e10 Infrared

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E10 INFRARED & RAMAN SPECTROMETRY : Principles &

INSTRUMENTATION

KEY NOTES

Principal:-Vibrational Transition In Molecules Cause Absorption In The Infrared

Region Of The Electromagnetic Spectrum. They may also be studied using the

technique of raman spectrometry, where they scatter exciting radiation with an

accompanying shift in its wavelength.

Group Frequencies:- Vibrational Spectra Give Information About The

Functional Groups In Molecules, And The Observed Group Frequencies Are

Affected By Molecular Interactions Such As Hydrogen Bonding.

Instrumentation:- Infrared And Raman Instruments Include A Radiation Source, A

Means Of Analyzing The Radiation And Detection And Data Processing System.

Additionally, sampling methods to deal with gases, liquids, solids, micro samples

and mixtures are available.

Related topics :-Infrared And Raman Spectrometry

Applications (E 11)

Gas chromatography infrared

Spectrometry (F4)

Principles:- The vibrational levels of molecules are separated by energies in the

infrared (1R) region of the electromagnetic spectrum. That is, in the wavenumberrange from 13000 to 10 cm, or between0.8 and 1000 um on the wavelength scale

.For convenience, this large region is divided into near IR, or NIR (13000-4000cm-

1), mid IR(4000-400cm-1) and far IR (400-10cm-1).


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Molecules contain bonds of specific spatial orientation and energy. These bonds

are seldom completely rigid, and when energy is supplied, they may bend, distort

or stretch. A very approximate model compares the vibration to that of a

harmonic oscillator, such as an ideal spring. If the spring has a force constant, k,

and masses m at the ends, then the theoretical vibration frequency v is given by:

V=(1/2n) (k/)

Where =m m /(m + m ) is called the reduced mass.

Each type of molecular vibration is characterized by a vibrational quantum

number, v. For a simple stretching vibration, there is a series of levels whose

energy is given approximately by

E=hv ,(v+1/2)

This means there is a set of levels spaced in energy by hv or in wavenumber by v .

The selection rule for an ideal harmonic oscillator allows transitions where

Av =+_1, giving a single, fundamental vibrational absorption peak.

However, when bonds are stretched they weaken, so a better model takes this

into account, and the molecules are treated as anharmonic oscillators. This where

high energies are involved, large energy transition may occur, where v = +2, +3,etc, giving the first overtone at a wavenumber approximately

double that of the fundamental, & so on.

The electrical field associated with the electromagnetic radiation will interact with

the molecules to change its electrical properties. Some molecules (for example,

HCI) have a dipole moment due to change separation & will interact with the field

others may acquire a dipole when they vibrate, for example, molecules, CH4 HAS

No Dipole, But When One Of The Ch Bonds Stretched, The molecules will develop

a temporary dipole.

even if the molecules does not have a dipole, the electric fields ,E, may distort the

electron distribution and polarize the molecules

=a E


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where is the dipole induced by fields ,E and a is the polarizability of the

molecules

The rules governing transitions in the infrared region of the spectrum require that

,in order to absorb, the dipole of molecule must change during the vibration.Such vibrations said to be IR active. For transitions to be active in the Raman

region, it is required that the polarizability must change using the vibration. The

transitions are then Raman active, or R active (fig. 1).

Consider two simple diatomic molecules, nitrogen and carbon monoxide. These

molecules have only one fundamental vibration frequency, vo. For nitrogen it is

2360 cm-1, and for carbon monoxide 2168 cm-1.

Since carbon monoxide has a permanent dipole, which will increase and decrease

as he molecule stretches and compresses, the vibration will interact with IR

radiation, and an absorption peak will be observed close to 2168 cm-1. Nitrogen

has no dipole, and vibration does not produce one. Therefore, it will not absorb IR

radiation. This is of great importance, since IR spectra may be recorded in air

without interference.

However, when the nitrogen molecule vibrates, he bonding electrons are

distorted and the polarizability is changed. Therefore, it will give a spectrum usingthe Raman technique.

In order to excite Raman transitions, energy comparable to the difference

between electronic energy levels must be supplied. This may be visible laser light


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or NIR radiation. If the exciting wavelength matches the energy difference

between the electronic levels of the sample, the Raman signal is greatly

enhanced by the resonance Raman effect. Rayleigh scattering re-emits the

exciting line. There ore intense emission due to fluorescence effects may mask

the weak Raman signal, but with NIR radiation fluorescent interference is much

less.

As molecules become more complex. The number of possible vibrations

increases. For example, carbon monoxide, co2 , has three atoms arranged in a

line: O=C=O. this molecule does not have a dipole and may vibrate in three ways.

I. The symmetric stretch, denoted by v is where both oxygen are equidistantfrom the central carbon, but the C-O bonds lengthen and contact together.

The dipole does not change, but the polarizability does . so this vibration is

IR inactive, but R active.

II. The antisymmetric stretch, v has one C-O bond stretching, while the othercontact. The carbon atom moves as well so that the centre of mass of the

molecules remains stationary. The dipole changes but the polarizability

does not so this is IR active but R active

III. The bending vibrations may be resolved into two identical and mutuallyperpendicular components corresponding to two transitions of the sameenergy ( degenerate ). It is necessary to think in three dimension:

considering the page as plane, then if the two oxygen go equally down the

page while the carbon goes up the page to balance this is in plane bending

.if the oxygen go into the page the carbon out of the page this is out of

plane bending. These changes will be reversed as the vibration progresses .

This vibration is IR active R inactive

The triangular molecule of water HO also has three different vibration

corresponding to the same vibrational types. However each involved change in

dipole so all three are IR active. The Raman spectrum shows only one line due

to the symmetric stretch. These vibrations are shown schematically in figure 2.

It is possible to extend these arguments to more complex molecules, but this is


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only of value for student of structural parameters such as the length and

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