IR spectroscopy

IR spectroscopy

1. INFRARED SPECTROSCOPY PRESENTED BY UTTAM PRASAD PANIGRAHY M.PHARM

2. CONTENTS 1. INTRODUCTION 2.PRINCIPLE 3. THEORY-MOLECULAR VIBRATION 4. INSTRUMENTATION 5.IMPORTANT FEATURES 6.APPLICATIONS

3. Definition: ● It is the study of absorption of infrared radiation which results in vibrational transitions. ● IR spectrum is an important record which gives sufficient information about the structure of a compound and also determine the functional group. IR spectroscopy is an useful tool to identify functional groups in organic molecules IR spectroscopy is a result of molecular vibrational transitions that occur when light interacts with matter Molecules are always vibrating For a molecule to be IR active, the vibrations should give rise to a net change in dipole moment Infrared spectroscopy

4. The absorption of IR radiations can be expressed either in terms of wavelength( ) or in wave number (  ). Relationship between wavelength( ) and wave number (  ). wave number (  )= 1/ wavelength( ) in cm suppose wavelength( ) is 2.5  = 2.5× 10 -4 cm, then wave number (  )= 1/ 2.5× 10 -4 cm=4000 2.5× 10 -4 cm   1  m = 10 -6 m ; cm -1 = no. of waves per cm of path = 1/[  (cm)]  energy of wave E = h  = hc 

5. Infrared region   LIMIT OF RED LIGHT : 800 nm, 0.8  m, 12500 cm -1 NEAR INFRARED : 0.8 -2.5  m, 12500 - 4000 cm -1 MID INFRARED : 2.5 - 25  m, 4000 - 400 cm -1 FAR INFRARED : 25 - 1000  m, 400 - 10 cm -1

6. Principle: IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

7. Theory of infra red absorption IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

8. Molecular rotations Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions. Molecular vibrations The positions of atoms in a molecules are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main catagories of stretching and bending . Stretching: Change in inter-atomic distance along bond axis

9. Bending: Change in angle between two bonds. There are four types of bend: Rocking Scissoring Wagging Twisting

10. Vibrational coupling In addition to the vibrations mentioned above, interaction between vibrations can occur ( coupling ) if the vibrating bonds are joined to a single, central atom. Vibrational coupling is influenced by a number of factors; 1.Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds 2.Coupling of bending vibrations occurs when there is a common bond between vibrating groups 3.Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration 4.Coupling is greatest when the coupled groups have approximately equal energies 5.No coupling is seen between groups separated by two or more bonds

11. Factors Affecting Frequency of Absorption Bond strength C=O stretching (1700 cm -1) vs C-O stretching (1200 cm-1) C=C stretching (1650 cm -1) vs C-C stretching (1200 cm-1) It takes more IR energy to stretch short strong bonds than it does to stretch long weak bonds It also takes more energy to stretch a bond between two heavy atoms than it does if the atoms are less massive Atomic Size C-H (3000 cm -1) C-C (1200 cm -1) C- Cl (800 cm -1) C-Br (550 cm -1) Bigger masses vibrate at lower energy

12. ♦ As a bond stretches, the atoms are moved apart from each other ♦ If the bond elongation changes the net dipole moment of the molecule, an IR peak is manifested Examples of large and small peaks ♦ Large peaks are observed for C=O bonds ♦ Small peaks are observed for C=C bonds ♦ If the atoms that stretch have different electro negativities, you are likely to see larger peaks

13. Ranges are broad, not exact • Peaks are generally broad, not sharp • Exact frequency depends upon – conjugation – proximity effects

14. Detection Electronics and Computer Infrared Source Determines Frequencies of Infrared Absorbed and plots them on a chart Sample Simplified Infrared Spectrophotometer NaCl plates Absorption “peaks” Infrared Spectrum frequency intensity of absorption (decreasing) focusing mirror

