IR Spectroscopy Modified

7/29/2019 IR Spectroscopy Modified

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Infrared Spectrometry

Principle

Instrument

Qualitative Analysis

Quantitative Analysis


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Infrared Light

Energy of IR photon insufficient to cause electronic

excitation but can cause vibrational or rotational excitation


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Selection Rule of Infrared Spectrum

Molecule must have change in dipole

moment due to vibration or rotation to

absorb IR radiation.

Homonuclear diatomic molecules will have

no IR spectrum.

Molecule dipole moment interacts with IR

photon electric field.

Absorption causes increase in vibration

amplitude/rotation frequency.


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Molecules with permanent dipole

moments () are IR active


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Types of Molecular VibrationsStretchchange in bond length

symmetric stretching

asymmetric stretching


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Types of Molecular Vibrations

Bendchange in bond angle

scissoring

wagging

rocking

twisting/torsion


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Vibrational Modes:

To determine the vibration of polymolecule we need to consider:

-motion of the entire molecule in space

-rotational motion of the molecule around its centre of gravity

-Motion of each atom (individual vibrations).

Molecule containing N atoms have 3N degree of freedom


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Normal Modes of Vibration

Linear molecule of N atoms: normal

modes = 3N - 5

Nonlinear molecule of N atoms: normal

modes = 3N - 6


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Only some modes may be IR active


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The three fundamental vibrations

for sulfur dioxide


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How many vibrational modes?

2 atoms (H2) - 1 vibration

3 atoms (H2O) - 3 vibrations

3 atoms (CO2) - 4 vibrations

4 atoms (H2CO) - 6 vibrations


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Vibrational coupling :

In addition to the vibrations mentioned above, interactionbetween 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

Coupling is greatest when the coupled groups have

approximately equal energies

No coupling is seen between groups separated by two or more

bonds


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Instrumentation: Sources


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Instrumentation: Transducers


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Fourier Transform Instruments

Michelson interferometer


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Fourier Transform

FrequencyTimeFT


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FTIR advantages

High S/N ratios

Rapid (


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Qualitative and Quantitative Analysis

Gases-fill gas cell

(a) transparent windows (NaCl/KBr)

(b) long pathlength (10 cm) - few molecules

Liquids-fill liquid cell

(a) solute in transparent solvent - not water

(b) short pathlength (0.015-1 mm)Solids-make semi-transparent pellet with

KBr


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Qualitative Analysis

Step One: Identify

functional groups

Step Two:

Compare withstandard spectra


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Quantitative Analysis

IR more difficult than UV-VIS

complex spectra

weak incident beam low sensitivity

solvent absorption

IR mostly used for rapid qualitative but notquantitative analysis

Documents

IR Spectroscopy Modified