Infrared Spectroscopy

Infrared Spectroscopy

2

2

Introduction

Introduction

Spectroscopy is an analytical

Spectroscopy is an analytical

technique which helps determine

technique which helps determine

structure

structure

It destroys little or no sample

It 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

varied

varied

3

3

Types of Spectroscopy

Types of Spectroscopy

Infrared (IR) spectroscopy

Infrared (IR) spectroscopy

–

measures the bond vibration frequencies in a molecule

measures the bond vibration frequencies in a molecule

and is used to determine the functional group

and is used to determine the functional group

Mass spectrometry (MS)

Mass spectrometry (MS)

–

fragments the molecule and measures the masses

fragments the molecule and measures the masses

Nuclear magnetic resonance (NMR) spectroscopy

Nuclear 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) spectroscopy

Ultraviolet (UV) spectroscopy

–

uses electron transitions to determine bonding patterns

uses electron transitions to determine bonding patterns

4

4

Electromagnetic Spectrum

Electromagnetic Spectrum

Frequency and wavelength are inversely proportional

Frequency and wavelength are inversely proportional

c

c

=

=





, where

, where

c

c

is the speed of light

is the speed of light

Energy per photon =

Energy per photon =

h

h





, where

, where

h

h

is Planck’s constant

is Planck’s constant

5

5

The Spectrum and Molecular

The Spectrum and Molecular

Effects

Effects

=
>

6

6

The IR Region

The IR Region

Just below red in the visible region

Just below red in the visible region

Wavelengths usually 2.5-25

Wavelengths usually 2.5-25





m

m

More common units are wavenumbers,

More common units are wavenumbers,

or cm

or cm

-1

-1

, the reciprocal of the wavelength

, the reciprocal of the wavelength

in centimeters (4000-400 cm

in centimeters (4000-400 cm

-1

-1

)

)

Wavenumbers are proportional to

Wavenumbers are proportional to

frequency and energy

frequency and energy

7

7

Molecular Vibrations

Molecular Vibrations

Light is absorbed when radiation frequency

Light is absorbed when radiation frequency

= frequency of vibration in molecule

= frequency of vibration in molecule

Covalent bonds vibrate at only certain

Covalent bonds vibrate at only certain

allowable frequencies

allowable frequencies

–

Associated with types of bonds and movement

Associated with types of bonds and movement

of atoms

of atoms

Vibrations include stretching and bending

Vibrations include stretching and bending

8

8

IR Spectrum

IR Spectrum

No two molecules will give exactly the same IR

No two molecules will give exactly the same IR

spectrum (except enantiomers)

spectrum (except enantiomers)

Simple stretching: 1600-3500 cm

Simple stretching: 1600-3500 cm

-1

-1

Complex vibrations: 400-1400 cm

Complex vibrations: 400-1400 cm

-1

-1

, called the

, called the

“fingerprint region”

“fingerprint region”

Baseline

Absorbanc
e/Peak

9

9

Interpretation

Interpretation

Looking for presence/absence of

Looking for presence/absence of

functional groups

functional groups

Correlation tables

Correlation tables

–

Wade: Ch. 12 and Appendices 2A and 2B

Wade: Ch. 12 and Appendices 2A and 2B

–

BTC: Chapter 11

BTC: Chapter 11

A polar bond is usually IR-active

A polar bond is usually IR-active

A nonpolar bond in a symmetrical

A nonpolar bond in a symmetrical

molecule will absorb weakly or not at all

molecule will absorb weakly or not at all

10

10

Carbon-Carbon Bond

Carbon-Carbon Bond

Stretching

Stretching

Stronger bonds absorb at higher frequencies:

Stronger bonds absorb at higher frequencies:

–

C-C 1200 cm

C-C 1200 cm

-1

-1

–

C=C 1660 cm

C=C 1660 cm

-1

-1

–

C

C





C 2200 cm

C 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 cm

1640-1680 cm

-1

-1

–

conjugated C=C

conjugated C=C

1620-1640 cm

1620-1640 cm

-1

-1

–

aromatic C=C

aromatic C=C

approx. 1600 cm

approx. 1600 cm

-1

-1

11

11

Carbon-Hydrogen Stretching

Carbon-Hydrogen Stretching

Bonds with more

Bonds with more

s

s

character absorb

character absorb

at a higher frequency

at a higher frequency

–

sp

sp

3

3

C-H, just below 3000 cm

C-H, just below 3000 cm

-1

-1

(to the

(to the

right)

right)

