Dhanya nair

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PRINCIPLES OF IR SPETROSCOPY PRESENTED BY, Dhanya KT, 1 st year M Pharm , Dept: of pharmaceutics, Nehru college of pharmacy, Pambady. Date of presentation:13-7-2011 1

Contents :

Contents Introduction to IR spectroscopy Range of IR spectroscopy radiation Principles of IR spectroscopy Modes of vibrations 3n Degrees of freedom Factors influencing vibrational frequencies References 2



Introduction :

Introduction IR spectroscopy ( vibrational spectroscopy) is connected with the study of of IR radiation, which result the vibrational transitions. IR spectra is mainly used in structural elucidation to determine the functional group 4

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Energy of molecule = Electronic energy + Vibrational energy + Rotational energy IR spectroscopy is the study focused on the change in the vibration of molecule and absorption of energy due to vibrations. 5

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Atoms or groups of atoms are connected by bonds. These are non rigid in nature. Due to continuous motion of the molecule they maintain vibration with frequency. Applied frequency = natural frequency of vibration. The absorption of IR take place and a peak is observed IR spectrum of a chemical substance is a finger print for its identification. 6 In any molecule,

Band positions in IR :

Band positions in IR Band positions in IR may be expressed conveniently by wave number( ν ), whose unit is cm-1. The relation between wave number( ν ) wave length( λ ) and frequency ( ν ) is as follows ν = C/ λ ν / (cm-1)= ν /c = 1/ λ C= velocity of light 7

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Band intensities of IR spectrum may be expressed either as transmittance or absorbences . Transmittance (T): It is defined as the ratio of radient power transmitted by a sample to the radient power incident on the sample. Absorbence (A): Absorbance is defined as the logaritham to the base10 of the reciprocal of the transmittance. Ie ; A= log 10 I/T 8

Range of IR spectroscopy region :

Range of IR spectroscopy region The IR radiation refers broadly to that region of electromagnetic spectrum which lies between the visible and microwave region. visible NEAR Infrared MID FAR Microwave λ cm ν cm-1 Energy 7.8x10-5 to 3x10-4 12820 to 4000 10-37 Kcal/mole 3x10-4 to 3x10-3 4000 to 400 1-10 Kcal/mole 3x10-3 to 3x10-2 400 to 33 0.1-1 Kcal/mole 9

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The infrared region constitutes 3 parts a) The near IR (12,820-4000cm-1) b) The middle (4000-400cm-1) c) The far IR (400-33cm-1) most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region. Wave number is mostly used measure in IR absorption because wave numbers are larger values & easy to handle than wave length which are measured in µm. 11

Theory of IR:

Theory of IR For a molecule to absorb IR radiation, it has to fulfill certain requirements : Correct wavelength of radiation : A molecule to absorb IR radiation the natural frequency of vibrations of some part of a molecule is the same as the frequency of incident radiation. Eg : Hcl 12

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Electric dipol A molecule can only absorb IR radiation when its absorption cause a change in its electric dipol . A molecule is said to have an electric dipol when there is a slight positive and a slight negative electric charge on its component of atom. Eg :- Ethylene, Bromoethylene 13


MOLECULAR VIBRATIONS There are 3 types of vibrations. Fundamental vibrations 2. Overtone vibrations 3. Combination vibrations 14

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There are 2 types of fundamental vibrations : Stretching vibrations Bending vibrations 1)Stretching vibrations: in this bond length is increased or decreased periodically. They are of 2 types symmetrical streching asymmetrical streching 15

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a) symmetrical stretching : 2 bonds increase or decrease in length symmetrically. b) Asymmetrical stretching : in this one bond length is increased and other is decreased. 16

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2)Bending vibrations: These are also called as deformations. In this bond angle is altered. These are of 2 types In plane bending → scissoring, rocking Out plane bending → wagging, twisting 17

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Scissoring: This is an in plane bending. In this bond angles are decreased,2 atoms approach each other. Rocking: In which bond angle is maintained ,but the bond move with in the plane . 18

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Wagging: It is an out of plane bending. In this 2 atoms move to one side of the plane. They move up and down the plane. Twisting: In this one atom moves above the plane and the other atom moves below the plane. 19

