INFRARED SPECTROSCOPY: FUNDAMENTALS AND APPLICATIONS

1. INFRARED SPECTROSCOPY: FUNDAMENTALS AND APPLICATIONS Dr. Jugun Prakash Chinta DST-INSPIRE Faculty National Workshop on Advanced Instrumentation (NWAI 2017)

2. STRUCTURE DETERMINATION How do we know: • How atoms are connected together? • Which bonds are single, double, or triple? • What functional groups exist in the molecule? • If we have a specific stereoisomer? ? Spectroscopy answers all the questions in structure determination

3. SPECTROSCOPY - Study of spectral information Physical Stimulus Interaction of Electromagnetic radiation Sample Response Spectroscopy is the study of interaction of electromagnetic radiation with matter Visual representation Or Spectrum

4. Two mutually perpendicular electric and magnetic fields, oscillating in single planes at right angles to each other ELECTROMAGNETIC RADIATION Velocity (c) =   - Wavelength  - Frequency Wavenumber (,in cm−1) = 1/ = /c (number of waves in a length of one centimetre)

5. Einstein, Planck and Bohr - Electromagnetic radiation could be regarded as a stream of particles (or quanta) for which the energy, E, is given by the Bohr equation E= h = hc/ ELECTROMAGNETIC RADIATION Molecules can exist at various energy levels and processes of vibration, rotation and electronic excitation are quantized The energy of the quantum is related to the frequency by the following: E = hν

6. INFRA RED RADIATION Infrared radiation induces stronger molecular vibrations in covalent bonds. The two atoms joined together by a chemical bond can be viewed as two balls joined by a spring. Specific bonds respond to (absorb) specific frequencies Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003

7. TYPES OF VIBRATIONS Two types of fundamental vibrations are possible Stretching modes are typically of higher energy than bending modes. Asymmetric stretching occur at higher frequencies than symmetric stretching

8. Complicating factors Overtone Bands Combination and Difference bands Two vibrational frequencies (1and 2) in a molecule couple to give rise to a new infrared active frequency(combination = 1+ 2, difference = 1- 2) Fermi resonance When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. Fermi resonance is often observed in carbonyl compound Vibration-Rotation Bands

9. VIBRATIONAL FREQUENCY The vibrational frequency of a bond can be calculated from the equation: Where c - the velocity of light k - force constant of bond (varies from one bond to another) μ = m1m2/(m1+m2) is the reduced mass m1, m2 are the masses of two atoms The vibrational frequency of a bond increases when bond strength increases and also when reduced the mass of the system decreases. For example, Stretching: C=O (1700 cm-1) > C-O (1100-1300 cm-1) O-H (3500 cm-1) > C-O (1100-1300 cm-1)

10. Typical IR instrument contains the following components: 1. Source - Globar(silicon carbide) and Nernst glower (Zirconium and Yttrium oxides) 2. Optical system - Monochromator (grating or prism) and Interferometer 3. Sample compartment 4. Detector - Photodiode Arrays or Pyroelectric transducers 5. Signal Processors and Readout Devices INSTRUMENTATION Two types of spectrometers are used in IR spectroscopy: 1. Dispersive 2. Fourier transform

11. DISPERSIVE IR SPECTROMETER The dispersive IR instruments are older double-beam instruments which uses grating as a monochromator The essential problem of the dispersive spectrometer lies with its monochromator. This contains narrow slits at the entrance and exit which limit the wavenumber range of the radiation reaching the detector to one resolution width.

12. FOURIER TRANSFORM IR SPECTROMETER Fourier transform IR instruments contain no dispersing element (monochromator), instead an interferometer Sensitivity-It detects a broad band of radiation all the time (the multiplex or Fellgetadvantage) The greater proportion of the source radiation passes through the sample(Jacquinotadvantage) Speed advantage-The mirror has the ability to move short distances quite rapidly

13. Recording spectrum Spectrum Analysis Sample preparation EXPERIMENTAL PROCEDURE Glass, quartz, and fused silica absorb strongly in IR region longer than 2.5 µm. Hence, optical elements and materials used for IR spectroscopy are typically made from halide salts such as NaCl (40,000-600 cm-1), KBr (43,500-400 cm-1), CsBr (42,000-250 cm-1) etc.

14. RECORDING SPECTRUM Precautions to be taken for recording the spectrum Purge the spectrometer with an inert gas or CO2 free air to reduce the background absorption from water vapour and CO2. The sample compartment should always be kept with desiccating material such as molecular sieves or silica gel etc. All the materials used in the experiments should be stored in the desiccator.

