2. Features
of an ideal IR Spectrum
Information
could be obtained….
Requirements
General
for the interpretation…
rules of interpretation
Illustration
3. 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
4.
5.
6.
7.
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. 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. Source of Data
Literature
(Books, Journals, Spectral library)
Correlation
Tables
Correlation
Charts
Reference
Web
Spectra (IP, BP etc.)
Database
12. 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
13.
14.
15. 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
16. 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.....
17. 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.
18. 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
19.
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
20. 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