INTERPRETATION OF INFRARED SPECTRA

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the spectral arrangement is good friends have a look at it

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INTERPRETATION OF INFRARED SPECTRA:

INTERPRETATION OF INFRARED SPECTRA PREPARED BY: SAINATH.K, Y12MPH0508. GUIDED BY : MR.B.PRAVEEN KUMAR, M.PHARM. LECTURER, CHEBROLU HANUMAIAH INSTITUTE OF PHARMACEUTICAL SCIENCES, CHOWDAVARAM, GUNTUR.

INTRODUCTION:

INTRODUCTION Infrared Spectroscopy or Vibrational spectroscopy is concerned with the study of absorption of IR radiation , which results in Vibrational transition. IR spectra is mainly used in structure elucidation to determine functional group. Energy of molecule = Electronic energy + Vibrational energy + Rotational energy Out of which for IR spectroscopy changes in vibration of molecule or absorption of energy due to vibration is important. 2

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Wavelengths usually used is 2.5-25 mm But in IR spectra, we use wave numbers(cm -1 ),the reciprocal of the wavelength in centimeters (4000-400 cm -1 ) Because wave numbers have larger values and easy to handle than wavelengths which will show only small differences between functional groups. Wave numbers is nothing but the number of waves present per cm, which can be calculated from the wavelength. Ν = 1 / λ Wave numbers are proportional to frequency and energy. 3

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The infrared regions may be categorized into three distinct zones based on their wave numbers and wavelengths as under: Region of IR Wave length(µm) Wave number(cm -1 ) Near IR (Overtone region) 0.8-2.5 12,500-4000 Mid IR(Vibration- rotation region) 2.5-50 4000-200 Far IR(Rotation region) 50-1000 200-10 Most used 2.5-25 4000-400 4

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Basic principle The principle of IR spectroscopy is based upon the molecular vibrations which is composed of the stretching and the bending vibrations of a molecule. In any molecule, it is known that atoms or groups of atoms are connected by bonds. These bonds are in a continuous motion in a molecule, as a result they maintain some vibrations with some frequency, characteristic to every portion of molecule. This is called natural frequency of vibration. 5

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When energy in the form of Ir radiation is applied and when, Applied IR frequency = Natural frequency of vibration, absorption of Ir radiation takes place and a peak is observed. 6

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Every bond or portion of a molecule requires different frequency for absorption. Hence characteristic peak is observed for every functional group or part of molecule. In other words IR spectra is nothing but a finger print of a molecule. Criteria for compound to absorb IR radiation: 1) change in dipole moment 2) applied IR frequency should be equal to the natural frequency of radiation. Otherwise compound do not give IR peak. E.G: No net change in dipole moment occurs during the vibration or rotation of homo nuclear species such as O 2 ,N 2 ,cl 2 ; such compounds cannot absorb IR radiation. 7

MOLECULAR VIBRATIONS:

MOLECULAR VIBRATIONS As we have seen that light is absorbed when radiation frequency = frequency of vibration in molecule Covalent bonds vibrate at only certain allowable frequencies associated with types of bonds and movement of atoms Vibrations include stretching and bending 8

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A molecule has a many degree of freedom as the total degree of freedom of its individual atom. Each atom has three degree of freedom corresponding to the Cartesian coordinates ( x,y,z ),so a molecule of n atoms therefore has 3n degree of freedom. For non-linear molecule three degree of freedom and three describe translation, the remaining 3n-6 degree of freedom are vibrational degree of molecule. Similarly, for linear molecule we have 3n-5 vibrational degree of freedom. 11

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The vibrations seen before are called fundamental absorption. They arise from excitation from the ground state to the lowest energy excited state. Usually spectra is complicated because of the presence of weak overtone, combination and difference bands. Overtones (multiples of given frequency) results from excitation from ground state to higher energy states. Combination bands is the sum of two vibrational frequency ( ν comb = ν 1 + ν 2 ) Difference bands is the difference of two vibrational frequency ( ν diff = ν 1 - ν 2 ). When a fundamental couples with an overtone or combination band, the coupled vibration is called Fermi Resonance. 12

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For simple diatomic molecules it is possible to calculate the vibrational frequency by treating the molecule as a harmonic oscillator and by applying Hooke’s law. ν = 1/2 c f/ μ Where ν = the vibrational frequency (cm -1 ) c = velocity of light (cm/s) f = force constant (dyne/cm) µ = m 1 m 2 / (m 1 + m 2 ) where m= mass of atom 13

IR SPECTRUM :

IR SPECTRUM Baseline Absorbance/ Peak No two molecules will give exactly the same IR spectrum (except enantiomers ) Simple stretching: 1600-3500 cm -1 Complex vibrations: 400-1400 cm -1 , called the “fingerprint region” 14

INTERPRETATION OF IR SPECTRA :

INTERPRETATION OF IR SPECTRA There are no rigid rules for interpreting IR spectrum. Thus, ir spectra must be interpreted from empirical comparison of spectra and extrapolation of studies of simpler molecules. Looking for presence/absence of functional groups A polar bond is usually ir -active A nonpolar bond in a symmetrical molecule will absorb weakly or not at all 15

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BOND TYPE OF VIBRATION FREQUENCY INTENSITY C-H Alkanes(stretch) CH 3 (bend) CH 2 (bend) Alkenes(Stretch) (out of plane) Alkyne(Stretch) Aldehyde 3000-2850 1450-1375 1465 3100-3000 1000-650 3300 S M M M S S C-C Alkanes Not useful C=C Alkenes Aromatic 1680-1600 1600 and 1475 M-w M-w C≡C Alkyne 2250-2100 M-w C=O Aldehyde Ketone Carboxylic acid Ester Amide Anhydride Acid chloride 1740-1720 1725-1705 1725-1700 1750-1730 1680-1630 1810 and 1760 1800 S S S S S S S C-O Alcohols,ethers,esters 1300-1000 S 17

