POINT TO DISCUSS :
-sample of solid
-sample of liquids
-sample of gases
3.INTERPRETATION OF I.R
Infrared spectrum is an important record which gives the
sufficient information about the structure of a compound.
The range of EMR between the visible and microwaves region
is called I.R region.
SAMPLE HANDLING IN IR SPECTROSCOPY
1 sapling of solid :-
a. pressed pellet technicque :
In this technique some amount of finely ground solid sample is
mixed with about 100 times its weight of powdered KBr . KBr
will eliminate the problem of additional bands because it won’t
absorb infrared light in the 2.5µm to 15 µm region and a
complete spectrum of sample is obtained . The mixture is
pressed under a high pressure(10000-15000 pounds/sq. inch)
in a die to form a small pellet.
1.The KBr pellet can be stored for longer time.
2.As the concentration of the sample can be suitably adjusted in
the pellet, it can be used for quantitative analysis.
3.The resolution of the spectrum in KBr is superior to that
obtained with Nujol mull technique.
1.It always has band at 3450cm-1 from the OH group of the
moister present in the sample.
2.The high pressure involved in the formation of pellet may bring
about polymorphic changes in crystallinity in the sample(specially
for inorganic complexes) which causes complication in IR
spectrum. In some causes even substitution of the ligand by
bromide may be possible in inorganic complexes.
3.This method is not successful for some polymers which are
difficult to grind with KBr.
b. Mull technique :
In this technique the finely ground solid sample is mixed with
Nujol (mineral oil) to make a thick paste which is then made to
spread between IR transmitting windows. This is then mounted
in a path of infrared beam and the spectrum is run. This method
is good for quantitative analysis.
1.It shows absorption of maximum at 2915 cm-1, 1462cm-
1, 1376cm-1, 719 cm-1
2.Polymorphic changes, degradation, and other changes may
occur during grinding.
c. Solid films :
If a solid is amorphous in nature the sample is deposited on the
surface of KBr or NaCl cell by evaporation of a solution of a
solid. This method is useful for rapid qualitative analysis and
become useless for carrying out quantitative analysis.
d. Solid run in solution :
The solid may be dissolved in a non-aqueous solvent provided
there is no chemical interaction with the solvent and also the does
not absorb in the studied range. The various solvent used are
acetone, acetonitrate, benzene, CCI4
,CS2, cyclohexane, terachloroethylene, methylenechloride. Among
these only CCI4 , CS2 are ideal as they shows very few absorption
band themselves CS gives two prominent bands, one in the
region of 200-2100 cm-1 and other at 1595-1460 cm-1 CCl4
gives only one bend 820-720 cm
2.Sampling of gases:
Gas samples are examined in the lR spectrometer after removal
of water vapour. The simplest gas cell consists of metal/gas
cylinder of 10cm long and closed with a appropriate window.The
gaseous sample is passed through a stopcock via a suitable gas
handing apparatus and partial pressure of 5 to 15mmHG gives a
reasonable level of absorption in most cases .The end wall of gas
cell is made of NaCl and for low concentrated gases long path
legths are required. Multi reflection can be used to make effective
path length as long as 40cm so that constituent of gas can be
3. Sampling of liquid:
Sample that are liquid at room temperature are usually put
frequently with no preparation into rectangular cell made up of
NaCl, KBr orThBr and their IR spectra is obtained directly. The
sample thickness is so selected that the transmittance lies between
15 and 20% . For most sample this will be represent a thin layer of
0.01 to 0.05mm in thickness. Some times the liquid samples can be
dissolved in suitable solvents and scanned in IR region using any
How to approch the analysis of spectra :
When analysing the spectra of unknown compound , concentrate
first on determining the presence or absence of few major
functional group .
The C=O , O-H , C-O , C=C , C≡C , C≡N, and NO2 peaks are
the most conspicuous and give immediate structural information
if they are present.
1.Look if carbonyl group (C = O) is present?
The C = O group gives strong absorption in the region 1820-
1600cm-1 , the peak is strong and medium width.
2. If C=O is present , check for the presence of the following groups:
3. If C=O is absent
4. Double bond and / or aromatic ring :
(i) C=C is a weak absorption near 1650 cm-1 .
(ii) Medium to strong absorption in the region of 1600 - 1550cm-1
often imply an aromatic ring.
(iii) Confirm the double bond or aromatic ring by consulting the C-H
region , if C-H stretch occurs to the left of 3000cm-1 ,then it is
aromatic or vinyl . If C-H stretch occurs to right of 3000cm-1 then it
5. Triple bond :
(i) C ≡ N is a medium sharp peak near 2250cm-1
(ii) C ≡ C is a weak sharp peak near 2150 cm-1
(iii) Check also for acetylinic C-H near 3300cm-1 , which gives an
idea if the triple bond is placed terminally.
