This document provides guidance on interpreting infrared spectra. It outlines the key features of an IR spectrum and the types of information that can be obtained, such as identifying functional groups present between 4000-1500 cm-1 and determining molecular fingerprints from 1500-400 cm-1. It reviews the requirements for interpretation and general rules for analysis, such as looking for carbonyl groups between 1820-1660 cm-1 and associated functional groups. Common absorption regions for functional groups like O-H, C=O, C-H and others are also presented to aid analysis. Examples of drug spectra are provided for illustration.

1. INTERPRETATION OF
INFRARED SPECTRA
GUIDED BY: PRESENTEDBY:
Dr.Sk.Abdul Rehman R.Keerthi
M.Pharm, Ph D M.Pharm 1st
year
DEPARTMENT: PhARMACEuTICAl ANAlySIS

2. Outline…
Features of an ideal IR Spectrum
Information could be obtained
Requirements for the interpretation
General rules of interpretation
Illustration

3. Features of an IR SpectrumAbsorption (Band) spectrum
Spectrum between percent transmittance (% T) and
wave number (ύ)
Wave number (ύ) represents frequency (υ) of IR
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

4. Different types of IR Bands

7. Information Obtained from an Unknown
IR Spectra
 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)

8. Interpretation of Spectra
Spectral Requirements
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.

9. Correlation Chart and Tables
9
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

11. Typical Infrared AbsorptionTypical Infrared Absorption
RegionsRegions
2.5 4 5 5.5 6.1 6.5 15.4
4000 2500 2000 1800 1650 1550 650
FREQUENCY (cm-1
)
WAVELENGTH (µm)
O-H C-H
N-H
C=O C=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*

12. Absorption Regions

14. BASE VALUESBASE VALUES
(+/- 10 cm-1
)
These are
the minimum
number of
values to
memorize.
O-H 3600
N-H 3400
C-H 3000
C N 2250
C C 2150
C=O 1715
C=C 1650
C O ~1100 large range

15. Simple Approach to the Analysis of Spectrum at a Glance
( General Rules)
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.....

16. Acids
(O-H)
a broad band near 3400-2400 cm-1
Amides
(N-H)
medium absorption near 3400 cm-1
& some times double
dips with equivalent halves
Ester
(C-O)
strong absorption near 1300-1000 cm-1.
Anhydride
s
(c-o)
Gives two C-O absorption near 1810-1760 cm-1
Aldehyde
(C-H)
show two weak absorption near 2850 and 2750 cm-1
.
Ketones
(C-R)
If none of the above is present, conforms the presence of
ketones

17. 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 1300-1
000cm-1
,and also absence of
O-H near 3400cm-1

18.  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

19. 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

20. Illustrations
Taking example of drugs

25. References :
www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRInt
erpretation.htm
chemwiki.ucdavis.edu › ... › Infrared Spectroscopy
chemistry.oregonstate.edu/courses/ch361-
464/ch362/irinterp.htm
infrared.als.lbl.gov/BLManual/IR_Interpretation.pd
f