Interpretation of IR spectra

IR Spectra

1. INTERPRETATION OF
INFRARED SPECTRA
GUIDED BY: PRESENTEDBY:
Dr.Sk.Abdul Rehman R.Keerthi
M.Pharm, Ph D M.Pharm 1
st
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 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=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 (1
O
– 2 bands, 2
O
-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