IR introduction

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1 INTRODUCTION TO INFRARED SPECTROSCOPY BY M.PRASANTHI

HISTORY: 

HISTORY Sir Frederick William Herschel Herschel was testing filters for the sun so he could observe sun spots. When using a red filter he found there was a lot of heat produced. Herschel discovered infrared radiation in sunlight by passing it through a prism and holding a thermometer just beyond the red end of the visible spectrum . This thermometer was meant to be a control to measure the ambient air temperature in the room. He was shocked when it showed a higher temperature than the visible spectrum . Further experimentation led to Herschel's conclusion that there must be an invisible form of light beyond the visible spectrum . 2

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EMR 4

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Electromagnetic spectrum 5

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INTRODUCTION IR spectroscopy is an absorption spectroscopy where absorption of radiation is studied. MOLECULAR spectroscopy where changes in energy takes place at molecular level on absorption. Mainly used in structure elucidation to determine the FUNCTIONAL GROUPS . 7

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Bonds in a molecule are analogous to springs and are rigid in nature. During continuous motion of molecule, they maintain some vibrations with some frequency, characteristic to every portion of molecule. This is called the Natural frequency of vibration. APPLIED FREQUENCY NATURAL FREQUENCY 9

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INSTRUMENTATION 10

We use wave numbers instead of wavelength for mentioning the characteristic peak, because wave numbers are larger value and easy to handle than wavelengths which will show only small difference between functional groups.: 

We use wave numbers instead of wavelength for mentioning the characteristic peak, because wave numbers are larger value and easy to handle than wavelengths which will show only small difference between functional groups. WHY TO USE WAVE NUMBER 1 × 10 4 wave length in µ 11

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CRITERIA FOR A COMPOUND TO ABSORB IR RADIATION For the molecule to absorb IR radiation, it has to fulfill certain requirements which are as follows WAVELENGTH DIPOLE MOMENT 12

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Dipole Moment Must Change for a vibration to be “IR active”! In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole moment changes. 13 Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active. Symmetrical stretching/bending are IR inactive.

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What is the intensity of an IR signal of: O 2 or N 2 or H 2 ? Ans : In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of the molecule O 2 , N 2 and H 2 DO NOT ABSORB IR LIGHT ! 14

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Does O=C=O absorb IR light? Ans : vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light 15

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16 No dipole generated Dipole generated

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MOLECULAR VIBRATIONS A molecule consisting of n atoms has a total of 3 n degrees of freedom, corresponding to the Cartesian coordinates (x,y,z) of each atom in the molecule. molecule degrees of freedom Nonlinear 3n-6 Linear 3n-5 17 Translation - the movement of the entire molecule while the positions of the atoms relative to each other remain fixed 3 degrees of translational freedom. Rotational transitions – interatomic distances remain constant but the entire molecule rotates with respect to three mutually perpendicular axes: 3 rotational freedom (nonlinear), 2 rotational freedom (linear). Fundamental-

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Fundamental Vibrations A molecule has as many as degrees of freedom as the total degree of freedom of its individual atoms. Each atom has 3 degree of freedom ( x,y,z ) A molecule of n atoms therefore has 3n degrees of freedom. Non linear molecules (e.g. H 2 O) Vibrational degrees of freedom or Fundamental Vibrations = 3n – 6 Symmetrical Stretching ( υ s OH) 3652 cm -1 Asymmetrical Stretching ( υ as OH) 3756 cm -1 Scissoring ( δ s HOH) 1596 cm -1

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For linear molecule (e.g. CO 2 ) : Vibrational degrees of freedom or Fundamental Vibrations = 3n – 5 Symmetrical Stretching ( υ s CO 2 ) 1340 cm -1 Asymmetrical Stretching ( υ as CO 2 ) 2350 cm -1 Scissoring (bending out of the plane of the paper) ( δ s CO 2 ) 666 cm -1 Scissoring (bending in the plane of the paper) ( δ s CO 2 ) 666 cm -1

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IR Stretching Frequencies: What Do they Depend On? Directly on the strength of the bonding between the two atoms (  ~ k) Inversely on the reduced mass of the two atoms (v ~ 1/m) Expect:  will increase with increasing bond strength (bond order) and decreasing mass 20

Asymmetric stretching: one atom approaches central atom while the other departs from it: 

21 stretching vibrations, two types: Symmetric stretching: movement of atoms in same direction with respect to a particular atom in a molecule in same direction Asymmetric stretching: one atom approaches central atom while the other departs from it

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22 Bending vibrations, four types: In plane bending: Scissoring : Two atoms approach each other Rocking: atoms move in same direction Out of plane bending: Wagging : two atoms move ‘up and down’ the plane with respect to the central atom Twisting : one atom moves up the plane while other atom moves down the plane with respect to the central atom

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Asymmetric Symmetric Scissoring Twisting Wagging Rocking 23

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VIBRATIONS IN ADRENALIINE 24

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MOLECULAR VIBRATIONS IN ACTEONE 25

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Stretching Vibrations The stretching frequency of a bond can be approximated by Hooke’s Law. In this approximation, two atoms and the connecting bond are treated as a simple harmonic oscillator composed of 2 masses (atoms) joined by a spring: 26

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= frequency k = spring strength (bond stiffness) m r = reduced mass (~ mass of largest atom) Hooke’s law 27

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Bending Symmetric Asymmetric 28

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