6. Nitrogroup :
Two strong absorption at 1600 - 1500cm-1 and 1390 - 1300cm-1
-They yield four stretching and bending vibration of C-H and C-C
1 .C - C BENDING VIBRATION :
2. C - H STRETCHING VIBRATION
3. CH3 BENDING VIBRATIONS:
4. CH2 BENDING VIBRATION :
Decane , cyclohexane
Alkenes show many more peaks than alkanes
(a) = C - H stretch for sp2 C-H occurs at region slightly greater
(b) = C - H out of plane (oop) bending occurs in ranges of 1000-
Example:1-hexene, cyclohexene, cis-2-pentene.
Terminali alkyne show prominent peak at about 3300cm-1 for
SP hybridized C-H.
≡C – H stretching vibration occurs at 3300cm-1
C≡C stretch occurs near 2150cm-1 this is only for terminal
alkynes (absent or very weak for non terminal alkynes)
The C-H stretching for SP2 carbon aapears at value greater
than 3000cm-1. since the C-H stretch for alkenes appears in the
same range ,it may be difficult to use the C - H stretching bands
to differentiate between alkenes and aromatic compounds.
Example: Toluene, o-diethylbenzene ,m-diethylbenzene, p-
diethylbenzene, and styrene.
5.ALCOHOL AND PHENOL:
Alcohol and phenol show strong and broad hydrogen bonded O-H
stretching bands centering between 3400cm-1 and 3300cm-1. In
solution it will also be possible to observe a free O-H stretching band
Example: 1-hexanol,2-butanol,para cresol.
Ether show prominent C-O stretching band at 1300 to 1000cm-1.
Absence of C=O and O-H is required to ensure that C-O stretch is
not due to an ester and phenol .phenyl alkyl ether gives two strong
bands at 1250cm-1 and 1040cm-1.
Example: dibutylether, anisole
Aldehydes show a very strong band for C=O that appears in the range
of 1740 to 1725cm-1 ,for simple aliphatic aldehydes. This band is
shifted to lower frequency with conjugation to a C=C or phenyl group
EXAMPLE : Nonanal , crotonaldehyde , benzaldehyde.
ketone show a very strong band for c=o group that appears in the
range of 1720 to 1708cm-1 for simple aliphatic ketones .this bond is
shifted to lower frequency with conjugation to a c=c or phenyl group
.ring strain moves the absorption to a higher frequency in cyclic
An overtones is seen at 3430cm-1
It shows very strong bands that appears in the range of 1730-
1700cm-1 for simple aliphatic carboxylic acids, this band is
shifted to lower frequency conjugation to a C=C or phenyl group.
Example : isobutyric acid
Esters show a vey strong band for the C=O group that appears in the
range of 1750-1735cm-1 for simple aliphatic esters. The C=O band is
shifted to lower frequency when it is conjugated to C=C or phenyl
Ex: ethyl butyrate, methyl methacrylate, vinyl acetate, methyl
benzoate, methyl salicylate.
Amides show a very strong band for the C=O group that appears in
the range of 1680 to 1630cm-1.
N-H stretch in primary amides (-NH2) gives two bands near
3350cm-1 (asymmetric), and 3180cm-1 (symmetric) secondary
amides (-NH) have one band at 3300cm-1. Tertiary amides will not
show an N-H stretch because they do not contain N-H.
EX: Propionamide, N-Methylacetamide
14. ACID CHLORIDES
-Acid chlorides show a very strong band for the C=O group that
appears in the range of 1810 to 1775cm-1 for aliphatic acid
-Acid chlorides and anhydrides are the most common functional
groups that have a C=O appearing at such a high frequency.
EX: Acetyl chloride, benzoyl chloride.
N-H stretch occurs in the range of 3500-3300cm-1.
Ex: butylamine, dibutylamine, tributylamine, n-methylaniline.
Nitro compounds shows two strong bands in the IR spectrum.
One appears near 1550cm-1
other near 1350cm-1.
They include single bonded compounds like mercaptansthiols and
S-H Stretch shows one weak band occurs near 2550cm-1 and
confirms the presence of this group , since few other absorptions
S=O stretch shows one strong band near 1050cm-1.
S=0 asymmetric stretch (strong) occurs at 1300cm-1.
Symmetric stretch (strong) at 1150cm-1.
1.Pharmaceutical drug analysis Ashutoshkar , New age international
publishers page 330-335.
2.Introduction to spectroscopy .Donald L. Pavia, Gray
M.Lampman, George S. Kritz. 3rd edition ,page 13-82.
3. Spectroscopy .B.K Sharma, Goel Publishing house, page 193-
4. Elementary organic spectroscopy .Y.R Sharma.S.Chand and
company Ltd,page 90.
5.Spectrometric identification of organic compound.Robert
M.Silverstein ,Francis X.Webster,David J.Kiemle. 7th edition page