Infra-red spectroscopy 1

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Infra-red spectroscopy 
 
Vibrational & Rotational 
Spectroscopy 
Introduction 
The term "infra red" or “ir”covers the range of the 
electromagnetic spectrum between 0.78 and 
1000 mm. 
• In ir spectroscopy, wavelength is measured in 
"wavenumbers", which have units cm-1. 
 wavenumber = 1/wavelength in cm 
• useful to divide the infra red region into three 
sections; near, mid and far infra red; 
 
Infra red regions 
Region wavelenght range (μm) Wavenumber range cm-1 
Near 0.78 -2.5 12800 – 4000 
Mid 2.5 - 50 4000 – 200 
far 50 - 1000 200 - 10 
• The most useful ir region lies between 4000 – 
600 cm-1 
Infra-red spectroscopy 
Vibrational & Rotational Spectroscopy 
• ir radiation does not have sufficient energy to induce 
electronic transitions as with UV. Absorption of ir is 
restricted to compounds with small energy 
differences in the vibrational and rotational states. 
• ir absorption arises from the interaction between the 
electrical component of EM and dipolar motion within 
molecules: can observe an ir spectrum for 
heteronuclear diatomic molecule e.g. H-Cl (H +-Cl -) 
but not homonuclear diatomic e.g. H2. 
• If the frequency of the radiation matches the 
vibrational frequency of the molecule then radiation 
will be absorbed, causing a change in the amplitude of 
molecular vibration. 
Molecular vibrations 
 
• The positions of atoms in a molecules are not fixed; 
they are subject to a number of different vibrations. 
Vibrations fall into the two main categories of 
stretching and bending. 
• Stretching: Change in inter-atomic distance along 
bond axis 
Molecular vibrations 
 • Bending: Change in angle between two bonds. There 
are four types of bend: 
– Rocking 
– Scissoring 
– Wagging 
– Twisting 
 
Vibrational coupling 
• In addition to bending & stretching, interaction 
between vibrations can occur (coupling) if the 
vibrating bonds are joined to a single, central atom. 
Vibrational coupling is influenced by a number of 
factors; 
• Strong coupling of stretching vibrations occurs when 
there is a common atom between the two vibrating 
bonds 
• Coupling of bending vibrations occurs when there is a 
common bond between vibrating groups 
• Coupling between a stretching vibration and a bending 
vibration occurs if the stretching bond is one side of 
an angle varied by bending vibration 
• No coupling is seen between groups separated by two 
or more bonds 
 
• Classically can consider 
chemical bond as a 
spring obeying Hooke’s 
law with two masses m1 
and m2 as the 
molecules. If one mass 
fixed can consider the 
potential energy U of 
the other vibrating 
particle. 
Theoretical model 
Hooke law model 
• consider the particle to be undergoing simple 
harmonic motion it is possible to calculate a natural 
frequency of vibration for each bond from: 
 
 
 
 
• k is force constant (5 x 105 dynes/cm for a 
single bond) 
• calculation of stretching frequencies possible with 
simplified form of equation 
 = 4.12 k/ 
21
21
mm
m.m
 where
k
c 2
1
• C-H bond 
 
 
 
 
 
C=C 
 
1cm 3032
923.0
10x5
12.4
5
923.0
112
)1)(12(
mm
.mm
Hc
Hc
6
1212
12.12
1cm 2168
6
10x10
12.4
6
1212
)12)(12(
5
ir instrumentation 
Ft-ir 
Sample preparation 
• Liquids 
– Drop of liquid between polished salt NaCl 
plates – organic samples must be H2O free. 
• Solids 
– KBr Disc. Dry KBr ground & mixed with 
sample and pressed to form pellet. 
– Nujol mull. Sample ground & dispersed as 
suspension in mineral oil. 
– Dissloution. Dissolve sample in ir 
transparent solvent such as CCl4. 
Interpretation of ir spectra 
• because chemical bonds (springs) vibrate with 
frequency determined largely by the strength of the 
bond and the atomic weights of the atoms involved an 
ir spectrum can be used as a diagnostic tool to 
identify particular functional groups. 
Interpretation of ir spectra 
• Sometimes bands 
absorb in same 
region but can usually 
distinguish between 
them on basis of 
strength of 
absorption, e.g. C=C 
& C=O 
 
• exact position of ir absorption band for particular 
functional group will vary with local chemical 
environment - but still considered diagnostic. 
• more accurate identification possible with 
correlation charts 
• mid- ir spectrum (4000-600cm-1) divided into 
three useful regions 
• 4000-1300cm-1 : functional group region. 
 Characteristic stretching vibrations for important 
functional groups e.g. C=O, OH, 
• 1300-1000cm-1 : fingerprint region - used to 
 assign fine structural details e.g. 1o, 2o, 3o 
alcohol 
• 1000 - 650cm-1: bending region - useful in 
 assigning substitution pattern i.e. ortho, meta, 
para etc on aromatic structures or cis, trans on 
vinyl compounds 
Absorption bands 
O-H 3400 cm-1 C C 2150 cm-1 
 
N-H 3400cm-1 
 
C=O 1715cm-1 
 
C-H 3000cm-1 
 
C=C 1650cm-1 
 
C N 2250 cm-1 
 
C-O 1100cm-1