Infrared spectroscopy ppt

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Theory principle and instrumentation of infrared spectroscopy

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THEORY AND INSTRUMENTATION OF INFRARED SPECTROSCOPY By T.SUJITH, Y11MPH476 1

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“Spectroscopy is an instrumentally aided study of the interactions between matter (sample being analyzed) and energy (any portion of the electromagnetic spectrum, EMS)” Introduction: 2 Energy of molecule = Electronic energy + Vibrational energy + Rotational energy Infrared spectroscopy is concerned with the study of absorption of infrared radiation, which results in vibrational transitions It is also called as Vibrational spectroscopy IR spectra mainly used in structure elucidation to determine the functional groups

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3 Electro Magnetic Radiation:

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4 Molecular effects:

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5 The Range of Infrared Radiation: The IR radiation refers broadly to that region of electromagnetic spectrum which lies between the visible and microwave regions IR region may be divided into four sections: The photographic region: This ranges from visible to 1.2µ The Very Near Infrared region: Also known as overtone region and ranges from 1.2-2.5µ The Near Infrared region: This is also known as vibration region and ranges from 2.5-25µ The Far Infrared region: This is known as the rotation region. This ranges from 25-400µ

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6 Principle: Molecules are made up of atoms linked by chemical bonds. The movement of atoms and chemical bonds like spring and balls (vibration) This characteristic vibrations are called Natural frequency of vibration When energy in the form of infrared radiation is applied and when, Applied infrared frequency= Natural frequency of vibration Absorption of IR radiation takes place and a peak is observed

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7 What happens when a sample absorbs IR energy? stretching and bending of bonds (typically covalent bonds) E vibration increases momentarily - O - H IR ( 3500 cm - 1 ) - O — H When an analytical chemist speaks of infrared spectroscopy, he usually means the range from 2.5-25µ This range gives important information of the vibrations of the molecules, and hence the structure of the molecule

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8 Theory of Infrared Absorption Spectroscopy Correct Wavelength of Radiation: A molecule absorbs radiation when only Natural frequency of vibration = Frequency of incident radiation Electric Dipole: i)A molecule can only absorb radiation when its absorption causes a change in its electric dipole. ii)A molecule is said to have electric dipole when there is a slight positive and a slight negative electric change on its component atoms

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9 MOLECULAR VIBRATIONS

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10 Modes of vibrations: Stretching: change in bond distance. Occurs at higher energy: 4000-1250 cm  1 H 2 O

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11 Bending: change in bond angle. Occurs at lower energy: 1400-666 cm  1 -CH 2 -

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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. i)Each atom has 3 degree of freedom (x,y,z) ii)A molecule of n atoms therefore has 3n degrees of freedom. ***For Non linear molecules (e.g. H 2 O) Vibrational degrees of freedom or Fundamental Vibrations = 3n – 6 Symmetrical Stretching Asymmetrical Stretching Scissoring 12

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***For linear molecule (e.g. CO 2 ) : Vibrational degrees of freedom or Fundamental Vibrations = 3n – 5 Symmetrical Stretching Asymmetrical Stretching Scissoring (bending out of the plane of the paper) Scissoring (bending in the plane of the paper) 13

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14 HOOKE’S LAW: It approximates stretching frequency In this approximation, two atoms and the connecting bond are treated as a simple harmonic oscillator According to Hooke’s law, the frequency of the vibration of the spring is related to the mass and the force constant of the spring k, by the following formula, Where, k is the free constant m is the mass v is the frequency of the vibration

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How does the mass influence the vibration? H 2 I 2 MM =2 g/mole MM =254 g/mole The greater the mass - the lower the wave number (ύ) 15

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16 Stretching Frequencies a)Frequency decreases with increasing atomic mass b)Frequency increases with increasing bond energy

IR Correlation Diagram: 

IR Correlation Diagram Transmittance (%) 100 80 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 2.5 3.0 4.0 5.0 6.0 10.0 Frequency (cm -1 ) Region I 3600-2700 cm -1 Region II 1800-1600 cm -1 / Wavelength (microns, mm) O-H N-H C-H bond stretching alcohols phenols carboxylic acids amines amides alkynes alkenes alkanes C=O acid chlorides anhydrides esters ketones aldehydes carboxylic acids amides Fingerprint Region (below 1500 cm -1 )  C-H =C-H -C-H 17

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18 IR SPECTRUM i)No two molecules will give exactly the same IR spectrum (except enantiomers) ii)Simple stretching: 1600-3500 cm -1 iii)Complex vibrations: 400-1400 cm -1 , called the “fingerprint region”

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19 Instrumentation The main parts of an IR spectrometer are as follows: IR radiation sources Monochromators Sample cells and sampling of substances Detectors

