Infrared / IR spectroscopy

IR/Infrared Spectroscopy
Instrumentation of IR/Infrared spectrophotometer
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1. “INFRARED SPECTROSCOPY” M.Zain Idrees

2. INFRARED SPECTROSCOPY IR spectroscopy is the  spectroscopy that deals with the  infrared region of the  electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional groups . The infrared region of the spectrum encompasses radiation with wave numbers ranging from about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200μ . Infrared region lies between visible and microwave region. Infrared Spectroscopy is the analysis of infrared light interacting with a molecule. It is based on absorption spectroscopy

3. INFRARED REGIONS INFRARED REGIONS RANGE Near infrared region 0.8-2.5 µ (12,500-4000 cm- 1 ) Main infrared region 2.5-15 µ (4000-667cm- 1 ) Far infrared region 15-200 µ (667-100 cm -1 )

4. The infrared region constitutes 3 parts a) The near IR (0.8-2.5μm) (12,500-4000cm-1) b) The middle IR (2.5 -15μm) (4000-667cm-1) i ) Group frequency Region (4000-1500cm-1) ii) Finger print Region (1500-667cm-1) c) The far IR (15-200μm) (667-50cm-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 . E = hν = hc /λ= hcν ̄ It gives sufficient information about the structure of a compound.

5. PRINCIPLE When infrared radiation is passed through an organic compound some of the frequencies are absorbed and appear as absorption band while other frequencies are do not intrect with the compound and are transmitted without being absorbed.Only those frequencies are absorbed which match with the vibrational frequencies of the bonds. IR sectroscopy is there for basically vibrational spectroscopy . The absorption of ir radiation is associated with the excitatio of the molecule from the ground state to higher vibrational state. The absorbed energy is eventually released as heat and the molecule revert from the excited state to the orignal ground state.

6. Conti... Fundamental Vibrations: vibrational frequencies which are concerned with the individual atom called Fundamental vibration. Non Fundamental vibration: Vibrations which apears as a result of fundamental vibrations. FUNDAMENTAL VIBRATIONS Stretching vib: Involves a continuous change in interatomic distance along the axis of the bond between 2 atoms Bending Vib: Involves the change in angle btw the two bonds

7. Stretching VIBRATIONS Symmetric vib: Interatomic distance btw the two atoms increase or decrease Asymmetric vib: Interatomic distance btw the two atoms is Opposite or alternate Bending VIBRATIONS In Plane Bending: All the atoms are in same plane Out of plane bending: 2 atoms are in same plane and 1 is on opposite plane

8. In Plane Bending: Scissoring: In this case, the two atoms connected to a central atom either move toward or away from each other with certain deformation of the valence angle. Rocking: In this case, the structural unit swings back and forth in the plane of the molecule. Out of plane bending: Wagging: In this case the structural unit swings back and forth out of the plane of the molecule. Twisting: In this case the structural unit rotates about the bond that joins it to the rest of the molecule.

9. Non-FUNDAMENTAL VIBRATIONS Over tone: Some additional absorption bands may be observed which are genrated by the modulation of the fundamental frequencies. This leads to the overtone that appear at almost twice and thrice the fundamental frequencies with greatly reduced intensties. Combination tone: Week band that apeares occasionally at frequencies that are sum or difference of 2 or more fundamental bands. Combination band arises when absorption by a molecule result in excitation of two fundamental vibrations simutaneously. This happen when a single photon has precisely the correct energy to excite the two vibrations. In such cases the frequency of the combination band must be the exact sum of the two fundamental absorption frequencies. More than two fundamental frequencies may be involved in combinaton band. The difference band arises when a molecule already existing in an excited vibrational state absorbes enough additional energy to be excited to another vibrational level of a different vibrational mode.

10. Fermi resonance: Interaction btw fundamental vib and combination tones overtones 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. We can also calculate an approximate value of the stretching vibrational frequency of a bond by treating the two atoms and their connecting bond, to first approximation, as two balls connected by a spring, acting as a simple harmonic oscillator for which the Hooke’s Law may be applied. According to Hooke’s Law , The Stretching frequency is related to the masses of the atom and the force constant(a measure of resistance of a bond to stretching) of a bond by the following equation

11. Factors influencing vibrational Frequencies 1)Electronic Effect: Conjugation: Any factor the tend to increase the contribution of the singe bond form will decrease the force constant of the carbon oxygen bond hence result in lowering the the frequency of C=O stretching vib. e.g. conjugation lowersthe C=O stretching frequency whether the conjugation is brought about by αβ-unsaturation or by an atomic ring. Mesomeric effect: Any substituent that enhance the singke bond character will decrease the bond order of C=O group and leads o lower the stretching frequency. A +M group lower the stretching freq.-M group have opposite effect. Inductive effect: substituent with an electron donating inductive effect(+I) will enhance the single bond character of the C=O group. (-I) group have opposite effect.

