IR Spectroscopy pdf

Definition:

Infrared spectroscopy is the measurement of wavelength and the absorption of light by sample.

It also deals with infrared region of electromagnetic spectrum such as the light which have longer long wavelength and a lower frequency than visible light All major functional groups absorb at their characteristics wavenumbers. From the data available due to absorption frequencies, the probable structure can be predicted. If some chemical data is available, it leads to the confirmation of the structure.

Parts of Infrared region:

The Infrared region divided into three parts which are given below;

• Near Infrared:

This region includes from 75um to 2.5um having wave numbers ranges from 12500-4000cm-1. In this region, usually the absorption bands due to overtones of hydrogen stretching vibrations.

• Middle or Fundamental infrared:

This region lies from 2.5 to 16um having wavenumber 4000-625cm-1. Rotations and molecular vibrations take place in it region.

• Far Infrared

This region extends from 16-1000um with wavenumber 625-10cm-1. In this region skeletal vibration and rotations occur.

Basic Principle:

IR radiations strike on sample or molecules then these vibrations caused vibrations in the sample and molecules excites from lower to higher vibrational levels. As we know that each vibrational level has rational energy levels. IR spectra is vibrational rotational spectra. Only those bond of molecules absorb IR energy their dipole moment is change they absorb IR energy in IR region.

For example:

The vibrational rotational transitions of C=O,N-H, O-H have change in dipole moment so they absorb IR energy.

Instrumentation:

• We use double beam photo spectrophotometer to study absorption in IR region.

Instrumentation consists of

• Radiation source

• Sample compartment

• Monochromator

• Amplifier

• Recorder

Radiation Source:

• The most important source of I.R radiations is a hot filament that scan the spectrum of an organic compound is called Nernst glower.

• It consist of rod of 2mm diameter.

• This rod is made by oxides of zirconium, yttrium and Erbium.

• When this rod is heated at 1300-1800 salcius then it produced I.R radiations.

• Glober is compose of silicon carbide also produced IR radiations when electrically heated.

• Sample compartment:

• When I.R radiations are emitted from radiation source then these radiations divided into two beams of equal intensity.

• To obtain monochromatic light then light of single wavelength optical prisms is used for this purpose.

• Then beam is passes through the sample and absorption by sample is measured from difference in between two beams which is usually measured by optical null method.

Types of Vibrations:

As we discuss we study the region of IR Spectroscopy which have range lies in the range of 625-4000cm-1 and this range of radiation also known as “Functional Group region”. When a molecule absorb radiations and it becomes excited and vibrations occur. When the molecule excite we classified its vibrations into basic two types;

1) Fundamental Vibrations

2) Overtone Vibrations

1) Fundamental Vibrations:

The fundamental vibrations basically gives information that, when a light of particular wavelength and frequency excite the vibration motion of two or more electrons at same energy level is known as fundamental vibrations.

 There is two types of  Vibrations.

I. Stretching Vibrations

II. Bending Vibrations

I. Stretching Vibrations

The motion in which the distance between two bonded atoms or the length of bond present between two atoms is increase or decrease by the continuous phenomenon is called is known as Stretching vibration.

The stretching vibrations also have two types;

a) Symmetric stretching vibrations

b) Asymmetric or anti-symmetric stretching vibrations

a) Symmetric stretching Vibrations

           In this type of stretching vibration, the bond lengths of two bonded atom with same atom is increase or decrease with a continuous phenomenon and with the same order is known as symmetric stretching vibrations.

     

                           

b) Asymmetric or anti symmetric stretching Vibrations

           In this type of stretching vibrations, the bond length of two bonded atom with a same atom is increase or decrease with the continuous phenomenon but with different order or with order less motion according to other atom attached to the same attachment point is known as asymmetric or anti symmetric stretching vibrations.

II. Bending Vibrations

                     In this type of vibration of molecule in which angle between two bonded atoms with another same common atom is increase or decrease is known as bending vibration. 

The bending vibration also classified into further two types;

A) In plane bending vibration

B) Out of plane bending vibration

A) In plane bending Vibration

                       The motion of bending vibrations, in which two bonded atoms move with same order in the plane, is known in plane bending vibrations. 

It has also two types;

a) Rocking vibrations

b) Scissoring vibrations 

a) Rocking Vibration

In rocking vibration two or more bonded atoms move same order parallel to each other.

b) Scissoring Vibration

                        In these types of vibration, two bonded atoms move towards each other or away from each other with same order like scissor is known as scissoring vibration.

B) Out of the Plane bending vibration

                             In this type of bending vibration, the movement of two bonded atoms with a common atom is out of plane then it is known as out of plane bending vibration.

It has further two types;

a) Twisting out of plane bending vibration

b) Wagging out of plane bending vibration

a) Twisting out of plane bending vibration

              In this type of motion, motion of two bonded atoms with a common atom in disorder or in order less motion is known as twisting out of plan bending vibration.

