infrared spectroscopy and factors effecting the IR

some basic about IR spectroscopy

1. IR SPECTROSCOPY PRESENTED TO DR SHAISTA GILLANI PRESENTED BY RIMSHA BAKHTAWAR RASHEED FA-19 MSC(CHEM) 006 12-4-2021 RIMSHA B.R MSC(006) 1

2. CONTANTS INTRODUCTION OF SPECTROSCOPY TYPES INFRARED SPECTROSCOPY PRINCIPLE OF IR MODES OF VIBRATION HOOKS LAW FACTORS APPLICATION RIMSHA B.R MSC(006) 2

3. WHAT IS SPECTROSCOPY Method of “Seeing the unseeable” EMR So by Using electromagnetic radiation we obtain information about atoms and molecules that are too small to seen. RIMSHA B.R MSC(006) 3

4. RIMSHA B.R MSC(006) 4 Spectroscopy is an instrumentally aided study of the interactions between matter (sample being analyzed) and energy (any portion of the electromagnetic spectrum)

5. IR SPECTROSCOPY Infrared spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. RIMSHA B.R MSC(006) 5

6. IR SPECTROSCOPY When infrared light radiations between 4000-400 cm-1 are passed through a sample of an organic compound, some of these radiations are absorbed by the sample and are converted into energy of molecular vibrations . So it is also called vibrational spectroscopy. Due to vibration there is a change of bond length and bond angle in the molecules.

7. TYPES Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region. RIMSHA B.R MSC(006) 7

8. PRINCIPLE OF IR SPECTROSCOPY The two atoms joined together by a chemical bond (may be single, double or triple bond), macroscopically can be composed as two balls joined by a spring These vibrations depend on the strength of the spring and also the mode (stretching or bending) in which the force is being applied . This characteristic vibration are called Natural frequency of vibration . Applied infrared frequency = Natural frequency of vibration RIMSHA B.R MSC(006) 8

9. RIMSHA B.R MSC(006) 9 Vibration or oscillation along the line of bond Change in bond length Occurs at higher energy: 4000-1250 cm-1 •Vibration not along the line of bond •Bond angle is altered • Occurs at low energy: 1400-666 cm-1

10. STRETCHING VIBRATION A stretching vibration changes the bond length. There are two types of stretching vibrations In  asymmetric stretching , some bonds are getting shorter at the same time as others are getting longer. RIMSHA B.R MSC(006) 10 In symmetric stretching, two or more bonds vibrate in and out together.

11. Bending Vibrations(in plane) Scissoring  is the movement of two atoms toward and away from each other Rocking  is like the motion of a pendulum on a clock, but an atom is the pendulum and there are two instead of one RIMSHA B.R MSC(006) 11

12. Bending vibration(out the plane) Wagging  is like the motion in which make a "V" sign as we make with our fingers and bend them back and forth from your wrist . Twisting  is a motion as if the atoms were walking on a treadmill . RIMSHA B.R MSC(006) 12

13. Graphical Representation RIMSHA B.R MSC(006) 13 Each type of vibration contributes to an infrared spectrum. For example, here is the spectrum of formaldehyde, H₂C=O.

14. Hooke’s Law It says that the amount of stress we apply on any object is equal to that amount of strain is observed on it, which means  Stress  ∝  Strain. RIMSHA B.R MSC(006) 14 Hooke’s Law Formula  is given as F = -K x Where, F is the amount of force applied in N, x is the displacement in the spring in m, k is the spring constant or force constant.

15. Hookes ' Law RIMSHA B.R MSC(006) 15   Consider a bond and the connected atoms to be a spring with two masses attached. Using the force constant  k  (which reflects the stiffness of the spring) and the two masses  m 1  and  m 2 , then the equation indicates how the frequency, u, of the absorption should change as the properties of the system change.

16. Trends RIMSHA B.R MSC(006) 16 absorption bands depends on bond order. Higher the bond order larger is the band frequency. . The force constant(K) is a measure of the strength of a bond. Thus the value of stretching frequency of a bond increases with increasing the bond strength and decreasing the reduced mass of the bonded atoms.

17. Continue.. As the atom attached to carbon a particular atom say carbon increases in mass, the reduced mass increases and hence the frequency of vibration decreases   compare: C-H   (3000 cm -1 ) C-0 (1100cm -1 ) C-C   (1000 cm -1 ) C-Cl  ( 800 cm -1 ) C-Br  (550 cm -1 ) C-I      (about 500 cm -1 ) RIMSHA B.R MSC(006) 17 Larger mass lesser absorption

18. T rends that can be accounted for using Hookes' Law. It also gives an approximate outline of where specific types of bond stretches may be found . RIMSHA B.R MSC(006) 18

19. Factors affecting the absorption frequencies. The vibrational frequency of a bond increase as a strength increase and reduce mass decrease µ Frequency shift also take place when working on same substance at different states ( solid , liquid , vapor) A substance usually absorb at higher frequency in a vapor state as compared to liquid or solid state. Factors responsible for shifting the vibrational frequencies to their normal value are Electronic effect Hydrogen effect Ring stain RIMSHA B.R MSC(006) 19

