IR spectroscopy

Infra-red spectrum is an important record which gives sufficient information about the structure
of a compound.

IMP Principle

The region from 0.8

to 2.5 is called Near infra-red and

that from 15 to 200 is called Far IR

Principle of Infra-red spectroscopy

The absorption of Infra-red radiations causes an excitation of
molecule from a lower to the higher vibrational level.

Theory—Molecular Vibrations

Absorption in the infra-red region is due to the changes in the vibrational and rotational levels.

less then 100cm-1 are absorbed molecular rotation take place in the substance

molecular vibration: 104 to 102 cm-1

fundamental vibration

(a) Stretching. In this type of vibrations, the distance between the two atoms increases or decreases
but the atoms remain in the same bond axis.

(b) Bending. In this type of vibrations, the positions of the atoms change with respect to the original
bond axis.

Types of stretching vibrations.

Symmetric stretching.

Asymmetric stretching.

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Scissoring. In this type, two-atoms approach each other.

Rocking. In this type, the movement of the atoms takes place in the same direction.

Wagging. Two atoms move ‘up and down’ the plane with respect to the central atom.

Twisting. In this type, one of the atoms moves up the plane while the other moves down the
plane with respect to the central atoms.

overtone combination and difference band;

Overtones results from excitation of ground state to higher energy states, which corresponds to
integral multiples of frequency of fundamental.

When two vibrational frequencies in a molecule couple to give rise to a vibration of new
frequency

Difference band is similar to combination bands. The observed frequency in this case results from
the difference between the two interacting bands.

vibrational frequency

Calculated by hookes law

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the value of vibrational frequency or wave number depends upon:

(i) Bond strength, and

(ii) Reduced mass.

Degrees of freedom

The number of degrees of freedom is equal to the sum of the co-
ordinates necessary to locate all the atoms of a molecule in space.

linear molecule, there are only two degrees of rotation.

non-linear molecule, there are three degrees of rotation

Factors Influencing
Vibrational Frequencies

Coupled Vibrations and Fermi Resonance.

asymmetric vibrations always occur at higher wave number compared with the
symmetric vibrations. These are called coupled vibrations

Electronic Effects.

inductive effect

mesomeric effect

Field effect

  1. Hydrogen Bonding

intermolecular hydrogen bonding: give rise to broad band

intramolecular hydrogen bonding: sharp dond

  1. Bond angles:

Normal angle 120

Bond angles

.

The more s character, the greater the bond angle. The shorter the bond, the stronger it is. The

greater the electron density in the region of orbital overlap, the stronger the bond. The more s

character, the shorter and stronger the bond.