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IR SPECTROSCOPYManu JoseFirst year M.Pharm Pharmaceutical Analysis Dept.National College of Pharmacy

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OVERVIEW• INTRODUCTION

• THEORY

• FACTORS AFFECTING VIBRATIONAL FREQUENCY

• SAMPLING TECHNIQUES

• INSTRUMENTATION

• INTERPRETATION

• APPLICATION

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Introduction

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Spectroscopy Spectroscopy is the measurement and

interpretation of EMR absorbed or emitted, when the organic molecule or

atoms or ions of a sample move from one energy state to another.

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Definition IR spectroscopy can be defined as a

method for the identification of substances based on their absorption of

IR wavelength.

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IR RADIATION

UVX-rays IRg-rays RadioMicrowave

Visible

Electromagnetic spectrum

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Wave number Wave length

Near IR 14000-4000cm-1 0.8-2.5µm

Mid IR 4000-400cm-1 2.5-25µm

Far IR 400-10cm-1 25-1000µm

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Theory •IR radiation in the range 10,000- 100 cm-1

is absorbed.•Frequency of absorption depends on

relative masses, the force constant, and geometry of atoms.

•Converted to energy of molecular vibration.

•The absorption is quantized.

•Vibrational spectra is appear as bands.

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•A molecule absorbs a selected frequencies of IR.

•Absorption corresponding to 8-40 KJ/mol

•Not have sufficient energy to cause the excitation of electrons, it can cause vibrations.

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Criteria for absorption•Not all bonds are capable of absorbing IR.

1. Dipole moment-▫To absorb IR, a molecule must undergo net

change in dipole moment as it vibrate.▫Electric field of radiation can interact with

molecule- change amplitude of vibration.▫Eg: Hydrochloride.

2. Correct wavelength▫ Applied IR frequency =Natural frequency of

vibration

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Calculation of stretching frequency:•By Hooks law;

▫v - vibrational frequency▫k - force constant▫µ= (m1.m2)/(m1+m2)▫m1 and m2 –mass of atom 1 and 2

respectively.

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Molecular vibrations

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•The relative positions of atoms in a molecule are not fixed but fluctuate continuously as a consequence of vibrations and rotations.

•Two types of fundamental vibrations are there,▫Stretching

▫Bending

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Stretching vibrations•Involves continuous change in inter

atomic distance along the axis.

•Two types:▫Symmetrical stretching-stretching/

compression in symmetrical way.

▫Asymmetric stretching-one bond is stretching while other is compressing.

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H

H

C

H

H

C

symmetrical

asymmetrical

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Bending vibrations:•Involves change in bond angle between

bonds with a common atom or the movement of group of atoms to the reminder of molecule.

•Two types:▫In-plane

▫Out-of plane

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•Scissoring- atoms move towards and away from each other.

•Rocking- The structural unit swings back and forth in the plane.

H

H

CC

H

H

CC

In plane bending vibrations

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•Wagging: The structural unit swings back and forth out of the plane

•Twisting: structural unit rotate about the bond which joins to the rest of the molecule

H

H

CC

H

H

CC

Out of plane bending vibrations

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• Each atom has 3 degrees of freedom• N-atom molecule there will be 3N degree of

freedom.

• Translation - the movement of the entire molecule while the positions of the atoms relative to each other remain fixed

• Rotational transitions – inter-atomic distances remain constant but the entire molecule rotates with respect to three mutually perpendicular axes.

Degree of freedom

Translation

Rotational

Non linear 3 3linear 3 2

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Number of vibrational degree of freedomFor molecules having N atoms in them

• Nonlinear molecules= 3N - 6

Eg : H2O (3x3)-6=3

• Linear molecules= 3N -5

Eg : N2 (3x2)-5=1

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Why not 3N-6/3N-5 bands in IR Spectrum?

•The theoretical number of fundamental vibrations (absorption frequencies) will seldom be observed.

•Overtone and combined tone increase the number of bands.

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Reasons for decrease in number of bands are-

•Bands which are so close that they coalesce.

•Bands which are too weak to be observed.

•Occurrence of a degenerate band.

•Lack of change in molecular dipole moment.

•Fundamental frequencies outside 4000-400 cm-1

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Eg:

• CO2 is linear 3x3-5 =4 degree of freedom.But▫ Symmetric stretching- no change in dipole▫ Bending vibrations are equivalent (degenerate)

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Factors influencing vibrational frequencies

•Calculated value of frequency of absorption for a particular bond is never exactly equal to its experimental value.▫Electronic effects – inductive and field effect

▫Ring size

▫Hydrogen bonding

▫Vibrational coupling

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•Inductive effect:

Electro negative/

electron withdrawing

Negative inductiv

e

Stronger bond

Increased force constant

Wave number rises

Wave no cm-1

Acetone 1715

chloroacetone 1725

Dichloroacetone 1740

Tetrachloroacetone

1778

Electro positive/

electron attracting

Positive inductive effect

Weaker bond

Decrease force constant

Wave number decreases

H-CHO 1750

CH3-CHO 1745

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Field effect:•Lone pair of electrons on two atoms

influences each other through space, can change vibrational frequency of both groups.