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20 IR radiation sources: The radiation source must emit IR radiation which must be Intense enough for detection Steady Extended over the desired wavelengths Various popular sources of IR radiation are: Incandescent Lamp: Has low spectral emissivity (b ) Nernst Glower: Glower is composed of rare earth oxides such as zirconia, yttria, thoria It is non conducting at room temperature Heated by external means to 1000-1800ºC to bring it into conducting state Provides maximum radiation at about 7100 cmˉ 1 Disadvantages: Emits IR radiation over wide wavelength range Frequent mechanical failure Its energy also concentrate in the visible and near IR regions of the spectrum

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21 Parts of Nernst Glower Globar source Mercury Arc

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22 (c) Globar Source: Contains a rod of sintered silicon carbide, when it is heated to 1300-1700ºC emits radiation in IR region at 5200 cmˉ 1 Unlike the Nernst glower, it is self starting As its temperature coefficient is positive, it can be conveniently controlled with a variable transformer Disadvantage: Less intense source than Nernst glower (d) Mercury Arc: In far IR region special high pressure mercury arc lamps are used Beckman devised the quartz mercury lamps At the shorter wavelengths, the heated quartz envelope emits the radiation whereas at longer wavelengths the mercury plasma provides radiation through the quartz

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23 Monochromators: (a) Prism Monochromator: Must be constructed of materials which transmit in the infrared Sodium chloride is most common prism salt Two types i) Single pass ii) Double pass Single pass prism monochromator

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24 The double pass monochromator produces more resolution than the monochromator in the radiation, before it finally passer on to the detector For 4000-650cmˉ 1 region in both mono and double pass monochromators NaCl prisms are used Prism of lithium fluoride or calcium fluoride give more resolution in the region where the significant stretching vibrations are located (b) Grating monochromator: High dispersion can be achieved Gratings offer linear dispersion and may be constructed for a wide variety of materials The grating is essentially a series of parallel straight lines cut into a plane surface Dispersion by grating follows the law of diffraction n λ = d(sin i ± sin θ ) Where, n is the order, λ the wavelength of the radiation, d the distance between the grooves, i the angle of incidence of beam of IR radiation and θ the angle of dispersion of light of a particular wavelength

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25 Grating monochromator

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26 Sample cells and sampling of substances: The only common point to the sampling of different phages is that the material containing the sample must be transparent to IR radiation This condition restricts our selection to only certain salts Eg: NaCl, KBr, ThBr etc Sampling of solids i) Solids run in Solution: Solids may also be dissolved in a non-aqueous solvent Provided - There is no chemical reaction between sample and solvent - Solvent does not absorb in the studied range A drop of solution is placed on an alkali metal disk and the solvent allowed to evaporate, leaving a thin film of the sample Or the entire solution is placed in a liquid sample cell Limitation: There is no single solvent which is transparent throughout IR region

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27 ii) Solid Films: If a solid is amorphous in nature the sample is deposited on the surface of a KBr or NaCl cell by evaporation of a solution of the solid Useful for rapid qualitative analysis but becomes useless for carrying out quantitative analysis iii) Mull Technique: The finely ground solid sample is mixed with Nujol(mineral oil) to make a thick paste which is then made to spread between IR transmitting windows Limitation: Nujol has the absorption maxima at 2915, 1462, 1376, and 719 cmˉ 1 , absorption bands of the sample happen to coincide with the absorption bands of Nujol mull This method is good for qualitative analysis but not for quantitative analysis iv) Pressed Pellet Technique: A small amount of finely ground solid sample intimately mixed with about 100 times its weight of KBr The finely ground mixture is then pressed under very high pressure in a press to form a small pellet The resulting pellet is transparent to IR radiation

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28 Advantages over Mull Technique: Use of KBr eliminates the problem of bands which appear in the spectrum due to the mulling agent KBr pellets can be stored for long periods of time As the concentration of sample can be suitably adjusted in the pellets, it can be used for quantitative analysis The resolution of the spectrum in the KBr is superior to that obtained with mulls Disadvantages: It always has a band at 3450 cmˉ 1 , from the OH group of moisture present in the sample High pressure involved during the formation of pellets may bring about polymorphic changes in crystallinity in the samples This method is not successful for some polymers which are difficult to grind with KBr

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29 b) Sampling of liquids: samples that are liquids at room temperature are usually put frequently with no preparation, into rectangular cells made of NaCl, KBr or ThBr and their IR spectra are obtained directly c) Sampling of gases: The gas absorption cell is similar to the cell for liquid samples To compensate for the small number of molecules of a sample that is contained in a gas, the cells are larger Multiple reflections can be used to make the effective path length Detectors : Thermal detectors are better choice except in the near infrared where photoconductivity cells are generally used Requirements for thermal detectors i) Short responsive time ii) Absorbed heat must be lost rapidly(As heat transfer is a slow process this is difficult requirement)

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30 Various types of detectors used in IR spectroscopy: Bolometer : A bolometer is based upon the fact tat the electrical resistance of a metal increases approximately 0.4% for every Celsius increase of temperature Usually consists of a thin metal conductor When radiation falls on this conductor, its temperature changes As the resistance of a metallic conductor changes with temperature, the degree of change in resistance is regarded as a measure of the amount of radiation that has fallen on the bolometer The response time for a bolometer is 4mSec

