Modes of Vibration Basic Principles OfIR Spectros
Modes of Vibration Basic Principles OfIR Spectros
Modes of Vibration Basic Principles OfIR Spectros
2. Introduction
The most frequent spectroscopic technique used by organic and inorganic chemists is infrared
(IR) spectroscopy. It deals with the absorption of radiation in the infrared region of the
electromagnetic spectrum. IR spectrum gives sufficient information about the structure
(identification of functional groups) of a compound and can also be used as an analytical tool to
assess the purity of a compound. The absorption of infrared radiation by a molecule causes
changes in their vibrational and rotational energy levels and hence IR-spectroscopy is also known
as vibrational-rotational spectroscopy. Unlike UV-spectroscopy which has very few peaks in their
spectrum, IR spectroscopy provides spectrum with a large number of absorption bands and hence
provide plenty of information about the structure of a compound. Different bands present in the
spectra correspond to various functional groups and bonds present in the molecule.
The infrared spectrum can be divided into three type of main regions: the far infrared (<400
cm−1), the mid-infrared (4000–400 cm−1) and the near-infrared (13000–4000 cm−1). The mid IR
region is of greatest practical use to the organic chemist, but the near- and far-infrared regions
also provide important information about many compounds.
Mid IR region: The mid-infrared spectrum extends from 4000 to 400 cm−1 and results from
vibrational and rotational transitions. This region is most useful for the organic chemist since
most of the organic molecules absorb in this region. The mid-infrared can be divided into two
regions viz functional group region (4000-1300 cm−1) and finger print region (1300-600 cm−1).
Functional group region (4000-1300 cm−1): Most of the functional groups present in organic
molecules exhibits absorption bands in the region 4000-1300 cm−1, hence this is known as
functional group region. In this region, each band can be assigned to a particular deformation of
the molecule, the movement of a group of atoms, or the bending or stretching of a particular
bond.
Finger print region (1300-600 cm−1): The region from 1300 cm-1 to 600 cm-1 usually contains a
very complicated series of absorptions. These are mainly due to molecular vibrations, usually
bending motions that are characteristic of the entire molecule or large fragments of the molecule.
Except enantiomers, any two different compounds cannot have precisely the same absorption
pattern in this region. Thus absorption patterns in this region are unique for any particular
compound that is why this is known as finger print region.
It is very difficult to assign individual bands in this region. Two molecules having the same
functional group but different bonding arrangements may show similar spectra in the functional
group region but their spectra differ in the finger print region. Therefore both the regions are very
useful for confirming the identity of a chemical substance. This is generally accomplished by
comparing the spectrum of an authentic sample. When two compounds show a good match
Near-infrared region (12500–4000 cm−1): The absorptions observed in the near-infrared region
(12500–4000 cm−1) are overtones or combinations of the fundamental stretching bands. Bands in
the near infrared are usually weak in intensity. They are often overlapped and hence are less
useful than the bands in mid-infrared region.
All the bonds in a molecule are not capable of absorbing infrared radiation but only those bonds
which are accompanied by a change in the dipole moment will absorb in the infra-red region.
Thus, vibrations which are associated with the change in the dipole moment of the molecule are
called infra-red active transitions otherwise the vibration is said to be IR-inactive and do not show
any absorption band in the IR-spectrum. Generally, larger the change in the dipole moment, the
higher is the intensity of absorption. Hence the vibrational absorption bands in simple
hydrocarbons are weak while bands associated with bonds connecting atoms with considerable
electronegativity difference give strong bands.
5. Selection Rule
IR-radiation is absorbed only when a change in dipole moment of the molecule takes place.
Complete symmetry about a bond may eliminate certain absorption bands.
Therefore number of absorption bands observed is not exactly equal to the fundamental
vibrations, some of the fundamental vibrations are IR-active while others are not. This is
governed by selection rule described below.
1) In a molecule with a centre of symmetry, the vibrations symmetrical about the centre of
symmetry are IR-inactive.
2) The vibrations which are not symmetrical about the centre of symmetry are IR-active.
Here are some examples which could explain the selection rule.
a) All the symmetrical diatomic molecules such as H2, N2 and Cl2 etc. are IR-inactive.
b) The symmetrical stretching of the C=C bond in ethylene (centre of symmetry) is IR-inactive.
c) The symmetrical stretching in CO2 is IR-inactive, whereas asymmetric stretching is IR-active.
d) Cis-dichloro-ethylene molecule shows C=C stretching bands whereas trans molecule does not
show this band.
6. Fundamental Vibrations
In the classical harmonic oscillator, E = 1/2kx2= hν, where x is the displacement of the spring.
Thus, the energy or frequency is dependent on how far one stretches or compresses the spring,
which can be any value. If this simple model were true, a molecule could absorb energy of any
wavelength.
However, vibrational motion is quantized: it must follow the rules of quantum mechanics, and the
only transitions which are allowed fit the following formula:
The lowest energy level is E0 = 1/2 hν, the next highest is E1 = 3/2 hν. According to the selection
rule, only transitions to the next energy level are allowed; therefore molecules will absorb an
amount of energy equal to 3/2 – 1/2 hν or hν. This rule is not inflexible, and occasionally
transitions of 2 hν, 3 hν, or higher are observed. These correspond to bands called overtones in an
IR spectrum. They are of lower intensity than the fundamental vibration bands. The IR spectrum
of a compound may show more than one vibrational absorption bands. The number of these bands
corresponds to the number of fundamental vibrations in the molecule which can be calculated
from the degree of freedom (DOF) of the molecule. A molecule comprising of n atoms has a total
of 3n DOF. In a nonlinear molecule, three of these degrees of freedom are rotational and three are
translational and the remaining (3n-6) correspond to vibrational degree of freedom or
fundamental vibrations. Whereas in a linear molecule, only two degrees of freedom are rotational
(because rotation about its axis of linearity does not change the positions of the atoms) and three
are translational. The remaining (3n-5) degrees of freedom are vibrational degree of freedom or
fundamental vibrations.
If the molecule is symmetrical such as hydrogen, nitrogen, and chlorine, the band is not observed
in the IR spectrum. Asymmetrical diatomic molecules, e.g. CO and iodine chloride absorb in the
IR spectrum.
It has been observed that in actual IR spectrum, the theoretical number of fundamental bands is
seldom observed because there are certain factors which may increase or decrease the number of
bands. Some fundamental vibrations lie outside the IR region (4000-400 cm-1), whereas some are
too weak to be observed. Few fundamental vibrations are too close that they merge into one
another. The occurrence of degenerate bands (bands of same frequency) also cause decrease in
the fundamental vibrational bands. This phenomena is called fermi resonance.
7. Summary
1. Absorption of electromagnetic radiation in infrared region can cause changes in the vibrational
and rotational energy states.
2. A molecule consisting of n atoms has a total of 3n degrees of freedom.
3. The number of fundamental vibrational bands in a molecule is equal to the degree of freedom
of a molecule however these numbers of bands is seldom obtained because of the occurrence of
certain non-fundamental bands such as overtones, combinations of fundamental vibrations or
difference of fundamental vibration bands.