Spectroscopy IR
Spectroscopy IR
Spectroscopy IR
1. Stretching
i. Symmetric
ii. Asymmetric
2. Bending
i. Scissoring
ii. Rocking
iii. Wagging
iv. Twisting
FACTORS INFLUENCING
ABSORPTION
1. Symmetry
◦ Symmetric compounds do not possess dipole moment
and are IR inactive.
◦ E.g. symmetric acetylene
2. Coupling
◦ There are so many factors which cause coupled
vibration in IR and it will influence the intensity and
shape of the absorption bands.
◦ E.g. normally the band due to C C bond is around at
1650 cm-1 but due to mechanical coupling of two C
C systems in allene give two bands at 1960 and 1970
cm-1
3. Fermi
resonance
◦ Fermi resonance results in an unexpected shift
in energy and intensity of the bands.
◦ E.g. the overtone of C--H deformation mode at
1400 cm-1 is always in Fermi resonance with
the stretch of the same bond at 2800 cm-1.
4. Hydrogen bonding
◦ It can change the shape and position of IR
bands.
◦ Stronger the H-bonding greater the absorption
shift.
Intermolecular- broad bands
Intra-molecular-sharp bands
5. Electronic effect
◦ Electronic effects such as inductive, mesomeric
and field effect may cause shift in absorption
bands due to the change in absorption frequency.
◦ E.g. inductive – acetone(1715 cm-1) and
chloroacetone(1725 cm-1)
mesomeric-acetophenone(1693 cm-1) and P-
aminoacetophenone(1677 cm-1)
6. Bond angles
◦ Difference in bond angles can also leads to the
changes is absorption bands.
◦ E.g.
SAMPLING
Samples of the same substance shows shift
in absorption bands as we pass from solid
to gases and hence the samples of different
phases have to be treated differently in IR
spectroscopy.
Sampling of solids
◦ Solids run in solution
◦ Mull technique
◦ Pressed pellet technique
◦ Solids films
1.Solid run in
solution
◦ Dissolve solid sample in non-aqueous solvent
(which should be IR inactive) and place a drop
of this solution in alkali metal disc and allow to
evaporate, leaving a thin film which is then
mounted on a sepectrometer.
◦ E.g. of solvents – acetone, cyclohexane,
chloroform, carbon tetrachloride etc.
2. Mull technique
◦ Finely powdered sample + mulling agent
(Nujol) and make a thick paste (mull). Transfer
the mull to the mull plates and the plates are
squeezed together to adjust the thickness it is
then mounted in spectrometer.
3.Pressed pellet
technique
◦ Finely powdered sample is mixed with about 100
times its weight of KBr in a vibrating ball mill and
the mixture is then pressed under very high
pressure in an evacuable die to form a small
pellet( 1-2mm thick and 1cm in diameter).
◦ Advantages:-
Eliminates bands which appear due to
mulling agent.
Pellets can be stored for longer period of time.
Concentration of sample can be adjusted.
◦ Disadvantages:-
Not suitable for polymers which are
difficult to bind with KBr.
High pressure may change the
crystallinity of the sample.
4. Solid films.
◦ Here amorphous solid is dissolved in volatile
solvents and this solution is poured on a rock
salt plate (NaCl or KBr), then the solvent is
evaporated by gentle heating.
Sampling of liquids
◦ Liquid sample can be sandwiched between
two alkali halide plates (NaCl , KBr ,
CaF2).
◦ The sample cell thickness is 0.01-0.05mm.
Sampling of gases
◦ Here gases sample is introduced into a glass
cell made up of NaCl.
◦ Very few organic compounds can be examined
as gases.
◦ E.g.: 1,4-dioxane.
Sampling of solutions
◦ Here 1-5% of solution is placed in a solution
cell made up of metal halides and a second
cell containing the pure solvent act as a
reference.
◦ Important solvents used are:-chloroform ,
CCl4, Carbon disulphide etc.
APPLICATIONS OF IR
SPECTROSCOPY
Butan-2-one Butan-2-ol
(1710 cm-1) (3300 cm-
1)
6. Study of keto-enol tautomerism
◦ Diketones and ketoesters exhibit keto-enol
tautomerism and this can be studied using IR
spectrum of the compound.
◦ E.g.: Ethyl acetoacetic etser.