9/18/2014

1

INFRARED SPECTROSCOPY: Overview

• The Infrared absorption process

• Bond properties and absorption trends

• The modes of stretching and bending

• The Infrared spectrometer

• Sample preparation

• Examining IR spectra, Correlation charts

• Survey of important functional groups

Basiese IR Absorpsie Bande

O-H 3500 cm-1 CC 2150 cm-1

N-H 3300 cm-1 C=O 1715 cm-1

C-H 3000 cm-1 C=C 1650 cm-1

CN 2250 cm-1 C-O 1100 cm-1

Jy moet hierdie basiese IR absorpsie bande ken

9/18/2014

2

Symmetric stretch Scissoring Wagging

(2853 cm-1) (1450 cm-1) (1250 cm-1)

Asymmetric stretch Rocking Twisting

(2926 cm-1) (720 cm-1) (1250 cm-1)

STRETCHING BENDING VIBRATIONS

VIBRATIONS

H

H

H

H

H

H

H

H

H H

H H

The modes of stretching and bending

OC HC-HStretching: Bending:

In-plane

out-of-plane

Symmetric Stretch Asymmetric Stretch

Methyl

Anhydride

Amino

Nitro

H

HH

H

HH

O

O O

O

O O

NH

HN

H

H

NO

ON

O

O

~ 2 8 7 2 c m - 1 ~ 2 9 6 2 c m - 1

~ 1 7 6 0 c m - 1 ~ 1 8 0 0 c m - 1

~ 3 3 0 0 c m - 1~ 3 4 0 0 c m - 1

~ 1 3 5 0 c m - 1 ~ 1 5 5 0 c m - 1

Symmetric and asymmetric stretching

9/18/2014

3

Fundamental absorptions: stretching and bending (ground

vibrational state to the first (lowest) vibrational level excited state)

Overtones: excitation to higher energy states (v¯ x 2,3…)

Combination bands: v¯ comb = v¯1 + v¯ 2

Difference bands: v¯ diff = v¯1 - v¯ 2

Rotational coupling (fine structure)

Additional absorptions

BOND PROPERTIES

= 1/2(K/)1/2

= frequency (sec-1)

= reduced mass = m1.m2 /(m1+m2)

K = force constant of the bond in dynes/cm (1 dyne = 1.02x10-3g)

-------------------------

M1 M2

For stronger force constants (stronger bonds), higher frequencies are required

C≡C C=C C-C

2150cm-1 1650cm-1 1200cm-1

Hooke’s law for vibrating

springs

9/18/2014

4

Absorption frequencies

= 1/2(K/)1/2

¯ = 1/2c(K/)1/2

-------------------------

M1 M2

For larger atoms, the absorption frequency decreases (larger reduced mass):

C-H C-C C-Cl

3000cm-1 1200cm-1 750cm-1

Bending occurs at lower frequency than stretching (force constant):

C-H stretch C-H bend

3000cm-1 1340cm-1

Bonds are stronger in the order sp>sp2>sp3:

≡C-H =C-H -C-H

3300cm-1 3100cm-1 2900cm-1

SAMPLE PREPARATION

Glass and plastic absorb in IR region, can’t be used for cell. Ionic substances used (NaCl, KBr)

Solid Samples• Mull techniques (suspend in mineral oil

between windows)• Pellet method (prepared in pellet with KBr

under P)• Film Techniques (solid films)

Liquid Samples• Direct liquid in cell• Dissolved in suitable apolar solvent (CCl4)

9/18/2014

5

Intepretation of IR spectra: What to look for

1. STRONG ABSORPTIONS

2. SHAPE AND INTENSITY

OF PEAKS

3. CORRELATION CHARTS

AND TABLES

Examples

O-H stretch (one broad absorption band ca. 3300cm-1)

N-H stretch (two sharp absorption bands ca. 3400 & 3300cm-1)

Comparison between intensities of C=O and C=C stretch absorption bands

9/18/2014

6

Analysis of IR spectrum

1. Check for C=O stretch (~1700)

2. If present – acid (OH stretch), amide (NH stretch), ester (C-O stretch), aldehyde (weak C-H~2850), ketone (none of above)

3. No C=O: alcohols (phenols, O-H), amines (N-H), ethers (C-O)

4. Double bonds and/or aromatic rings (1600-1450, C-H above3000)

5. Triple bonds (C≡C (w), C≡N (m), ≡C-H 3300)

6. Nitro groups (1630-1530, 1390-1300)

7. Hydrocarbons (simple, none of above, C-H < 3000)

Voorbeelde

3-methyl-2-butanone

para-cresol

9/18/2014

7

Alkanes

C-H Stretch occurs around 3000 cm-1.

In alkanes (except strained ring compounds), sp3 C-H absorption always occurs at frequencies less than 3000 cm-1.

If a compound has vinylic, aromatic, acetylenic or cyclopropylhydrogens, the C-H absorption is greater than 3000 cm-1. These compounds have sp2 and sp hybridizations.

CH2 Methylene groups have a characteristic bending absorption of approximately 1465 cm-1.

CH3 Methyl groups have a characteristic bending absorption of approximately 1375 cm-1.