12. 3) Angle strain: Double bond carbon atom in unstrained molecules have planer trigonal geometry with a bond angle of 120° Increase in absorption freq with the decrease in bond angle can be explained in terms of bond angular strain.Reduction in bond angle below 120 leads to an increased in s character in double bond resulting in shortening and strengthening of a double bond. 4) Coupling It is observed in compounds containing -CH2 and -CH3. e.g Carboxlic acid anhydride,amides

13. Instrumentation There are 2 types of infrared spectrophotometer, characterized by the manner in which the ir frequencies are handled. 1) dispersive type (IR) 2) interferometric type(FTIR) In dispersive type the infrared light is separated into individual frequencies by dispersion, using a grating monochromator . In interferometric type the ir frequencies are allowed to interact to produce an interference pattern and this pattern is then analyzed, to determine individual frequencies and their intensities.

14. Dispersive Infrared spectrophotometer It is double beam instrument.The main components of Dispersive ir spectrophotometer are: Radiation source Sample compartment opticle chopper monochrometer detector amplifier readout device

15. 1)Infrared Sources The most common source is either Nernst filament or Globar The Nernst filament is composed of a mixture of oxide of zirconium,thorium and yattrium, which is formed into a hollow rod about 2mm in diameter and 2-5 cm in length. Globar is a bonded silicon carbide rod about 7mm in diameter and 5 cm in length. On heating electrically (1000-1800C) these rods emit infrared radiations. 2)Sampling Area(Optical null method): The radiation is divided into two beams of equal intensties which are focused into the sampling area. One beam pass through the sample while the other serves as the reference beam.Absorption by the sample is measured directly from the difference in intensties of two beams. The intensity difference is usually measured by optical null method inthis instrument. In this method afer passing through the sampling area the reference beam pass through an optical wedge which limits the amount of radiation that passes through it. 3)Chopper: The two beams are now reflected to a light chopping device.As the chopper rotates it causes the sample beam and reference beam to be reflected aternately to the monochromator.

16. 4)Monochromators Three types of substances are normally employed as monochromators, namely : (i) Metal Halide Prisms : Various metal halide prisms, such as : KBr (12-25 μm), LiF (0.2-6 μm) and CeBr (15-38 μm) have been used earlier, but they have become more or less obsolescent nowadays. (ii) NaCl Prism (2-15 μm) : Sodium chloride prism are of use for the whole of the region from 4000- 650 cm–1. First, it offers low resolution at 4000-2500 cm–1, and secondly, because of its hygroscopic nature the optics have got to be protected at 20 °C above the ambient temperature. (iii) Gratings : In general, gratings are commonly employed in the design of the instruments and offer better resolution at higher frequency than the prisms. They offer much better resolution at low frequency, viz., typical rulings are 240 lines per nm for the 4000-1500 cm–1 region and 120 lines per nm for the 1500-650 cm–1 region.

17. 5)Detectors a)Thermocouples (or Thermopiles) : The underlying principle of a thermocouple is that if two dissimilar metal wires are joined head to tail, then a difference in temperature between head and tail causes a current to flow in the wires. In the infrared spectrophotometer this current shall be directly proportional to the intensity of radiation falling on the thermocouple. Hence, the thermocouples are invariably employed in the infrared region, and to help in the complete absorption of ‘available energy’ the ‘hot’ junction or receiver is normally blackened. b)Golay Detector: In this specific instance the absorption of infrared radiation affords expansion of an inert gas in a cell-chamber. One wall of the cell-chamber is provided with a flexible mirror and the resulting distortion alters the intensity of illumination falling on a photocell from a reflected beam of light. Thus, the current from the photocell is directly proportional to the incident radiation.

