Ir spectroscopy slide

Infrared spectroscopy is technique to identify the functional group of the molecule.

1. Khulna University of Engineering & Technology (KUET) Presentation on Presented by…. Shuvodip Mondal Dept. of Chemistry, KUET

2. What is Spectroscopy? Spectroscopy is the Study of interaction between matter and electromagnetic radiation e.g ., IR Spectroscopy is concerned with the study of absorption of infrared radiation , which causes vibrational transition of bonds in molecules. So it is also called vibrational Spectroscopy . IR Spectroscopy mainly used to identify the functional group, but also gives information likes No. of bond, Specific stereo-isomer, Atomic arrangement in the molecule.

3. What is Spectroscopy? Spectroscopy is the Study of interaction between matter and electromagnetic radiation e.g., IR Spectroscopy is concerned with the study of absorption of infrared radiation , which causes vibrational transition of bonds in molecules. So it is also called vibrational Spectroscopy . Why NIR & FIR are not used in IR Spectroscopy???

4. Wave Length: 0.8 – 2.5 μ m Wave number: 12,500 – 4,000 cm -1 Wave Length: 2.5 – 5.0 μ m Wave number: 4,000 – 200 cm -1 Wave Length: 50 – 1000 μ m Wave number: 2 00 – 10 cm -1 Interfered with UV radiation and gives extra IVCT band on IR spectra due to delocalization of electron FIR is also closer to the radio-wave region and sometimes bandly interact & formed extra unwanted band s pectra on the IR spectrum. MIR is appropriate for the IR spectroscopy. Because it gives fully IR peak on IR spectrum. So, We used 2.5 – 5.0 μ m wavelength in IR Spectroscopy

5. Principle of IR Spectroscop y When IR radiation interact with sample molecule, the sample molecule will absorb the specific applied quantized IR light which is matched with the internal vibrational frequency of the molecule and other residual frequencies are transmitted via the sample. Due to absorption of IR radiation, the net change of dipole moment in molecule is occurred and causes vibration of bonds in the molecule like Stretching and bending vibration. The transmitted light is detected by the detector and IR spectrum interpreted on the computer screen by analysis of the transmitted light. IR Radiation (Mid IR) Range: 4000 – 400 cm -1 Sample (absorbs Specific frequency) Transmitted Light   Wave number, cm -1 Position Change of Atoms in a molecule due to net change of dipole Moment Change in amplitude of molecular Vibration Stretching & Bending Vibration

6. Figure: FOURIER TRANSFORM SPECTROPHOTOMETER Radiation source: NERST glower or Globar Source which produce Mid IR Working Function: The source energy strikes the beamsplitter and produces two beams of roughly the same intensity. One beam strikes the fixed mirror and returns to the beamsplitter . The other beam goes to the moving mirror. When these two beams meet up again at the beamsplitter , they recombine. T he recombined beam passes through the sample……..

7. Stretching Vibration Higher Frequency Bending vibration Lower Frequency Molecular Vibration Symmetric Stretching Asymmetric Stretching In-Plane Bending Out of Plane Bending Scissoring Bending Rocking Bending Twisting Bending Wagging Bending NOTE: No. of bending vibration is usually more than the no. of stretching vibration in IR spectrum.

8. a) Stretching Vibration involves a continuous change in interatomic distance along the axis of bond between 2 atoms. In Symmetric Stretching , the bond length increase & decrease symmetrically. In Asymmetric Stretching, length of one bond increase & the other one decrease at the same time without changing bond angle. b) Bending vibration involves the change of angle between two atoms Scissoring is the movement of two atoms toward and away from each other. So bond angle decrease and increase with time. Rocking in which the bond angle is maintained but bonds moves within the same plane. Changing angle b/w a group of atoms. Wagging in which both atoms move to one side of the plane. Changing angle b/w the plane a group of atoms Twisting in which one atom is above the plane and the other is below the plane. Changing angle b/w the plane 2 groups of atoms

9. What are the conditions or criteria to absorb IR radiation?? Those are, When applied infrared frequency in equal to natural frequency of vibration, then the absorption take place and a peak is observed. Only molecule gives IR absorption. Molecules must have a dipole moment. The mode of vibration of the molecule should not be center of symmetry. The molecule should be polar. Note: It is important to remember that it is not necessary for a compound to have a permanent dipole moment to be IR active.

10. Modes of molecule with respect to electromagnetic radiation ? Modes are, Translation Rotational Vibrational Now, The total no. of degrees of freedom = ( Translation + Rotational + Vibrational ) degrees of freedom Vibrational degree of freedom = 3N - ( Translation + Rotational ) degrees of freedom Vibrational degree of freedom = 3N - ( 3 + Rotational ) degrees of freedom   For Linear molecule, Vibrational degrees of freedom = 3N - ( 3 + 2 ) = 3N - 5 For Non-Linear molecule, Vibrational degree of freedom = 3N - ( 3 + 3 ) = 3N - 6 No. of atoms in the molecule E.g. Vibrational degrees of freedom of CO = 3   Vibrational degrees of freedom of H 2 O = 3   For explain this, We should know about degrees of freedom ??

