THEORY OF I.R. SPECTROSCOPY AND FT-IR

Theory of I.R. spectroscopy and FT-IR

1. THEORY OF I.R.SPECTROSCOPY & FT-IR ANJALI TERESA FIRST YEAR M.PHARM THEORY OF I.R. SPECTROSCOPY & FT-IR Presented by : ANJALI TERESA FIRST YEAR M.PHARM 1

2. INTRODUCTION Spectroscopy is the branch of science dealing with the study of interaction of electromagnetic radiation with matter . IR spectroscopy is Absorption spectroscopy in which molecular vibrations observed due to absorption of IR radiation. Infrared radiation was discovered in 1800 by William Herschel . 2 THEORY OF I.R. & FT-IR.

3. The range of EMR between the visible and microwaves region is called INFRARED region(14000-40 cm -1 ). 3 THEORY OF I.R. & FT-IR.

4. I.R. spectroscopy is also known as vibrational spectroscopy since it causes vibrational transitions. The vibrations in the I.R. spectroscopy is known as Fundamental vibrations. I.R. spectrum is mainly used in structural elucidation to determine the functional groups. The infrared region of the spectrum encompasses radiation with wavenumbers ranging from about 12800 to 10cm -1 . 4 THEORY OF I.R. & FT-IR.

5. REGION WAVELENGTH (µm) WAVE NUMBER (cm -1 ) NEAR 0.78-25 12800-4000 MIDDLE 2.5-50 4000-200 FAR 50-1000 200-10 5 THEORY OF I.R. & FT-IR.

6. Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region . THEORY In any molecule, atoms or group of atoms are connected by bonds which are similar to springs & not rigid in nature. 6 THEORY OF I.R. & FT-IR.

7. This characteristic vibration are called Natural frequency of vibration. When energy in the form of infrared radiation is applied then it causes the vibration between the atoms of the molecules and when, Applied infrared frequency = Natural frequency of vibration 7 THEORY OF I.R. & FT-IR.

8. Then , Absorption of IR radiation takes place and a peak is observed. Different functional groups absorb characteristic frequencies of IR radiation. Hence gives the characteristic peak value. Therefore, IR spectrum of a chemical substance is a finger print of a molecule for its identification. CRITERIA FOR A COMPOUND TO ABSORB I.R RADIATION Correct wavelength of radiation Change in dipole moment 8 THEORY OF I.R. & FT-IR.

9. 1. 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 . 2. Change in dipole moment: A molecule can only absorb IR radiation when its absorption cause a change in its electric dipole A molecule is said to have an electric dipole when there is a slight positive and a slight negative charge on its component of atoms. 9 THEORY OF I.R. & FT-IR.

10. For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule. 10 THEORY OF I.R. & FT-IR.

11. MODE OF VIBRATION Degree of freedom is the number of variables required to describe the motion of a particle completely.   For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three.   Its motion is purely translational.   If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom.   11 THEORY OF I.R. & FT-IR.

12. In defining the motion of the molecule , we need to consider, Translational motion :The motion of the entire molecule through space. Rotational motion :The motion of the entire molecule around the centre of gravity. Vibrational motion :The motion of each atom relative to the other atom. 12 THEORY OF I.R. & FT-IR.

13. There are two cases, Linear molecule Non linear molecule LINEAR MOLECULE :It is a special case since by definition all of the atoms lie on a single straight line.Rotation about the bond axis is not possible and two degrees of freedom suffice to describe rotational motion.Thus the number of vibrations for a linear molecule is given by (3N-5). NON LINEAR MOLECULE :For non-linear molecule all rotational degrees of freedom is three and the remaining(3N-6) degree of freedom constitute vibrational motion. 13 THEORY OF I.R. & FT-IR.

14.   Non linear molecule Linear molecule TRANSLTIONAL 3 Degrees of freedom 3 Degrees of freedom ROTATIONAL 3 degrees of freedom 2 Degrees of freedom FUNDAMENTAL 3N-6 3N-5 BENDING 2N-5 2N-4 14 THEORY OF I.R. & FT-IR.

15. Types of molecular vibrations There are 2 types of vibrations: Stretching vibrations Bending vibrations Stretching vibrations : Vibration or oscillation along the line of bond Change in bond length Occurs at higher energy: 4000-1250 cm -1 2 types: Symmetrical stretching Asymmetrical stretching 15 THEORY OF I.R. & FT-IR.

16. Symmetric stretching 2 bonds increase or decrease in length simultaneously. H H C In this, one bond length is increased and other is decreased. Asymmetrical stretching 16 THEORY OF I.R. & FT-IR.

17. 2 .BENDING VIBRATIONS H H C Vibration or oscillation not along the line of bond These are also called as deformations In this, bond angle is altered Occurs at low energy: 1400-666 cm -1 17 THEORY OF I.R. & FT-IR.

18. 2 types : In plane bending : scissoring, rocking Out plane bending : wagging, twisting In plane bending Scissoring : This is an in plane blending 2 atoms approach each other Bond angles decrease 18 THEORY OF I.R. & FT-IR.

