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a review of infrared spectroscopy


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INFRARED SPECTROSCOPY Presented by: Ms. Ashritha Narikimalli , B.Pharm., (M.Pharm.,) Under the guidance of Mr. D. Sathis kumar, M.Pharm.,(Ph.D) Associate professor, Aditya Institute Of Pharmaceutical Sciences.


Electromagnetic spectrum is the arrangement of all types of electromagnetic radiations in order of their increasing wavelengths or decreasing frequencies. In the spectrum, the portion above visible region and below microwave region is INFRARED region. The region is Infra (less) energetic to visible red radiation. Hence its name. IR radiations have longer wavelength and are thus less energetic. The absorption of radiation by an organic compound in this region causes molecular vibrations. Hence IR spectroscopy is also called vibrational spectroscopy. INTRODUCTION

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THE ELECTROMAGNETIC SPECTRUM UV X-rays IR g -rays Radio Microwave Energy (kcal/mol) 300-30 300-30 ~10 -4 > 300 ~10 -6 Visible Frequency , n in Hz ~10 15 ~10 13 ~10 10 ~10 5 ~10 17 ~10 19 Wavelength , l 10 nm 1000 nm 0.01 cm 100 m ~0.01 nm ~.0001 nm nuclear excitation (PET) core electron excitation (X-ray cryst.) Electronic excitation( p to p *) molecular vibration molecular rotation Nuclear Magnetic Resonance NMR (MRI)


REGIONS OF IR The Infrared (IR) region of the electromagnetic spectrum extends from 0.8µm (800nm) to 1000µm (1mm). It is subdivided into near infrared (0.8 to 2.5µm), middle infrared (2.5 to 50µm), and far infrared (50 to 1000µm). Near IR Mid IR Far IR 12500 4000 200 10cm -1 Wave number 0.8 2.5 50 200µ Wave length


THEORY & PRINCIPLE The absorption of IR radiations causes the excitation of molecule from a lower to the higher vibrational level. Vibrational spectra appear as bands rather than as lines because a single vibrational energy is accompanied by a number of rotational energy changes. (vibrational-rotational bands). Band intensities can be expressed as transmittance (T) or absorbance (A). ‘T’ is the ratio of the radiant power transmitted by a sample to the radiant power incident on the sample. A=log 10 (1/T)

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All bonds in a molecule are not capable of absorbing IR energy but only those bonds which are accompanied by change in dipole moment will absorb in the IR region. Such vibrational transitions are called IR active transitions . Those transitions which are not accompained by change in dipole moment of molecule are not directly observed and are called IR inactive . The compounds transparent (inactive) to IR radiation are primarily monoatomic & homonuclear diatomic gases such as He, Ne, Cl 2 , N 2 ,O 2 .

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Electronic transitions Vibrational transitions Rotational transitions

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Energy levels in Infrared Absorption Infrared absorption occurs among the ground vibrational states, the energy differences, and corresponding spectrum, determined by the specific molecular vibration(s). The infrared absorption is a net energy gain for the molecule and recorded as an energy loss for the analysis beam. h n Excited states Ground (vibrational) states h( n 1 - n 0 ) h( n 1 - n 0 ) h( n 2 - n 1 ) (overtone) Infrared Absorption and Emission n 1 n 2 n 0 n 3


TYPES OF MOLECULAR VIBRATIONS The molecular vibrations are categorised into 2 types. Stretching vibrations, Bending vibrations . symmetrical asymmetrical scissoring rocking Out of plane

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They correspond to an oscillation along internuclear axis. Bond angle is not altered. They are either symmetrical or asymmetrical. Symmetrical - two nuclei approach or move away from the same location. Asymmetrical - one nucleus moves towards a point while other moves away from the point. Stretching vibration Symmetric Stretching Asymmetric Stretching Isolated vibration Without change in bond axis Without change in bond angle Stretching vibrations

Bending vibrations: : 

Bending vibrations: These are nuclear motions that cause a change in angle between two vibrating bonds, so require a molecule that contain at least 3 atoms. Classified as scissoring, rocking, twisting & wagging depending upon the motion of 2 outer nuclei in comparision with central nucleus. They are 2 types in plane and out plane .

Bending vibrations:: 

In plane: Scissoring - (motion during the operation of a pair of scissors). Two nuclei rotate toward & away from each other while remaining in same plane. Rocking - (motion of a rocking chair). Two nuclei rotate in the same direction & in the plane about a common nucleus. Bending vibrations: Bending Vibration In plane Scissoring Rocking

Bending vibrations: : 

Bending vibrations: Out of plane: Twisting - (twisting of wires naround a central point). One of the atoms move up the plane while the other moves down the plane with respect to the central atom. Wagging - (back and forth motion of a wagging tail). Two nuclei move up & down the plane with respect to the central atom. Out of plane Twisting Wagging Bending Vibration


NO. OF FUNDAMENTAL VIBRATIONS For a molecule with ‘N’ no. of atoms, the total no.of coordinates that are required to completely specify the position & orientation in space of that molecule at a particular instant is 3N, i.e., three coordinates are required for each atom in a molecule. Each coordinate is a degree of freedom for the molecule. The number of possible vibrational motions must be - For linear molecules, 3N – 3 (for translational motion) – 2 (for rotation), or 3N – 5 - For nonlinear molecules, 3N – 3 – 3 , or 3N – 6.