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31 b) Thermocouple: The thermocouple detector is based upon the fact that an electrical current will flow when two dissimilar metal wires are connected together at both ends and a temperature differential exists between two ends The end exposed to the infrared radiation is called the “hot junction”. In order to increase the energy gathering efficiency, it is usually a “black body” The other connection, “cold junction” is thermally insulated and carefully screened from stray light A thermocouple is closed in an evacuated steel casing with a KBr window to avoid losses of energy by convection When hot junction is exposed to IR radiation which increases the temperature of the junction The temperature difference between the two junctions generates potential difference which depends on how much IR radiation falls on the hot junction The response time a thermocouple is about 60mSec

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32 c) Golay cell: It consists of a small metal cylinder filled wit xenon gas. It is sealed with blackened metal plate at one end and other end by a flexible metalized diaphragm When radiation fall on the metal plate, it heats the gas in the cylinder which causes it to expand The resulting pressure increases in the gas deforms to metalized diaphragm Light from a lamp is made to fall on the diaphragm which reflects the light on to a photocell Motion of the diaphragm changes the output of cell The signal seen by the phototube is modulated in accordance with the power of the radiant beam incident on the golay cell Advantages: -Useful wavelength range is very wide - Response time is much faster than bolometer or thermocouple Disadvantage: More expensive and bulky

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33 d) Photoconductivity cell : Non thermal detector of greater sensitivity It consists of a thin layer of lead sulphide or lead telluride supported on glass and enclosed into an evacuated glass envelope When IR radiation is focused on lead sulphide, its conductance increases ans causes more current to flow Response time is 0.5mSec Disadvantage: When operated at room temperature, it has a very restricted range(limited to near infrared ) *** The range can be broadened by drastic cooling e) Thermistors: Made of fused mixture of metal oxides As the temperature of the mixture increases, its electrical resistance decreases(As opposed to the bolometer) Response time is slow

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34 Speed and accuracy of analysis Small sample requirement IR spectra are information rich; the peak position, intensities, widths, and shapes in a spectrum all provide useful information Can be applied to solids, liquids, gases and polymers Advantages of IR spectroscopy: Can’t be applied to single atomic entities as they don’t contain chemical bonds and hence don’t absorb IR radiation Nobel gases such as helium and argon don’t have infrared spectra Disadvantages:

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35 Difficult to be applied to samples of complex composition because the spectrum will also be complex to be interpreted and it will be very hard to know which infrared bands are due to which molecules Monoatomic ions such as Pb +2 dissolved in water aren’t chemically bonded to anything and don’t have an infrared spectrum Aqueous solutions are difficult to analyze using infrared spectroscopy

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36 Applications: Identification of functional group and structure elucidation Identification of drug substance Identifying the impurities in a drug sample Study of hydrogen bonding Study of polymers Ratio of cis-trans isomers in a mixture of compounds Quantitative analysis

Dealing with mixtures:: 

Dealing with mixtures: Library searching: In this technique, the mixture spectrum is mathematically compared to a collection of known spectra kept in a library. A number called the hit quality index (HQI) describes how similar or different the spectra are to each other. It takes place by one of two ways: 1. Spectral subtraction: This process can remove the bands of unwanted components from a spectrum. Subtraction involves taking the spectrum of a mixture and subtracting from it the spectrum of a pure compound that is present in the mixture. 37

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In this part we make use of a reference sample in addition to the unknown sample we are working on We obtain the spectra of the sample and the reference and then compare them Plot the two spectra using the same scale and then compare the two spectra with each others Properly performing identities: 38

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39 Fourier Transform system(FTIR)

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40 Dispersive IR Fourier Transform IR Many moving parts; results in mechanical slippage Only mirror moves Calibration against reference spectra is required Use of laser provides high frequency accuracy Stray light results in spurious results Stray light does not effect, since all signals are modulated A small amount of IR beam may be allowed to pass through slits A much larger beam may be used Only radiation of a narrow frequency range falls on the detector All frequencies of radiation fall on the detector Slow scan speeds Rapid scan speeds Sample is subjected to thermal effects Sample is not subjected to thermal effects Any emission of IR radiation by the sample will fall on the detector Any emission of IR radiation by the sample will not be detected Comparison of Fourier Transform and Dispersive IR

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41 Conclusion Infrared spectroscopy is one of the most powerful analytical techniques which offers the possibility of chemical identification One of the most important advantage of infrared spectroscopy over other usual methods of structural analysis is that it provides useful information about the structure of molecule quickly, without tiresome evaluation method Infrared spectroscopy itself established as a valuable tool for determination of organic, and to a lesser extent, inorganic, structure IR spectrum of a chemical substance is a fingerprint for its identificatio n

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42 References

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