18. c)Bolometer: Working:

19. d)Thermistors •It is made up of metal oxides. •It functions by changing resistance when heated. •It consists of two closely placed thermistor flakes, one of the 10 um is an active detector, while the other acts as the compensating / reference detector. •A steady voltage is applied, due to the temperature increase there is change in resistance which is measured and this gives the intensity of the IR radiation

20. 6)Recording: When the sample has absorbed radiation of a perticular frequency,the detector will be receiving alternately an intense reference beam and weak sample beam.This will in effect lead to an alternating current which is flown to an amplifier.The amplifier is coupled coupled to small sevo-meter which drives the optical wedge into the reference beam untl the detector receives radiations of equal intensties from the sample and the reference beams until the difference in intensties of the two beams is zero or in other words the instrument is at an optical null. The wedge is coupled to a recorder pen so that the movement of the wedge in and out of the reference beam is accompained by movement of a pen on the printed paper which thus records the absorption bands.

21. Sample Handling in IR Spectrophotometer a)Gas Sample: A gas sample is introduced into an evacuated gas cell.Gas cells are available in lenghts ranging from a few cm to some meters. The sampling area of standard dispersive IR spectrophotometer is about 10cm. So long path are achieved by using multiple pass gas cells in which internal mirrors permit the beam to be reflected several times through the sample before it exits the gas cell. b)Liquid Sample: It is examined as thin film squeezed btw two optically polished circular flat plates(about 5mm thick and 25mm in dm) of rock salt(NACl). Pressing a small drop of liquid sample btw the plates produces a film of about 0.1mm thick. The plates are held together by capillary action are mounted in sample beam. Plates are cleaned and dried immediately after use by rinsing in suitable solvent such as chloroform.

22. c)Solid Sample: Mull technique: Mulls are prepared by grinding 2-5mg of sample in smooth mortar with pestle of agate,adding 1 or 2 drops of mulling agent and continue grinding to get a homogenous paste.The mull is examined as thin film btween the NACl plates. Nujol is commonly used as mulling agent. Pressed Pellet Technique: Grind and mix about 1.0mg of solid sample with about 100mg of dry powdered KBr in mortar. to prepare KBr discs. Then this solid solution is compressed under pressure (about 20,000 psi) in vacuo with a special die to form transparent disc. Discs are usually 10mm in diameter and 1mm thick. KBr does not absorb in the IR region so complete spectrum of sample is obtained.

23. d)Sample As Solution: Sample can also be examined as solution in solvents such as CCl 4 or Chloroform. Solutions(1-5%) are handled in cells of 0.1-1.0mm pathlength. A 2nd cell containing pure solvent is placed in reference beam. Calibration of Infrared Spectrophotometers The wavelength (or wave number) scale calibration of infrared spectrophotometers is usually carried out with the aid of a strip of polystyrene film fixed on a frame. It consists of several sharp absorption bands, the wavelengths of which are known accurately and precisely. Basically, all IR-spectrophotometers need to be calibrated periodically as per the specific instructions so as to ascertain their accuracy and precision.

24. Interpretation of IR spectra An infrared spectrum may be divided into 2 parts. the part ranging from 4000-1600cm -1 is known as Functional group region, while that ranging from 1600-625cm -1 is called Fingerprint region. Functional group region: In this region most compounds have only a few strong bands due to the characteristic stretching vibration of their functional groups. For example the functional group of X-H type (C-H,O-H) absorb in the region 3700-2500 cm -1 . Absorption due to triple bond occur 2300-2100cm -1 and due to double bond at 1900-1600cm -1 . Fingerprint region: It is much more difficult to pick out individual bonds in this region than it is in the "cleaner" region at higher wavenumbers. The importance of the fingerprint region is that each different compound produces a different pattern of troughs in this part of the spectrum. e.g C haracteristic absorption bands corresponding to aromatic ring system fall in the 1600-1450cm -1 . Aromatic compounds display strong adrosption bands in 900-700cm -1 .

25. FOURIER TRANSFORM IR SPECTROPHOTOMETER FT-IR A method of obtaining an Infrared spectrum by measuring the interferogram of a sample using an interferometer, then performing a Fourier Transform upon the interferogram to obtain the spectrum. It is single beam instrument. The Interferometer: An device in which two or more radiation beams interfere with each other after passing through different optical paths. The interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it. The output is an interferogram or interference record. The two domains of Distance and Frequency are inter convertible by the mathematical method of Fourier transformation. Majority of commercially avaiable FT-IR are based on an inetrferometer commonly Michelson interferometer.