15. Sources • Tungsten incandescent lamp – black body source for measurements in NIR • Nichrome (or rhodium) wire – Coiled, heated by resistance to incandescence. Black oxide layer forms on surface. Temperature 1100°C. Requires little maintenance and no cooling required. Emits in Mid-IR but less power than other sources. Cheaper instruments • Nernst Glower (rare earth oxides) – More intense emitted radiation. Constructed from mixture of fused oxides of Zr , Th and Cs. Non-conducting at ambient temperatures but at temperatures >800 °C it is electrically conducting, maintains high temperature by resistive heating. Good energy output (intensity 2x nichrome wire or globar) • Globar – A rod of silicon carbide 6-8 mm in diameter. Characteristics between nichrome wire and Nernst Glower. Self starting and operates at 1300 °C. Globar must be water cooled – brass jacket surrounds globar. • Carbon Dioxide Laser – Useful for narrow radiation bands

16. Instrumentation-Components ♦ Sample Cells and Preparation • Solids • Mull - suspend ground solid in oil of similar refractive index (Nujol, perfluorocarbon) • KBr Pellet - few mg sample + 0.5 to 1 g dry KBr ground + compressed at very high pressure • Disposable polyethylene film strips (dissolve solid in volatile solvent, “paint” on the film or on a salt plate) • Liquids • Gases ♦Optics - dessicated salts such as NaCl , CsBr , LiF , KBr and front surface mirrors. Glass and quartz lenses cannot be used because they absorb IR radiation ♦Chopper (modulation and tuned amplifier)

17. Most flexible system for analyzing all 3 states of matter (solid, liquid, gas) “Neat” (analysis of liquids/oils) Pellet (analysis of solids) Thin Cell (analysis of dissolved solid samples - solutions) Long Cell (analysis of gases)

18. Preparing a “Neat” IR Sample

19. Preparing a KBr Disk

20. Apparatus for KBr Disk

21. Sample cells To obtain an IR spectrum, the sample must be placed in a “container "or cell that is transparent in the IR region of the spectrum. Sodium chloride or salt plates are a common means of placing the sample in the light beam of the instrument. These plates are made of salt ( NaCl or KBr) and must be stored in a water free environment

22. http://www.chem.vt.edu/chem-ed/scidex.html Photon detectors

23. Analyzing IR Spectra Look for C=O peak (1820-1660 cm -1 ) If C=O check for OH (3400-2400 cm -1 ) indicates carboxylic acid If C=O check for NH (3500 cm -1 ) indicates amide If C=O check for C-O (1300-1000 cm -1 ) indicates ester If no OH, NH or C-O then ketone

24. Analyzing IR Spectra If no C=O check for OH (3600-3300 cm -1 ) indicates alcohol If no C=O check for NH (3500 cm -1 ) indicates amine If no C=O & no OH check C-O (1300 cm -1 ) indicates ether Look for C=C (1650-1450 cm -1 ) then aromatic

25. IR Characteristic Vibrations

26. Sample IR Spectrum #1 A - CO-OH stretch (3000) B - CH stretch (2800) C - C=O ester (1757) D - C=O carboxy (1690) E - C=C aromatic (1608) F - C=C aromatic (1460)

27. Sample IR Spectrum #2 A B C C CH 3 O Acetophenone A) C=O (1730) B) C=C aromatic (1590) C) C-H aromatic (3050)

28. Applications Qualitative “fingerprint” check for identification of drugs Used for screening compounds and rapid identification of C=O groups Can be used to characterize samples in solid states (creams and tablets) Can detect different crystal isoforms (polymorphs) Water content measurement

29. Applications Analysis of urine and other biofluids (urea, creatinine, protein)

30. Applications Used in non-invasive measurement of glucose

31. Applications of Near IR (NIR) Quality control of pharmaceutical formulations Determination of particle size Determination of blend uniformity Determination or identification of polymorphic drugs