15. ANALYSIS OF SPECTRUM IR source Transmitted light Sample Detector The sample absorb (retain) specific frequencies and allow the rest to pass through it (transmitted light)

16. ABSORBANCE Vs CONCENTRATION Beer–Lambert law - The absorbance of a sample is directly proportional to the thickness and the concentration of the sample, as follows: A = cl A - Absorbance of the sample c - Concentration l - Path length of the sample ε - Molar absorptivity The absorbance is equal to the difference between the logarithms of the intensity of the light entering the sample (I0) and the intensity of the light transmitted (I) by the sample: A = log I0 − log I = log (I0/I ) Transmittance is defined as follows: T = I/I0 A = −log (I/I0) = −log T

17. Selection rule: In order a molecule to show infrared absorption it must exhibit a change in dipole moment during the vibration. Therefore a heteronuclear diatomic molecule is ‘infrared active’ whereas homonuclear diatomic molecule is ‘infrared inactive’ because its dipole moment remains zero even the bond expands or contracts. For example, molecules such as N2, O2, or F2 do not absorb infrared radiation. INFRA RED ACTIVE BONDS Not all covalent bonds display bands in the IR spectrum

18. INFORMATION OBTAINED FROM IR SPECTRA Infrared radiation extends from 2.5 μm - 25 μm (4000 cm-1 - 400 cm-1) • IR gives information about the presence or absence of specific functional groups • MOLECULAR FINGER PRINT - no two unique molecular structure produce the same IR spectrum • IR does not provide detailed information or proof of molecular formula or structure • IR must be used in conjunction with other techniques to provide a more complete picture of the molecular structure

19. INFRA RED SPECTRAL RANGE Infrared spectral range can be divided in to three regions Near Infrared (NIR, 14000-4000 cm-1) - Allows the study of overtones and harmonic or combination vibrations Mid Infrared (MIR, 4000-400 cm-1) - Gives information about fundamental vibrations of small molecules. Useful for functional group identification and material characterization Far Infrared (FIR 400-10 cm-1) - Region covers backbone vibrations of large molecules as well as fundamental vibrations of molecules containing heavy atom (inorganic or organometallic compounds)

20. INFRA RED SPECTRAL RANGE Infrared spectral range can be divided in to four regions based on absorption of different bonds Light green zone (3500 to 2700 cm-1) - O-H (alcohol), C-H (alkyne, aryl, vinyl, aldehyde and acid), N-H (amine or amide) Green zone (1850-1600 cm-1) - C=O (aldehyde, ketone, carboxylic acid, ester or amide functional groups), C=C (alkene, aromatic < 1600 cm-1) Orange Zone (1400-600 cm-1) - C-O, C-C and C-N. This region also called as finger print region. This region is commonly too complex but can be used for the identification of the compound Blue zone (2300 to 2100 cm-1) - CC (alkyne), CN (nitrile) and S-H (thiol) Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003

21. WORKING EXAMPLE 1 Propionic acid

22. WORKING EXAMPLE 2 Ortho-, Meta- and Para-Xylene

23. ADVANCES IN INFRA RED SPECTROSCOPY Reflectance Methods - Reflectance techniques may be used for samples that are difficult to analyse (Attenuated Total Reflectance Spectroscopy) MicrosamplingMethods - FTIR microscopy, with samples of the order of microns being characterized (IR imaging) Chromatography-IR - Allows for the identification of the components eluting from a gas chromatograph Thermal Analysis-IR – Gives a complete picture of the chemical and physical changes occurring in various thermal processes

24. APPLICATIONS OF INFRA RED SPECTROSCOPY Identification - Citronellal is the terpenoid responsible for the characteristic aroma of lemon oil, and is used in perfumes and as a mosquito repellent Infrared spectrum of Citronellal

25. APPLICATIONS OF INFRA RED SPECTROSCOPY Multicomponent analysis - Analysis of a component in a complex mixture Infrared spectrum of commercialxylene in cyclohexane

26. APPLICATIONS OF INFRA RED SPECTROSCOPY Polymers - Identify and characterize simple polymers, copolymers and blends, Quantitative analysis of copolymers and additives or contaminants, Surface properties of polymers and the monitoring of the Degradation processes in polymers For example, the extent of cross-linking of epoxy resins with amines can be examined by using the C–O stretching and C–H stretching bands because the cross-linking process involves opening of the epoxy ring. The absorbancesof the 912 and 3226 cm-1 bands may be measured as a function of time to follow the reaction.

27. APPLICATIONS OF INFRA RED SPECTROSCOPY Biological Applications • To understand lipid conformation and quantification of lipids in blood serum • To estimate the secondary structure of proteins • To recognize the major infrared bands of nucleic acids • To use changes in the infrared spectra of animal tissues to characterize disease • To differentiate microbial cells • Quantitativeanalysis in clinical chemistry

28. APPLICATIONS OF INFRA RED SPECTROSCOPY • Industrial and Environmental applications • To identify and characterize pharmaceutical materials • To carry out quantitative analysis of food samples (oils, fats, carbohydrates, protein, ethanol in wine) • To quantify oil, fibre, minerals and carbohydrates etc. in grains and in the pulp and paper industries • To identify the different components of paints • Infrared spectroscopy can be used in environmental applications

29. INFRA RED SPECTROSCOPY APPLICATIONS IR spectroscopy has wide variety applications ranging from biological chemistry, pharmaceutical, nanotechnology, food science, paint industry, agriculture, paper and pulp industry and environmental sample analysis. Further reading 1. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 9th Edition, Brooks/Cole, Cengage Learning, 2014. 2. Banwell, C.; McCash, E. Fundamentals of Molecular Spectroscopy, 4th Edition, Tata Mcgraw Hill Education Private Limited, 2001. 3. Stuart, B. H. Infrared Spectroscopy: Fundamentals and Applications John Wiley & Sons, 2004.

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