Principles Behind Infrared Spectroscopy

Infrared Spectroscopy 1
INFRARED SPECTROSCOPY
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Infrared Spectroscopy 2
L02
1. Summarise the principles behind infrared spectroscopy.
The infra-red region of the spectrum has radiations with energy enough to stretch or
bend bonds in a molecule. In the IR region, each functional group of molecules has a
specific range of characteristic absorption frequencies. Infra-red absorption
spectroscopy involves detection of the frequencies of IR light absorbed by a given
molecule to determine the basic functional groups of molecules (Meislich 2014).
Atoms in a molecule are subject to different vibrations (stretching or bending) since they
are not fixed in the molecular structure. During IR absorption, a net change is caused in
the dipole moment of the molecule due to the vibrations or rotations within the
molecules of compounds with polar bonds. The IR radiation is absorbed when the
radiation’s frequency and the vibrational frequency of the molecules matches thus
causing a change in amplitudes of the vibrations. (Jones, Mulloy and Thomas
2013).The concept of absorption of specific frequencies of light by molecules is used to
deduce the corresponding molecular structure from the molecule’s characteristic
absorption. The absorbed frequencies of the infrared light correspond to the resonant
frequencies of vibration of the molecular bonds (Meislich 2014). The resonant
frequencies relate to the molecular bonds’ strength and the molecular atomic mass and
thus absorbed frequencies can be associated with a specific type of bond and functional
group.
2. IR Spectrum
i. Explain why the region between 1000cm-1 and 1550cm-1 is
described as the ‘fingerprint region’.
The region between 1000 cm-1 and 1550 cm-1 is known as the fingerprint region
because it consists of bands that are unique to each molecule. It is specific to each
molecule and is full of small peaks showing that IR absorption results from vibrations of
the whole molecule (Thornton 2009).
ii. Identify the functional groups that could be responsible for the peaks in
the non-fingerprint region of the spectrum.

Infrared Spectroscopy 3
a) alkyl C−H group
b) carboxylic acid C=O group
c) ketone C=O group
The non-fingerprint region of the IR spectrum generally has five zones namely Zone 1
(4000-3200 cm-1), zone 2 (3200-2600 cm-1), zone 3 (2400 -2000 cm-1), zone 4 (1850-
1650 cm-1) and zone 5 (1680-1550 cm-1). Each zone has distinct peaks corresponding
to the possibility of presence different molecular functional groups.
Performing the five-zone analysis, the peaks found in zone 2 only occur after 3000 cm-1
hence there are alkyl C−H functional group in this region.
The peak around 3000 cm-1 as well as the peak occurring at around 1720 cm-1 indicates
that carboxylic acid C=O group is present.
The strong peak occurring at around 1720 cm-1 indicates that ketone C=O group is
present.
iii. Is this the spectrum of butan-2-ol or butanone? Explain your answer.
This is butanone spectrum since butanone has ketone group which are present.
Butan-2-ol has alcohol group which is absent (Academics.eckerd.edu 2020).
3. Explain how IR spectroscopy can be used to distinguish between alcohols,
carboxylic acids and esters.
The five-zone analysis of the non-fingerprint regions of IR spectrum is used (Meislich
2014).
Alcohol (O=H ) groups demonstrate strong and broad peaks between 3650-3200 cm-1.
Carboxylic acid (C=O) groups demonstrate broad peaks that are very strong between
3000-2500 cm-1 and also similar peaks between 1725- 1700 cm-1 (Ibarra, Valencia and
Pérez 2015).
Ester group demonstrates strong peaks between 1750-1735 cm-1.
L04