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O-H Alcohols,esters,ethers 1300-1000 S N-H Primary and secondary amines(stretch) bend 3500-3100 1640-1550 M M C-N Amines 1350-1000 M-s C=N Imines and oximes 1690-1640 W-s C≡N Nitriles 2260-2240 M X=C=Y Allenes,isothiocyanates,isocyanates 2270-1940 M-S N=O Nitro 15550 and 1350 S S-H Mercaptans 2550 W 18

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S=O Sulphoxides 1050 S C-X Fluoride 1400-1000 S Chloride 785-540 S Bromide, Iodide 667 S 19

CARBON-CARBON BOND STRETCHING:

CARBON-CARBON BOND STRETCHING Stronger bonds absorb at higher frequencies: C-C 1200 cm -1 C=C 1660 cm -1 C C 2200 cm -1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C 1640-1680 cm -1 conjugated C=C 1620-1640 cm -1 aromatic C=C approx. 1600 cm -1 20

HYDROCARBONS: ALKANES:

HYDROCARBONS: ALKANES Examples: Decane, Cyclohexane. STRUCTURAL UNIT WAVE NUMBER (CM -1 ) Sp 3 C—H 2850-2950 Sp 2 C—H 3000-3100 Sp C—H 3310-3320 C-h2 1465 Ch3 1375 21

ALKENES:

ALKENES Structural unit Wave number(cm -1) Examples:1-hexene,cyclohexene C C 1620-1680 —C N —C C— 2100-2200 2240-2280 22

IR SPECTRA OF ALKYNES:

IR SPECTRA OF ALKYNES Structural unit Wave number Cm -1 ≡C-H 3300 C≡C 2150 23

IR SPECTRA OF ALCOHOL:

IR SPECTRA OF ALCOHOL Examples:1-butanol,1-hexanol. Structural unit Wave number(cm -1 ) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 24

IR SPECTRA OF ETHERS AND EPOXIDES:

IR SPECTRA OF ETHERS AND EPOXIDES Structural unit Wave number cm -1 C-O 1300-1000 Phenyl alkyl 1250 and 1040 Aliphatic 1120 25

IR SPECTRA OF ALDEHYDES:

IR SPECTRA OF ALDEHYDES EXAMPLES: Butyraldehyde, Benzaldehyde. Structural unit Wave number cm -1 C=0 1745-1725 C=C 1640 CHO 2860-2800 26

IR SPECTRA OF KETONES:

IR SPECTRA OF KETONES Examples: 2-tanone,acetophenone Structural Unit Wave number Cm -1 C=O 1720-1708 C=C 1644-1617 27

IR SPECTRA OF CARBOXYLIC ACIDS:

IR SPECTRA OF CARBOXYLIC ACIDS Examples: Hexanoic acid, Benzoic acid. Structural Unit Wave number cm -1 O-H 3400-2400 C=O stretch 1730-1700 C-O 1320-1210 28

IR SPECTRA OF ESTERS:

IR SPECTRA OF ESTERS Structural Unit Wave Number Cm -1 C=O 1750-1735 C-O stretching 1300-1000 29

IR SPECTRA OF ACID CHLORIDES:

IR SPECTRA OF ACID CHLORIDES Structural Unit Wave number Cm -1 C=O 1810-1775 Conjugated 1780-1760 C=O Stretch 730-550 30

IR SPECTRA OF AMINES:

IR SPECTRA OF AMINES Examples: Butyl amine, N-methyl aniline Structural unit Wave number cm -1 N-H 3500-3300 1 0 amines 1640-1560 2 0 amines 1500 3 0 amines 1350-100 31

IR SPECTRA OF ANHYDRIDES:

IR SPECTRA OF ANHYDRIDES Structural unit Wave number Cm -1 C=O 1830-1800 and 1775-1740 32

IR SPECTRA OF NITRILES:

IR SPECTRA OF NITRILES Examples: Butyronitrile, Benzonitrile. Structural unit Wave number Cm -1 -C≡N- 2250 33

IR SPECTRA OF ISOCYANATES :

IR SPECTRA OF ISOCYANATES Structural unit Wave number Cm -1 =R=C=O 2270 34

IR SPECTRA OF NITRO COMPOUNDS:

IR SPECTRA OF NITRO COMPOUNDS Structural unit Wave number Cm -1 NO 2 1550 1350 Aliphatic 1600-1530(asymmetric) 1390-1300(symmetric) Aromatic 1550-1490(asymmetric) 1355-1315(symmetric) 35

IR SPECTRA OF SULFUR COMPOUNDS:

IR SPECTRA OF SULFUR COMPOUNDS Structural unit Wave number Cm -1 S-H 2250 36

IR SPECTRA OF HALIDES :

IR SPECTRA OF HALIDES Structural unit Wave number Cm -1 C-F 1400-1000 C- Cl 785-540 C-Br 650-510 C-I 600-485 37

REFERENCES:

REFERENCES SPECTROSCOPY BY PAVIA, LAMPMAN, KRIZ, VYVYAN. INDIAN EDITION, PAGE NO.26-93. SPECTROMETRIC IDENTIFICATION OF ORGANIC COMPOUNDS, ROBERT M. SILVERSTEIN, FRANCIS X. WEBSTER, 6th EDITION, PAGE NO. 71-143 ELEMENTARY ORGANIC SPECTROSCOPY BY Y.R.SHARMA, PAGE NO.69-143 38

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THANK YOU 39

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