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Eg: CO 2 is a linear molecule & contains 3 atoms. Therefore it has 4 fundamental vibrations [(3*3)-5]. 1) one symmetrical stretching @ 1340 cm -1 2) one asymmetrical stretching @ 2350 cm -1 3 & 4) two scissoring bending @ 666cm -1 Here ‘1’ is inactive in IR, since no change in dipole moment of molecule. Bending vibrations in ‘3 & 4’ are equivalent & have same frequency & are said to be doubly degenerate.

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The theoretical number of fundamental vibrations will seldom observed because overtones (multiples of a given frequency) & combination tones (sum of two other vibrations) increase the number of bands. Where as the following decrease . 1. Fundamental frequencies that fall outside the 4000- 400cm -1 region. 2. Fundamental bands that are too weak to be served. 3. Fundamental vibrations that are so close that they coalesce. 4. The occurrence of a degenerate band from several absorptions of the same frequency in highly symmetrical molecules. 5. Due to no change in dipole few fundamental vibrations do not appear in the spectrum.

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The nuclei are assumed to be known masses that are connected to each other by springs. The springs represent chemical bonds between atoms. The value of vibrational frequency of a bond can be calculated fairly accurately by the application of ‘ Hooke’s law’ which may be represented as: Eg: HCl Cl - H + where (Reduced mass)



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SOURCE IR source consist of an inert solid that is heated electrically to a temperature between 1500 & 2200 K. These sources produce continuum radiation approximating that of a black body.

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NERNST GLOWER: The nernst glower is composed of rare earth oxides ( zirconia, yttria & thoria ) formed into a cylinder having a diameter of 1 to 3 mm & a length of 2 to 5 cm. Platinum leads are sealed to the ends of the cylinder to permit electrical connections. It is generally heated to a temperature between 1000 to 1800. It produces max radiation at about 7100 cm -1 .

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THE GLOBAR: It is a silicon carbide rod , usually about 5cm long & 5mm in diameter. it also is electrically heated (1300-1500K) & has the advantage of a positive coefficient of resistance. On the other hand, water cooling of the electrical contacts is required to prevent arcing.


INCANDESCENT WIRE SOURCE A tightly wound spiral of nichrome wire heated to about 1100K by an electrical current. A rhodium wire heater sealed in a ceramic cylinder has similar properties, although it is more expensive. Nichrome wire sources are less intense than many IR sources. However the incandescent wire source requires to cooling & is nearly maintenance free. For this reason the nichrome wire source is often where reliability is paramount, such as in process analyzers.


THE MERCURY ARC For far IR region of the spectrum (wavelength > 50µm) none of the thermal sources provides sufficient radiant power for convenient detection. Here a high pressure mercury arc is used. This device consists of a quartz jacketed tube containing Hg vapor at a pressure greater than 1 atm . Passage of electricity through the vapor forms an internal plasma source that provides continuum radiation in far IR region.


THE TUNGSTEN FILAMENT LAMP An ordinary TUNGSTEN filament lamp is a convenient source for the near IR region of 4000-12800cm -1 (2.5 - 0.78 µm )


SAMPLE CELL & SAMPLING Gases: The spectra of gases or low boiling point liquids may be obtained by expansion of the sample into an evacuated cell. Gas cells are available in lengths of a few cm to 40m. Sampling area of a std IR spectrophotometer will not accommodate cells much longer than 10cm; long path lengths are achieved by multiple reflection optics. IR spectrum can be obtained for gases, liquids, or solids. 1m

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Liquids: Neat liquids are examined between salt plates , usually without a spacer. Pressing produces a film 0.01mm or less, plates are held together by capillary action . Volatile liquids are examined in sealed cells with very thin spacers. AgCl plates may be used for samples that dissolve NaCl plates.

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A compensating cell, containing pure solvent , is placed in the reference beam. The spectrum thus obtained is that of the solute except in those regions in which the solvent absorbs strongly. The solvent selected must be dry & transparent in the region of interest. Solvent - solute combinations that react must be avoided. Eg: CS 2 cannot be used as a solvent for 1 0 & 2 0 amines. When very small amount of sample is available, Ultra micro cavity cells are used in conjugation with a beam condenser.