20. Electronic effect   Changes in the absorption frequencies for a particular group take place when the substituents in the neighbourhood of that particular group are changed. It includes : Inductive effect Resonance effect RIMSHA B.R MSC(006) 20

21. INDUCTIVE EFFECT   The introduction of alkyl group causes +I effect which results in the lengthening or the weakening of the bond Hence the force constant is lowered and wave number of absorption decreases. Introduction of an electronegative atom or group causes –I effect which results in the bond order to increase . Hence the force constant increases and the wave number of absorption rises. RIMSHA B.R MSC(006) 21 1750cm-1 1745cm-1 1715cm-1 More +I less wavenumber More –I more wavenumber

22. Resonance effect Resonance effect has significant influence on the absorption frequency in the infrared spectra the conjugation lowers absorption frequency or wave number This is because conjugation tends to increase the bond length of the group so that its bond strength decrease. this in turn lowers the force constant of the bond and the absorption frequency decrease. RIMSHA B.R MSC(006) 22

23. Hydrogen bonding: T he infrared spectra of organic compounds such as alcohol ,phenols, carboxylic acid and amines are strongly influenced by the presence of hydrogen bond. Hydrogen bonding results in the weakening of bond involved so that force constant decrease and absorption shift toward lower frequency or wave number. Hydrogen bonding to a C=O group withdraws electrons from oxygen and lowers the C=O double bond character. This results in lowering of C=O absorption frequency. More effective is the hydrogen bonding, higher will be the lowering in C=O absorption frequencies. RIMSHA B.R MSC(006) 23

24. CONTINUE …. The monomeric carboxylic acids absorb at the frequency of 1760 cm-1 where dimers at 1710cm-1 due to the presence of hydrogen bonding with carbonyl group. I n methyl salicylate lowers the C=O stretching frequency to 1680 cm-1 than observed at 1700 cm in case of methyl p- hydroxybenzoate . RIMSHA B.R MSC(006) 24 1710cm-1

25. Ring stain RIMSHA B.R MSC(006) 25 As the size of the ring reduce their strength is continuously increasing And due to size reduction their mass also reduce so frequencies increase .

26. APPLICATION Identification of functional group and structure Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic peaks are observed. This region of IR spectrum is called as finger print region of the molecule. Detection of impurities Detection of hydrogen bonding & distinguish inter & intramolecular Identification of drugs • Ibuprofen in IP, BP and USP identified by IR- because it contains COOH group so, chemical tests are not useful for identification. Identify odor & taste components of food Determine atmospheric pollutants from atmosphere itself. Examination of old paintings & artifacts To determination of raw materials. Quality control of pharmaceutical formulation Determination of particle size RIMSHA B.R MSC(006) 26

27. QUIZ 1 . On which factors the vibrational stretching frequency of diatomic molecule depend? Force constant Atomic population Temperature Magnetic field 2 . What is the order of decreasing vibrational frequency for C — Cl, C — Br, C — C, C — O and C — H? C-H, C-C, C-O, C- Cl, C-Br C- Cl, C-Br, C-C, C -H, C-O C-O, C-H, C-Br, C- Cl, C-C C-Br, C- Cl, C-C, C-O, C-H RIMSHA B.R MSC(006) 27

28. 3 . Infrared spectroscopy is based on Absorption spectroscopy Emission spectroscopy Elastic scattering Reflection spectroscopy 4 . What is the relation between restoring force, f to the displacement q in Hooke’s law? a) f = - kq b) f = kq c) f = kq2 d) f = -kq2 RIMSHA B.R MSC(006) 28

29. 5 . What is the correct increasing order of stretching frequencies for C ≡ C, C = C and C — C ? a ) C — C > C = C > C ≡ C b ) C ≡ C > C = C > C — C c ) C — C > C = C < C ≡ C d ) C ≡ C < C — C > C = C 6 . Which of the following molecules will not show infrared spectrum? a) H2 b) HCI c) CH4 d) H20 RIMSHA B.R MSC(006) 29

30. Q NO 1 The carbonyl stretching frequency in RCOCH3 (~1720 cm-1) is lower than acid chloride RCOCl (1750-1820 cm-1)?why Q NO 2 Why asymmetric stretching is higher than symmetric stretching? Q N0 3 What factors influence IR? Q NO 4 why C-C not show peaks in IR spectrum ? RIMSHA B.R MSC(006) 30

31. REFERANCES https:// socratic.org/questions/what-are-the-differences-between-stretching-vibration-and-bending-vibrations . https://byjus.com/hookes-law-formula / https:// www.slideshare.net/VrushaliTambe2/factors-affecting-ir-absorption-frequency http:// epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S000005CH/P000667/M009109/ET/1459227637CHE_P12_M8_e-Text.pdf https :// www.slideshare.net/rashmiraveendran92/factors-affecting-vibrational-frequencies-in-ir https :// classroom.google.com/u/0/c/Mjk2NDA0NDM2Mzk3 https ://www.youtube.com/watch?v=8SVHLzs35lI RIMSHA B.R MSC(006) 31

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