19. H H C C H H C C 19 THEORY OF I.R. & FT-IR.

20. b) Out plane bending i . Wagging: 2 atoms move to one side of the plane. They move up and down the plane . Twisting: One atom moves above the plane and another atom moves below the plane . H H C C H H C C 20 THEORY OF I.R. & FT-IR.

21. MECHANICAL MODEL OF A STRETCHINGVIBRATION IN A DIATOMIC MOLECULE The characteristics of an atomic stretching vibration can be approximated by a mechanical model consisting of two masses connected by a spring.A disturbance of one of these masses along the axis of the spring results in a vibration called simple harmonic motion . Let us first consider the vibration of a single mass attached to a spring that is hung from an immovable object. If the mass is displaced a distance ‘y’ from its equilibrium position by the application of a force along the axis of the spring, the restoring force F is proportional to the displacement. 21 THEORY OF I.R. & FT-IR.

22. That is, F = - ky -----------1 Where, k is the force constant which depends on the stiffness of the spring - ve sign indicate that F is a restoring force.ie, the direction of the force is opposite the direction of the displacement. 22 THEORY OF I.R. & FT-IR.

23. Potential energy of a harmonic oscillator The potential energy E of mass and spring can be arbitrarily assigned a value of zero, when the mass is in its rest or equilibrium position.As the spring is compressed or stretched, however P.E. of this system increases by an amount equal to the work required to displace the mass. If for example the mass is moved from some position y to y+dy , the work & hence the change in Potential energy dE is equal to the force F times the distance dy. Thus, dE = - Fdy -----------------2 Substituting 1 in 2 we get, dE = kydy 23 THEORY OF I.R. & FT-IR.

24. Integrating between the equilibrium position y=o &y gives E ∫ dE = y ∫ydy E=1/2 ky 2 --------------------------3 The potential energy curve for a simple harmonic oscillation is parabola. P.E. is maximum when the spring is stretched or compressed to the maximum amplitude A &it decreases to zero at the equilibrium position. Vibrational Frequency The motion of the mass as a function of time t can be deduced from classical mechanics as follows Newtons second law states that, F = ma Where, ‘m’ is the mass & ‘ a’is the acceleration 24 THEORY OF I.R. & FT-IR.

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29. FTIR 29 THEORY OF I.R. & FT-IR.

30. FT-IR stands for Fourier Transform Infrared , the preferred method of infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passed through (transmitted). The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. 30 THEORY OF I.R. & FT-IR.

31. THEORY . Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the limitations encountered with dispersive instruments. The main difficulty was the slow scanning process. A method for measuring all of the infrared frequencies simultaneously, rather than individually, was needed . A solution was developed which employed a very simple optical device called an interferometer. The interferometer produces a unique type of signal which has all of the infrared frequencies “ encoded ” into it. 31 THEORY OF I.R. & FT-IR.

32. The signal can be measured very quickly , usually on the order of one second or so. Thus, the time element per sample is reduced to a matter of a few seconds rather than several minutes. Most interferometers employ a beamsplitter which takes the incoming infrared beam and divides it into two optical beams. One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat mirror which is on a mechanism which allows this mirror to move a very short distance (typically a few millimeters) away from the beamsplitter . 32 THEORY OF I.R. & FT-IR.

33. The two beams reflect off of their respective mirrors and are recombined when they meet back at the beamsplitter . Because the path that one beam travels is a fixed length and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams “interfering” with each other . The resulting signal is called an interferogram which has the unique property that every data point (a function of the moving mirror position) which makes up the signal has information about every infrared frequency which comes from the source. 33 THEORY OF I.R. & FT-IR.

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36. This means that as the interferogram is measured, all frequencies are being measured simultaneously . Thus, the use of the interferometer results in extremely fast measurements . Because the analyst requires a frequency spectrum (a plot of the intensity at each individual frequency) in order to make an identification, the measured interferogram signal can not be interpreted directly . A means of “ decoding ” the individual frequencies is required. This can be accomplished via a well-known mathematical technique called the Fourier transformation . This transformation is performed by the computer which then presents the user with the desired spectral information for analysis. 36 THEORY OF I.R. & FT-IR.

37. Fourier transform infrared spectroscopy is preferred over dispersive or filter methods of infrared spectral analysis for several reasons: • It is a non-destructive technique. • It provides a precise measurement method which requires no external calibration. • It can increase speed , collecting a scan every second. • It can increase sensitivity . • It has greater optical throughput . • It is mechanically simple with only one moving part. 37 THEORY OF I.R. & FT-IR.

38. REFERENCE 1)Instrumental Analysis By Skoog,Holler,Crouch , Indian edition(2009),Pg.no:477-490 2)Instrumental analysis of chemical analysis By Gurdeep.R.Chatwal & Sham.K.Anand , Himalaya Publishing house, 5 th edition, Pg.no:2.29-2.40 3)Pharmaceutical Drug Analysis By Ashuthosh Kar Page No: 315 – 318 4) Introduction to FT-IR Spectrometry By Thermo Nicolet corporation . 38 THEORY OF I.R. & FT-IR.

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