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Gas cuvette Liquid cuvette

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Solids: Solids are examined as a mull , as a pressed disk , or as a deposited glassy film . Mulls are prepared thoroughly grinding 2 - 5mg of a solid in a smooth agate motor. Grinding is continued after the addition of 1 or 2 drops of the mulling oil . The suspended particles must be less than 2µm to avoid excessive scattering of radiation. Mull is examined as a thin film between flat salt plates . Nujol (a high boiling point petroleum oil) is commonly used as mulling agent. When hydrocarbons interfere with the spectrum, Fluorolube (a completely halogenated polymer containing F & Cl) or hexachlorobutadiene may be used.

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It depends upon the fact that dry, powered KBr can be compacted under pressure in vacuo to form transparent disks. The sample is intimately mixed with 100 mg of dry powdered KBr. Mixing can be effected by thorough grinding in a smooth agate motor, or with a small vibrating ball mill or by lyophilisation. The mixture is pressed with special dies under a pressure of 10,000- 15,000 psi into a transparent disk. The quality of spectrum depends on intimacy of mixing & reduction of the suspended particles to 2µm or less. Bands near 3448 & 1639 cm -1 resulting from the moisture frequently appear in spectra obtained by the pressed disk technique.



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Dispersion films They are useful only when the material can be deposited from the solution or cooled from a melt as microcrystals or as a glassy film. Crystalline films generally lead to excessive light scattering. This technique is particularly useful for obtaining spectra of resins and plastics.


MONOCHROMATOR The radiation source emits radiations of various frequencies. As the sample in IR spectroscopy absorbs only at certain frequencies, it therefore becomes necessary to select its desired frequencies from the radiation source & reject the radiations of other frequencies


PRISMS: They are constructed of materials of various metal halide salts which transmit in infrared. While glass & quartz were utilsed in visible & UV, they absorb & unsatisfactory in the IR. Because of its high dispersion in the region of 4 -15µm, a region which is of special importance of functional group studies, NaCl is probably the most common prism salt Its limitations due to thermal instability &/or water solubility.


GRATINGS : Prisms replaced with gratings. The gratings offer linear dispersion and may be constructed from a wide variety of materials. Several gratings, each with different rulings (lines/cm) are necessary to cover the wide wavelength (energy) range associated with IR radiation. Various combinations of transmission or interference filters with gratings, or filters without grating are utilized. Grating is essentially a series of parallel St. lines cut into a plane surface. Dispersion of a grating follows law of diffraction .

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Concave mirrors Diffracting grating Entrance slit Exit slit Light source Detector GRATING

DETECTOR Detection of IR signal is of prime importance. Different detectors:: 

DETECTOR Detection of IR signal is of prime importance. Different detectors: THERMOCOUPLE: It is made by welding together two wires of metals 1& 2 (like bismuth & antimony ) in such a manner that a segment of metal 1 is connected to two terminal wires of metal 2. One junction between metals 1 & 2 is heated by the IR beam & the other jn. is kept at constant temperature; small changes inn ambient temperatures are thus minimized.

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To avoid losses of energy by convection, the couples are enclosed in an evacuated vessel with a window transparent to IR radiation. The metalic junctions are also covered with a black deposit to decrease reflection of the incident beam. Response time is 30ms. To enhance the selectivity several thermocouples are connected in series to give a thermopile. Metal A Metal B welded junction (cold) welded junction (hot)


BOLOMETER It is a thin strip of blackened platinum or nickel in an evacuated glass vessel with a window transparent to the IR rays. It is connected as one arm of a wheatstone bridge . Any radiation absorbed raises the temperature of the strip & changes its resistance. Two identical elements are usually placed in the opposite arms of a bridge; one of the elements is in the path of IR beam & the other compensates for variations in ambient temperature. Thermocouples & bolometer give very small DC, which may be amplified by special methods to drive a recorder. Response time is few milli sec.


GOLAY PNEUMATIC DETECTOR It consists of gas filled chamber which undergoes a pressure rise when heated by radiant energy. Small pressure changes cause deflections of one wall of the chamber. This movable wall also functions as a mirror & reflects an incident light beam towards a photocell ; The amount of light reflected is directly related to the expansion of the gas chamber, hence to the radiant energy of the light from the monochromator.


THE PYROELECTRICAL DETECTOR It use ferroelectric materials operating below their curie- temperatures. When IR radiation is incident on the detector there is a change in polarization which can be employed to produce an electrical signal. The detector will only produce a signal when the intensity of the incident radiation changes. They are of a special value in FTIR, where rapid response times are needed; they use Deuterium triglycine sulphate as the detecting medium in an evacuated chamber . For high sensitivity a Hg cadmium telluride (MCT) detector is used, cooled by liquid N 2 .