CH2 The bending (rocking) motion associated with four or more CH2

groups in an open chain occurs at about 720 cm-1 (long chain band).

Alkanes

Examples: Decane, Mineral oil and Cyclohexane

sp3 C-H stretch

CH2 bend

CH3 bend

9/18/2014

8

Aromatic rings

=C-H Stretch for sp2 C-H occurs at values greater than 3000cm-1.

=C-H Out-of-plane (oop) bending occurs at 900 - 690cm-1. These bands can be used with great utility to assign the ring substitution pattern.

C=C Ring stretch absorptions often occur in pairs at 1600 and 1475 cm-1. For alkenes C=C is at 1650cm-1

Overtone/combination bands appear between 2000 and 1667 cm-1. These weak absorptions can be used to assign the ring substitution pattern.

Alkynes:

C-H at 3300, CC at 2150

Aromatic rings

Examples: Toluene, ortho-Diethylbenzene, meta-Diethylbenzene, para-Diethylbenzene and Styrene

sp2 C-H stretch

C=C stretch vinyl

Aromatic C=C

9/18/2014

9

Alcohols and Phenols

O-H The free O-H stretch is a sharp peak at 3650-3600 cm-1. This band appears in combination with the hydrogen-bonded O-H peak when the alcohol is dissolved in a solvent. The hydrogen-bonded O-H band is a broad peak at 3400 - 3300 cm-1. This band is usually the only one present in an alcohol that has not been dissolved in a solvent (neat liquid). When the alcohol is dissolved in a solvent, the free O-H and hydrogen-bonded O-H bands are present together, with the relatively weak free O-H on the left.

C-O Stretching vibration usually occurs in the range 1260 - 1000 cm-1; This band can be used to assign a primary, secondary or tertiary structure to an alcohol.

Alcohols and Phenols

Examples: The hydrogen-bonded O-H stretch is present in the pure liquid (neat) samples of 1-hexanol, n-butanol and para-Cresol.

Free O-H stretch

O-H H-bonded

C-O stretchsp3 C-H

stretch

9/18/2014

10

Aldehydes

C=O Stretch at approximately 1725 cm-1 is normal. Conjugation moves the absorption to a lower frequency.

C-H Stretch, aldehyde hydrogen (-CHO), consists of weak bands at about 2750 and 2850 cm-1; Note that the C-H stretch in alkyl chains does not usually extend to frequencies as low as 2750 cm-1.

Examples:

Nonanol, Crotonaldehyde and Benzaldehyde

Ketones

C=O Stretch at approximately 1715 cm-1 is normal. Conjugation moves the absorption to a lower frequency. Ring strain moves the absorption to a higher frequency in a cyclic ketone.

C=O Bending appears as a medium-intensity peak in the range 1300 - 1100 cm-1.

Examples:

3-Methyl-2-butanone, Mesityl Oxide, Acetophenone, Cyclopentanone and 2,4-pentanedione

No C-H of aldehyde @ 2750!

9/18/2014

11

Carboxylic acids

O-H Stretch, usually very broad (strongly H-bonded), occurs at 3400-2400cm-1.

C=O Stretch, broad, occurs at 1730-1700cm-1. Conjugation lowers the frequency.

C-O Stretch occurs in the range 1320-1210cm-1, medium intensity.

Examples:

Isobutyric acid and benzoic acid.

C-O rek

C=O rek

O-H rek

Esters

C=O Stretch at approximately 1750-1735cm-1 is normal. Conjugation moves the absorption to a lower frequency. .

C-O stretch in two or more bands in the range 1300-1100cm-1.

Ethers

C-O Most prominent band at 1300-1100cm-1. Absence of C=O and O-H (not due to aldehyde, ester or alcohol)

9/18/2014

12

Amines

N-H Stretch occurs in the range 3500-3300cm-1. Primary amines have 2 bands. Secondary amines have one band: a vanishingly weak one for aliphatic compounds and a stronger one for aromatic secondary amines. Tertiary amines have no N-H stretch.

N-H Bending in primary amines results in a broad band in the range 1640-1550 cm-1. Secondary amines absorb near 1500cm-1.

N-H Out-of-plane bending absorption can sometimes be observed near 800cm-1.

C-N Stretch occurs in the range 1350-1000cm-1.

AminesExamples:

Butylamine, Dibutylamine, Tributylamine and N-methyl-aniline

N-H stretch (two bands!)

N-H bend

9/18/2014

13

Amides

C=O Stretch occurs at approximately 1680-1630cm-1

N-H Stretch in primary amides (-NH2) gives 2 bands near 3350 and 3180cm-1

Secondary amides (-NH) have one band at about 3300cm-1

N-H Bending occurs around 1640-1550cm-1 for primary and secondary amines

Examples:

Propionamide and

N-Methylacetamide

N

O

O

N

O

O

++

-

-

Nitro Compounds

Aliphatic Nitro Compounds: Asymmetric stretch (Strong), 1600-1500cm-1; Symmetric stretch (Medium), 1390-1300cm-1

Aromatic Nitro Compounds (Conjugated): Asymmetric stretch (Strong), 1550-1490cm-1; Symmetric Stretch (Strong) 1355-1315cm-1

Examples:

1-Nitrohexane and

Nitrobenzene