11. Calculation of Vibrational frequency For Single bond, K= 5 dyne / cm Double bond, K = 10 dyne / cm Triple bond, K = 15 dyne / cm  

12. What is IR Spectrum? IR Spectrum is the graphical plot of % transmittance verses vibrational frequency in wavenumbers. % T Wave Number, cm -1 T =   IR Spectrum Functional Group Region 4,000 – 1500 cm -1 Finger-print Region 1500 – 500 cm -1 Identifies the functional group with the consequence of changing stretching vibration Identifies the exact molecule with the consequence of changing bending vibration Only stretching mode of vibration, so this region contains few peaks. Stretching and bending both are occurred, but bending is more than stretching The pattern of peaks is easy to analyze because stretching vary within a narrow range. The pattern of peaks is more complicated due to complex vibration which create a unique IR band to distinguished compounds. Sharp, Broad, Strong, Medium Peaks Sharp & Weak Peaks

13. Peaks of Functional Group Shape (Width of peak) Weak Broad Sharp Intensity (height of peak) Strong Medium Due to H-bonding or Presence of water molecules A medium band falls to about half of the y-axis A strong band covers most of the y-axis A weak band falls to about one third or less of the y-axis Factors affecting the Peak intensity : Electronegativity difference Polarity α Dipole moment α More Intense Peak  

14. Factors affecting for Vibrational frequencies of the functional group 3. More Vibration frequency = More Force-Contact (K) 1. Effect of different mode of vibration: V. frequency of stretching is higher than the bending mode of bond 4 . Electronic effect: Conjugation / Resonance α vibrational frequency 5 . H-bonding effect α   6 . Multiplicity of bond: No. of bond α Vibrational frequency 7 . Nature of Hybridization: % of S- character α Bond Strength α α V. frequency   2. Reduced Mass of atoms α  

15. How to interpret / analyze IR spectra?? IR spectra isn’t generally used to determine the whole structure of an unknown molecule. But IR spectra is great for identifying certain specific groups. Here an overview of the IR window from 4000 to 500 cm -1 with various region of interest highlighted. Basically 80% of the most useful information for our purpose can be obtained by looking at 3600 to 1600 cm -1 specific area of spectrum. Reaming forest of peak from 500 – 1400 cm -1 (fingerprint region) comparatively less useful.

16. NO. Peak take place between (cm -1 ) Common Symbols of bond Examples of bond of functional group ZONE 1 3600 – 2700 X –H O-H bond, C–H bond, N-H bond ZONE 2 2700 – 1900 X ≡ X C≡N or C≡C bond ZONE 3 1900 – 1500 X=O, X=X C=O bond, C=C bond ZONE 4 1500 -- 1000 X –O , X –X C-O bond, C – C Bond Here, X symbol will be Oxygen [O] or Carbon [C] or Nitrogen [N] An even more compressed overview looks like this… For Zone 1: a) O-H bond stretching at 3650 – 3200 cm -1 with strong intensity & broad ‘U’ elongated Shape b) Terminal alkynes (≡C–H bond) stretching at 3340 – 3250 cm -1 with stronger & sharper peak

17. c) N-H bond stretching mainly occurred for Primary & Secondary amine: For primary amine, stretching at 3500 – 3200 cm -1 with two signal. These are medium in intensity & broad shape. For 2ndary amine, stretching at 3500- 3200 cm -1 with one signal . These are weak intensity & sharp shape d) =C–H Stretching at 3100 cm -1 with weak intense & sharp peak e) - C–H Stretching at 2900 – 2800 cm -1 with medium intense & sharp peak NOTE : for aromatic compound, out of plane =C–H bending peak at 900 – 690 cm -1

18. For Zone 2: a) C≡C bond stretching at 2200 – 2000 cm -1 with variable intense & shape peak b ) C ≡N bond stretching at 2260 – 2220 cm -1 with variable intense & shape peak For Zone 3: C=O bond stretching peak for aldehyde at 2200 – 2000 cm -1 , Ketone at 1700- 1725 cm -1 , Carboxylic acid at 1715 cm -1 , Ester at 1750 – 1735 cm -1

19. b) C=C bond stretching peak at 1660 – 1600 cm -1 c ) C=C bond stretching for aromatic compound peak at 1510 – 1450 cm -1 For Zone 4: a ) C–C bond stretching: peak at 1200 – 1000 cm -1 b) C–O bond stretching: peak at 1150 – 1000 cm -1

20. Think for 1 min; How can you detect compound from IR Spectra ??

21. Application of IR Spectroscopy Identification of an organic compound Structure determinations Identification of functional group Studying the progress of reaction Distinction between two types of hydrogen bonding Study of keto-enol tautomerism Study of Ciss -trans Isomerization Conformation analysis Determination of impurities in compound Presence of water in sample

22. Limitations of IR Spectroscopy Can’t determine the molecular weight of the compound. Doesn’t give information about the relative position of different functional groups in a molecule. From the single IR spectra of an unknown substance, it is not possible to know whether it is pure compound or a mixture of compound. Sample cells are made of halogen salts which are susceptible to moisture Gas samples can’t be analyzed as the lack of sensitivity.

23. Thanks For having me…