26. Michelson Interferometer

27. Michelson interferometer: It has three basic components a beam splitter,fixed mirror, and a moveable mirror. Beam splitter is semi-reflecting device so that 50% oof light impinging on it is transmitted while the remaining 50% is reflected. It is either a Partially silvered mirror or thin film of germanium (Ge) sendwiched btw two KBr plates. IR light received from source is split into two identical beams by the beam splitter such that one beam is reflected to the fixed flat mirror while the other beam is transmitted to the moveable flat mirror. The two beams are reflected back by their respective mirrors to recombine at the beam splitter to again form one beam which passes through the sample and is then focused on detector. Of course,each of the two beams reflected back by their respective mirror is again split into two equal halves by the beam splitter.one half of each beam goes to detector while the half of each beam goes back to the source.The sample can be use directly without the prepration of mull or KBr discs or solution.

28. Detectors of FT-IR: 1. Pyroelectric detectors a thin, Pyroelectric crystals such as deuterated triglycine sulfate (DTGS) or LITA (lithium tantalate), When a pyroelectric material is polarized by an electric field, it remains polarized after the field is removed due to an effect called residual electric polarization. This residual polarization is sensitive to changes in temperature. 2. Photoconductive detectors show an increase in electrical conductivity when illuminated with IR radiation, They have a rapid response and high sensitivity. The most commonly used photoconductive detector is the MCT (Mercury Cadmium Telluride), which must be cooled to liquid nitrogen temperatures for proper operation.

29. Advantages of Fourier transform IR over dispersive IR Improved frequency resolution Improved frequency reproducibility (older dispersive instruments must be recalibrated for each session of use) Faster operation Computer based (allowing storage of spectra and facilities for processing spectra) Energy limiting accessories such as diffuse reflectance or FT-IR microscopes High resolution experiments (as high as 0.001 cm-1 resolution) Trace analysis of raw materials or finished products Depth profiling and microscopic mapping of samples Kinetics reactions on the microsecond time-scale Analysis of chromatographic and thermogravimetric sample fractions

30. Interferogram: The light signal measured by the detector is known as Interferogram. An interferogram is time domain interference signal,and represents the detector's response to the itensity changes against time within the mirror scan. Movement of the moveable mirror back and forth once is known as scan. Fourier Transformation: The time domain interferogram obtained as detector's response cannot be interpreted as such.It is subjected to furier transformation (a mathematical operation performed by an attached computer) which converts it into a frequency-domain spectrum.

31. SINGLE BEAM IR ABSORPTION SPECTROPHOTOMETER A = Infrared source B = Sample beam C = Chopper D = Monochromator grating E = Detector thermopile F = Amplifier G = Servo-motor H = An optical Wedge I = Prism J = Ink-pen recorder K = Slits

32. APPLICATIONS OF IR SPECTROSCOPY Structure determination: IR spectroscopy i mainly used for determination of structure of organic compounds by correlation and interpretation of IR spectra. Identification of Substances: To compare spectrums. No two samples will have identical IR spectrum. Criteria: Sample and reference must be tested in identical conditions, like physical state, temperature, solvent, etc. Determination of Molecular Structure Used along with other spectroscopic techniques. Identification is done based on position of absorption bands in the spectrum. Eg .: C=O at 1717 cm -1 . Absence of band of a particular group indicates absence of that group in the compd.

33. Detection of Impurities Determined by comparing sample spectrum with the spectrum of pure reference compound. Eg .: ketone impurity in alcohols. Detection is favoured when impurity possess a strong band in IR region where the main substance do not possess a band. Progress of Reactions Observing rate of disappearance of characteristic absorption band in reactants or Rate of increasing absorption bands in products of a particular product. Presence of Water in Sample If lattice water is present, spectra will contain 3 characteristic bands  at 3600-3200cm -1 , 1650cm -1 and 600-300cm -1 . Application In Industry Determine impurities in raw materials (to ensure quality products). For Quality Control checks; to determine the % of required product. Identification of materials made in industrial research labs, or materials of competitors. E.g.: Impurity in bees wax (with petroleum wax)

34. Reference: Books Organic spectroscopy and Chromatography by M.Younas Pharmaceutical Drug Analysis By Ashutosh Kar Web https://www.chemguide.co.uk https://www.slideshare.net