. Please indicate

your combination and explain your reasons.

• CH2H2C

CH2

OO

O

Chapter 12 38

習題• The C=O vibration frequencies are varied to different

molecules which reveals the influence of conjugation

and other factors. Show the order of the carbonyl

absorption of the following molecules and explain

your reasons for full credits.

O

O

O

OOH

O

O

CH3

O

CH3

OO CH3

O

(I) (II) (III)

(IV)

(V)

Chapter 12 39

An Amide

IR Spectrum

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Chapter 12 40

習題

• The absorption of carbonyl group in amide is usually in

the range from 1680 to 1630 cm-1. However, the

following compound has the C=O band appearing about

1700cm-1. Please explain this observation.

•

N

O

C=O:1700cm-1

Chapter 12 41

Carbon - Nitrogen

Stretching (C~N)

• C - N absorbs around 1200 cm-1.

• C = N absorbs around 1660 cm-1 and is

much stronger than the C = C

absorption in the same region.

• C N absorbs strongly just above 2200

cm-1. The alkyne C C signal is much

weaker and is just below 2200 cm-1 .

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Chapter 12 42

A Nitrile

IR Spectrum

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Chapter 12 43

Chapter 12 44

Summary of IR Absorptions

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Chapter 12 45

TODAY’S CHEMIST AT WORK

Chapter 12 46

• In addition to the physical symptoms, diseases cause changes in the chemical composition of the organs,tissues, or fluids they affect; these differences are the basis of everyday clinical chemical tests, tissue staining, and medical imaging techniques.

• IR spectroscopy not only probes the chemical composition

of a sample but also determines the precise position and

amplitude of IR absorption bands that reflect interactions

among the matrix constituents. Because of its sensitivity to

both molecular structure and molecular interactions, the

spectrum is often referred to as a molecular fingerprint of the

sample;

• the specificity of that fingerprint is the basis for biomedical applications.

Chapter 12 47

Strengths and Limitations

• IR alone cannot determine a structure.

• Some signals may be ambiguous.

• The functional group is usually indicated.

• The absence of a signal is definite proof

that the functional group is absent.

• Correspondence with a known sample’s

IR spectrum confirms the identity of the

compound. =>