Interpretation of Infra red spectra

Presented
by
Y. Narayudu

(Pharm. Analysis & Q.A)

 Features

of an ideal IR Spectrum

 Information

could be obtained….

 Requirements
 General

for the interpretation…

rules of interpretation

 Illustration

 Absorption

(Band) spectrum
 Spectrum between percent transmittance (% T) and
wave number (ύ)
 Wave number (ύ) represents frequency (υ) of lR
absorption
 Two regions of absorption:
FGR (4000-1500 cm-1): Identification of groups
FPR (1500-400 cm-1): Molecular fingerprint (Spectral
matching)
 Nature (Intensity and Shape) of spectral bands: Strong
(s), medium (m), weak (w); Broad and Narrow



IR is most useful in providing information
about the presence or absence of specific
functional groups (FGR)



IR can provide a molecular fingerprint that
can be used for the identification of samples
(FPR)

The spectrum must be adequately resolved and of
adequate intensity.
 The spectrum should be of a pure compound.
 The spectrophotometer should be calibrated so that the
bands are observed at their proper frequencies or
wavelength.
 The method of sample handling must be specified. If a
solvent is employed, the solvent concentration, and the
cell thickness should be indicated.


Source of Data
 Literature

(Books, Journals, Spectral library)

 Correlation

Tables

 Correlation

Charts

 Reference
 Web

Spectra (IP, BP etc.)

Database

O-H

3400cm-1

C≡C

2150cm-1

N-H

3400cm-1

C=O

1715cm-1

C-H

3000cm-1

C=C

1650cm-1

C≡N

2250cm-1

C-O

1100cm-1

10

Typical Infrared Absorption
Regions
WAVELENGTH (µm)
2.5

4

O-H

C-H

5

6.1

C=O

C N

N-H

5.5

C C
X=C=Y

Very
few
bands

(C,O,N,S)
4000

2500

2000

6.5

C-Cl
C-O
C=C
C-N
C-C
N=O N=O *
C=N

1800 1650

FREQUENCY (cm-1)

15.4

1550

650

BASE VALUES
-1
(+/- 10 cm )

O-H
N-H
C-H

3600
3400
3000

C N
C C

2250
2150

C=O
C=C

1715
1650

C O

~1100

These are
the minimum
number of
values to
memorize.

large range

 Look

if carbonyl group is present?...... The C=O
gives strong absorption in the region 1820-1660
cm-1

 If

C=O is present, check for the presence of
following group.....

Acids (OH)

a broad band near 3400-2400 cm -1

Amides(N- medium absorption near 3400 cm-1 & some times
H)
double dips with equivalent halves
Ester(C-O) strong absorption near 1300-1000 cm-1.
Anhydride
s
(c-o)
Aldehyde
(C-H)

Gives two C-O absorption near 1810-1760 cm-1

Ketones
(C-R)

If none of the above is present, conforms the presence
of ketones

show two weak absorption near 2850 and 2750 cm-1.

If C=O is absent,
 Alcohols and Phenols: Check for O-H, show
broad absorption near 3400-3300 cm-1. confirm this
by finding C-O near 1300-1000cm-1.
 Amines: N-H stretch, show medium absorption
near 3400 cm-1 (1O – 2 bands, 2O-1 band)
 Ethers: C-O near 1300-1000cm-1,and also absence
of O-H near 3400cm-1



Double bonds and / or Aromatic ring:


C=C is a weak absorption near 1650 cm-1

 Medium to strong absorption 1600-1450 cm-1




Aromatic C-H occurs to left of the 3000 cm-1
Aliphatic C-H occurs to right of 3000 cm-1

Triple bonds:


C≡N medium, sharp absorption near 2250 cm-1



C≡C weak sharp absorption near 2150 cm-1

 Nitro

groups

 two absorptions at 1600-1530 cm-1 & 1390-1300 cm-1
 Hydrocarbons

 None of the preceding is found
 Major absorptions are in C-H region near 3000 cm-1
Very simple spectrum

Illustrations
Taking example of drugs

Interpretation of Infra red spectra

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Interpretation of Infra red spectra (20)

Recently uploaded (20)

Interpretation of Infra red spectra

Editor's Notes