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Incoming light Sensing circuit Pyroelectric disc Front surface elcetrodes Rear surface elcetrodes


FTIR Varying distances between two path lengths result in a sequence of constructive & destructive interferences & hence variations in intensities: an interferogram. Fourier transformation converts this interferogram from the time domain to one spectral point on the more familiar form of the frequency domain . Smooth and continuous variation of length of piston adjust the position of mirror B and varies the length of beam B; Fourier transformation at successive points through out this variation gives rise to complete IR spectra. ADVANTAGES: since monochromator is not used, the entire radiation range is passed through the sample simultaneously and much time is saved. (Felgett’s advantage).

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The data undergo analog to digital conversion. FTIR can be used in conjugation with HPLC and GC FTIR have very high resolution (≤ 0.001 cm -1 ).


MICHEALSON’S INTERFEROMETER It takes the radiation from an infrared source and splits it into two beams ( one beam is fixed the other of variable length) using a half silvered 45 0 mirror so that the resulting beams are at right angles to each other. If an absorbing material is placed in one of the beams, the resulting interferogram will carry the spectral characteristics of the sample in the beam.

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Fixed mirror C Movable mirror Same-phase interference wave shape Opposite-phase interference wave shape Same-phase interference wave shape l 0 Movable mirror Continuous phase shift Signal strength I (X) -2 l - l 0 l 2 l -2 l - l 0 l 2 l FTIR seminar Interference of two beams of light Fixed mirror A Movable mirror Fixed mirror B Movable mirror

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He-Ne gas laser Fixed mirror Movable mirror Sample chamber Light source (ceramic) Detector (DLATGS) Beam splitter FT Optical System Diagram

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4000 400 SB Fourier transform Optical path difference [x] (Interferogram) (Single beam spectrum) Wavenumber[cm -1 ] Single strength Time axis by FFT Wave number Fourier Transform

Attenuated total reflection: 

Attenuated total reflection Useful for obtaining qualitative spectra of solids regardless of thickness. It depends on the fact that a beam of light that is internally reflected from the surface of a transmitting medium passes a short distance beyond the reflecting boundary and returns to the transmitting medium as a part of the process of reflection. An extension of the technique provides for multiple internal reflections along the surface of the sample. This technique results in intensities comparable to transmission spectra.


FACTORS Hydrogen bonding Coupling interactions Electronic effects Bond angles


APPLICATIONS Identification of an organic compound. Structure determination Qualitative analysis of functional groups Distinction between two types of hydrogen bonding Quantitative analysis Study of a chemical reaction Study of Keto-enol tautomerism Study of complex molecules Confirmational analysis Geometrical isomerism Rotational isomerism Detection of impurity in a compound


INTERPRETATION OF SPECTRA Functional Group Region Fingerprint Region


HOW TO APPRAOCH THE ANALYSIS OF A SPECTRUM Is a carbonyl group (C=O) present? [1820-1660cm -1 ] If (C=O) Present If (C=O) Absent Check for Check for Acids (O-H) [3400-2400 cm -1 broad] Amides (N-H) [3400 cm -1 medium] Esters (C-O) [1300-1000 cm -1 strong] Anhydrides (2C=O) [1810 & 1760 cm -1 ] Aldehydes (C-H) [2850 & 2750 cm -1 2 weak] Ketones If preceeding 5 choices are eliminated Alcohols, Phenols (O-H, C-O) [3400-3300cm -1 broad], [1300-1000cm -1 ] Amines (N-H) [3400cm -1 medium] Ethers (C-O) [1300-1000cm -1 & absence of O-H] Double bond &/ aromatic ring [C=C 1650 w cm -1 ;1600-1450cm -1 m to s Ar ring] Triple bonds [C Ξ N-2250, C Ξ C- 2150 & C-H near 3300cm -1 ] Nitro groups [2 strong at 1600-1530 & 1390-1300 cm -1 ] Hydrocarbons [None of the above is found Major absorptions in C-H near 3000cm -1 ]


REFERENCE : VOGEL’S Text Book Of Quantitative Chemical Analysis , J Mendham, R C Denney, J D Barnes, M J K Thomas; Sixth edition; Pearson Education. Introduction To Instrumental Analysis , Robert D. Braun; PharmaMed Press. Instrumental Methods of Chemical Analysis , Gurudeep R. Chatwal & Sham K. Anand; Fifth edition; Himalaya Publishing House. Elementary Organic Spectroscopy, Y. R. Sharma; Fourth Edition; S. Chand & company ltd. Practical Pharmaceutical Chemistry , A H Beckette, J B Stenlake; fourth edition; Part – II; CBS Publishers & Distributors.

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Instrumental Analysis , Skoog , Holler, Crouch; India edition; Cengage learning. Spectrometric identification of organic compounds, Robert M Silverstein, Francis X Webster; sixth edition, John Wiley & Sons INC. Spectroscopy, Pavia, Lampman; India edition. Animations, Videos & images – from web source,



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