–

sp

sp

2

2

C-H, just above 3000 cm

C-H, just above 3000 cm

-1

-1

(to the

(to the

left)

left)

–

sp

sp

C-H, at 3300 cm

C-H, at 3300 cm

-1

-1

12

12

An Alkane IR Spectrum

An Alkane IR Spectrum

13

13

An Alkene IR Spectrum

An Alkene IR Spectrum

14

14

An Alkyne IR Spectrum

An Alkyne IR Spectrum

15

15

O-H and N-H Stretching

O-H and N-H Stretching

Both of these occur around 3300 cm

Both of these occur around 3300 cm

-

-

1

1

, but they look different

, but they look different

–

Alcohol O-H, broad with rounded tip

Alcohol O-H, broad with rounded tip

–

Secondary amine (R

Secondary amine (R

2

2

NH), broad with one

NH), broad with one

sharp spike

sharp spike

–

Primary amine (RNH

Primary amine (RNH

2

2

), broad with two

), broad with two

sharp spikes

sharp spikes

–

No signal for a tertiary amine (R

No signal for a tertiary amine (R

3

3

N)

N)

16

16

An Alcohol IR Spectrum

An Alcohol IR Spectrum

17

17

An Amine IR

An Amine IR

Spectrum

Spectrum

18

18

Carbonyl Stretching

Carbonyl Stretching

The C=O bond of simple ketones,

The C=O bond of simple ketones,

aldehydes, and carboxylic acids absorb

aldehydes, and carboxylic acids absorb

around 1710 cm

around 1710 cm

-1

-1

Usually, it’s the strongest IR signal

Usually, it’s the strongest IR signal

Carboxylic acids will have O-H also

Carboxylic acids will have O-H also

Aldehydes have two C-H signals around

Aldehydes have two C-H signals around

2700 and 2800 cm

2700 and 2800 cm

-1

-1

19

19

A Ketone IR

A Ketone IR

Spectrum

Spectrum

20

20

An Aldehyde IR Spectrum

An Aldehyde IR Spectrum

21

21

O-H Stretch of a Carboxylic

O-H Stretch of a Carboxylic

Acid

Acid

This O-H absorbs broadly, 2500-3500 cm

This O-H absorbs broadly, 2500-3500 cm

-

-

1

1

, due to strong hydrogen bonding

, due to strong hydrogen bonding

22

22

Variations in C=O Absorption

Variations 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 cm

frequency to ~1680 cm

-1

-1

The C=O group of an amide absorbs at an even

The C=O group of an amide absorbs at an even

lower frequency, 1640-1680 cm

lower frequency, 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)

Carbonyl groups in small rings (5 C’s or less)

absorb at an even higher frequency

absorb at an even higher frequency

23

23

An Amide IR

An Amide IR

Spectrum

Spectrum

24

24

Carbon - Nitrogen

Carbon - Nitrogen

Stretching

Stretching

C - N absorbs around 1200 cm

C - N absorbs around 1200 cm

-1

-1

C = N absorbs around 1660 cm

C = N absorbs around 1660 cm

-1

-1

and is much

and is much

stronger than the C = C absorption in the same

stronger than the C = C absorption in the same

region

region

C

C





N absorbs strongly just

N absorbs strongly just

above

above

2200 cm

2200 cm

-1

-1

.

.

The alkyne C

The alkyne C





C signal is much weaker and is

C signal is much weaker and is

just

just

below

below

2200 cm

2200 cm

-1

-1

25

25

A Nitrile IR Spectrum

A Nitrile IR Spectrum

26

26

Summary of IR Absorptions

Summary of IR Absorptions

27

27

Strengths and Limitations

Strengths and Limitations

IR alone cannot determine a structure

IR alone cannot determine a structure

Some signals may be ambiguous

Some signals may be ambiguous

The functional group is usually indicated

The functional group is usually indicated

The

The

absence

absence

of a signal is definite proof

of a signal is definite proof

that the functional group is absent

that the functional group is absent

Correspondence with a known sample’s IR

Correspondence with a known sample’s IR

spectrum confirms the identity of the

spectrum confirms the identity of the

compound

compound