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Overtone vibrations: when a molecule absorbs a quantum of energy( E), corresponding to the vibration's frequency (ν) according to the relation E = h ν The overtone band is arise due to the absorption of a photone that leads to doubly excited vibrational state. 20

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Combination vibrations Combination modes involve a simultaneous increase in the vibrational quantum number of two or more vibrational modes by the absorption of a single photon However, in order to “combine”, the vibrations must a) involve the same functional group, and b) have the same symmetry properties 21

NUMBER OF VIBRATIONAL MODES (3n degrees of freedom) :

NUMBER OF VIBRATIONAL MODES (3n degrees of freedom) 22 x y z (usually the axis of highest symmetry)

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A molecule can vibrate in many ways, and each way is called a vibrational mode. If a molecule contains ‘N’ atoms, total number of vibrational modes For linear molecule it is (3N-5) For non linear molecule it is (3N-6) 23

The Distribution of Degrees of Freedom for Polyatomic Molecules :

The Distribution of Degrees of Freedom for Polyatomic Molecules Degrees of Freedom Linear Molecules Non-Linear Molecules Translational 3 3 Rotational 2 3 Vibrational 3n-5 3n-6 Total 3n 3n 24

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Eg : 1. H 2 O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom or modes. 2. CO 2 ,linear molecule, will have 3× 3-5=4 degrees of vibrational modes. 25

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Stretching vibrations = the number of bonds in the molecule the no of Bending vibrations = total number of vibrations - stretching vibrations = ( 3 N – 5 for a linear molecule ) - the no of stretches = ( 3 N – 6 for a non-linear molecule ) - the no of stretches 26 Vibrations in a molecule can be classified as

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In polyatomic molecules, the actual number of modes of vibrations will be altogether different from those calculated theoretically. Reasons The over tones and combination of tones may increase the number of modes of vibrations. Some other phenomenon may reduce the number of bonds: 1.Vibrations that do not fall in the IR region cannot be observed in the spectrum. 2.Some weak vibrational bonds, if present cannot be seen in the IR 27

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3. Some vibrational bands, which are having the same or slight different frequencies, overlap each other and appear as one band in the IR spectrum. 4.Some vibrational bands may degenerate and appear at the same place in IR spectrum. 5.If there occurs no required change in dipol character of the molecule, no band will appear in the IR spectrum. 28


VIBRATIONAL FREQUENCY occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion. The frequency of the periodic motion is known as a vibration frequency. The value of stretching vibrational frequency of a bond can be calculated by the application of hooke’s law. ν /c = ν = 1/2 п c[k/m1m2/m1+m2]1/2 = 1/2 п c√k /µ Where, µ →reduced mass m1&m2 →masses of the atoms k →force constant c →velocity of radiation 29

Factors influencing vibrational frequencies:

Factors influencing vibrational frequencies Calculated value of frequency of absorption for a particular bond is never exactly equal to its experimental value. There are many factors which are responsible for vibrational shifts Vibrational coupling and fermi resonance it is observed in compounds containing –CH2 & -CH3. EG. Carboxylic acid anhydrides amides aldehydes 30

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Fermi resonance Fermi resonance results in the splitting of two vibrational bands that have nearly the same energy and symmetry in IR spectroscopies . The two bands are usually a fundamental vibration and either an overtone or combination band The wave functions for the two resonant vibrations mix, and the result is a shift in frequency and a change in intensity in the spectrum. As a result, two strong bands are observed in the spectrum, instead of the expected strong and weak bands. 31

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2) Hydrogen bonding: Hydrogen bonding brings about remarkable downward frequency shifts. Stronger the hydrogen bonding, greater is the absorption shift towards lower wave length than the normal value. There is 2 types of hydrogen bonding a) inter molecular →broad bands b) intra molecular → sharp bands hydrogen bonding in O-H and N-H compounds deserve special attention. Eg: alcohols&phenols enols & chelates 33

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3) Electronic effects: In this the frequency shifts are due to electronic effects which include conjugation, mesomeric effect, inductive effect . a) conjugation: conjugation lowers the absorption frequency of C=O stretching whether the conjugation is due to α , β - unsaturation or due to an aromatic ring. b) mesomeric effect: a molecule can be represented by 2or more structures that differ only in the arrangement of electrons. c) inductive effect: depends upon the intrinsic tendency of a substituent to either release or withdraw electrons. 34

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