In this, one bonded atom motion is out of plane and other atom motion into plane and after twisting their position will change.

b) Wagging out of plane bending vibration

                   In this type, the motion of one atom movement is in to plane and other is out of plane with same order are known as Wagging out of plane bending motion.

2) Overtone Vibration:

                              The overtone vibration is a type of vibration in which when a light of particular wavelength irradiated on a molecule and the two or more electrons of that molecule excite those electrons at different energy levels is known as Overtone vibration.

Due to the excitation of electron at different energy level they form two types of bands or spectrum bands;

i) Combination band

ii) Difference band

Double Beam Spectrophotometer:

The design of double beam spectrophotometer splits coming light or radiation into two equal half from which one passing through the reference and two beams rejoin again at entrance slit and enters into entrance slit of the Monochromator. It reduces disadvantage of single beam spectrophotometers and a horizontal base line is obtained. 

Components of Spectrophotometer:

• Radiations  source.

• Monochromator  and  optical material.

• Sampling  area.

• Detectors.

• Amplifiers and  Recorders..

Radiation Source:

Radiations are produced due to electrical heat up of Nernst filament or a Globar to 1000-18000c.  Globar 5 cm in length made up of silicon carbide and 0.5 cm in diameters. The maximum  radiations  for  Globar occur in the 5500-500 cm-1 IR region. For light source nichrome wire, carbon arc, rhodium wire and tungsten filament lamp are also used.

Monochromator:

Monochromator is used for the separation of desired  frequencies. For this purpose we can use gratings as well as prism. Prisms that are made up of alkali halides used for IR spectrophotometer.  In the region prisms of CaF2 gives more resolution. But now mostly grating spectrometer are used. 

Sampling area:

We can examine any compound as in gaseous, pure liquid or solid phase.

Detector:

Detector converts  thermal  energy into electrical energy. Infrared detector may be selective or non-selective. The Selective detector are those whose response is markedly dependent upon wavelength of the incident radiations. 

There are two types of detector 

 Thermal detector 

It depends on heating effect of radiation. There are many types of thermal detector used:

 Thermocouple 

 Thermistor 

 Pyroelectric detector

 Pneumatic detector

 Golay cell

         Merits of thermal detector:

• Used for wide wavelength range

• Linearity in response

        Demerits of thermal detector:

o It shows slow response time

o It lowers sensitivity

 Quantum detectors

It detects all types of radiations. It consists of intrinsic and extrinsic detectors: there are many types of these detector used: 

• Photoconductive cells

• Photographic plates

• Photo voltaic cells

• Photocells

 

Amplifier and Recorder:

Increasing signals are used to transmit a certificate that reduces the radiation emitted from the reference beam until the energy balance is restored. This is achieved by a  motor comb into the reference beam when an absorbing band is encountered and out of the beam when band is passed over.

         An alternating current amplifier is used. The radiation beam is      chopped at a suitable frequency to provide the A. C. signal for the detector and rectified and used to drive the recorder.

Spectrophotometer:

Applications of IR Spectroscopy:

i. Detection of impurity in a compound

ii. Identification of an organic compound

iii. Structure determination

iv. Quantitative analysis

v. Qualitative analysis of functional group

vi. Conformational analysis

vii. Progress of a reaction

Infrared Spectra of different Compounds:

Infrared Specta of Alcohol:

Structural Determination:

All major functional groups absorb at their characteristics wavenumbers. From the data available due to absorption frequencies, the probable structure can be predicted. If some chemical data is available, it leads to the confirmation of the structure.

For example:

 The infrared spectra of amino acid exhibit bands of ionized carboxylic acid and amino salts. No band for free ---NH2 and ---COOH group is observed. Clearly amino acid exists as Zwitter ion.

Quantitative analysis:

It helps to make a qualitative estimation of an organic mixture. The estimation of component of a mixture can be done by:

• Measuring the intensities of absorption band characteristics of each component.

• Knowing the optical density of the absorption band for a pure component.

For example: it is known that commercial xylene exists as a mixture of three isomers, Ortho meta and para xylene.

Qualitative analysis of Functional group:

The presence of oxygen reveals that the group may be –OH, -COOH, COOR, C=O, and anhydrides. But an absorption band between 3600-3200per cm limits the possibilities. The band in this region may be due to O-H bond.

Conformational analysis:

The technique of IR spectroscopy is quite useful in determining the relative stability  0f various conformations 0f cyclic  compounds. Cyclohexane has two conformations as chair and boat form. In chair from six H-atoms are axial and other six H-atoms are equatorial. By infra-red spectroscopy, axial and equatorial substituents in chair form of cyclohexane can also be distinguished.

Progress of a reaction:

The progress of most reaction cab be followed readily by examining the infrared spectra of aliquots withdraw from the reaction mixture.

 For example,

 The oxidation of a secondary alcohol to ketone is accompanied by the disappearance of the O-H band near 3600cm-1and appearance of C=O band near 1715cm-1.

Use in Conformational Analysis:

The technique of I.R spectroscopy has great importance for determance the relative stability of conformations of cyclic compounds. As we know that cyclohexane has two conformation one is chair and other is boat form.