•Eg: O- haloacetophenoneC = O

X : :

: :

:

CH3Electrostatic repulsion due to lone pair off electrons

Change the state of hybridization of C=O

Absorption occurs at higher wave number

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Ring size

7 membered

ring

6 membered

ring

5 membered

ring

4 membered

ring

3 membered

ring

1702 cm-1 1715 cm-1 1745 cm-1 1780 cm-1 1850 cm-1

Increased ‘s’ character

•Ring strain causes large shift to higher frequency

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Hydrogen bonding •Remarkable downward frequency shift.

•Stronger H bond- greater the shift towards lower wave number.

▫Free alcohol - sharp OHstr band

▫H bonded alcohol - broad OHstr band

•OH of phenol – condensed to polymeric

wide envelope of O-Hstr

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Vibrational coupling• Isolated C-H bond – only one str frequency

• For methylene (-CH2-) –two coupled vibrations of different frequency.

( asymmetric and symmetric)

Fundamental+ fundamental

Amide

1600- 1700cm-1C=O str

N-H def

Fundamental + overtone

Aldehydes 2800- 2700 cm-1C-H str

C-H def

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Sampling techniques

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1) Solid samples:•Mull technique•Pressed pellet technique•Solids run in solutions•Solid film

2) Liquid samples

3) Gaseous sample

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Mull technique:•Grinding 2-5mg of sample in smooth

agitate mortar.•Powdered sample + Nujol paste•Paste between the two plates of salt.

•The oil has few absorption bands at 2857, 1449 and 1389 cm-1

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Pressed pellet technique:1mg Sample+ 100mg KBr powder

pressed in hydraulic press

transparent disc inserted in sample holder.

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Solids films:•Amorphous solid samples melted between

salt plates allowed to form solid film.•For qualitative purpose.

Solids run in solution•Solid + suitable solvent solution •Kept in cells for liquids•Solvents- non associated

solvents- CS2, CCl4,

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Liquid sample:

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Gaseous sample:

•Vapors in specially designed cells.

•End walls made up of Sodium chloride.

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Instrumentation

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Dispersive infra red spectrometers•Instrument produce a beam of IR radiation•Mirrors divide it into 2 parallel beam of

equal intensity.•Sample is placed in one beam, other is

used for reference.•The beam is then passed through mono-

chromator which disperse into a continuous spectrum.

•The detector senses the ratio b/w intensities of sample and reference beam.

•Signals is amplified and recorded.

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Dispersive infra red spectrometers

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Radiation sources•Consist of inert solid•Heated electrically

▫Nernst glower▫Globar source▫Mercury arc▫Incandescent wire source▫Tungsten filament

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Nernst glower• Composed of rare earth oxides- zirconium,

yttrium and thorium• Hollow tube (2-5cm x 1-3mm)• Platinum leads at one end

• Large negative temp. coefficient.• Emit radiation over wide range and remains

steady over a long period.

Disadvantages:• Fragile• Auxiliary heater • Over heating

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Globar source•Rod of sintered silicon carbide. (5cm x

5mm).•Positive coefficient of resistance.•Self starting and electrically heated•Enclosed in water cooled brass tube

•Less intense.

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Mercury arc•For far IR region

•High pressure Hg arc, enclosed in quartz jacketed tube, at 1 atm

•Passage of electricity through vapor internal plasma source IR radiation

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Incandescent wire loop•Tightly wound spiral of Nichrome wire.•No water cooling•Less maintenance•Less intense than other sources.

Tungsten filament lamp•Tightly wound spiral of tungsten wire.•For near IR

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Sample holders• Constructed of rock salt.

• Path length is adjusted with Teflon.

• Filled and emptied with hypodermic needles.

• Foggy due to moisture.

Care: ▫Moisture free samples

▫Fingers should not be come in contact

▫Prevent contamination with silicones

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Monochromator•To select desired frequency from radiation

source.▫Prism monochromator

▫Grating monochromator

•Material used: Halogen salt

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Detectors •Thermal detectors(IR heating potential difference ∞ amt of

radiation)

▫Thermocouple

▫Bolometer

▫Golay cell

•Photo detectors

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Thermocouple

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Bolometer

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Golay cell

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Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector

(DLATGS)

Beam splitter

FT Optical System Diagram

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Michelson interferometer FT-IR

IR Source

Moving mirror

Stationary mirror

Beamsp

litte

r

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Constructive and destructive interferences

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Comparison• The dispersion Spectrometer takes several minutes to measure an IR spectrum

•Also the detector receives only a few % of the energy of original light source.

•FT-IR takes only a few seconds, to measure an IR spectrum.

•The detector receivesup to 50% of the energy of original light source.

Dispersion Spectrometer

FT-IR

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Spectrum

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IR spectrum

• Each dip- band• 100% transmittance- no absorption.• Scale of spectrum is not entirely linear.

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Interaction of IR Radiation with Organic Molecules

•Primary use is to detect functional group.

•All organic functional groups are made up of multiple bonds and therefore they will show multiple IR bands.

•Capable of giving sufficient information about the structure of a compound.

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ALKANESGeneral structure : R- CH2- R

Wave number

C-H (str) 3000-2840 cm-1

C-H (def) 1470-1450 cm-1

C-H (rock) 1370-1350 cm-1

In long chain alkanes

C-H (rock) 725-720 cm-1

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Eg: Octane

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ALKENES

General structure : R-C=C-R

Wave number

=C-H str 3100-3000 cm-1

=C-H def 1000-650 cm-1

C=C str 1680-1640 cm-1

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Eg: 1-Octene

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ALKYNESGeneral structure : R- C ≡ C - R

Wave number

- C ≡ C - H str 3333-3267 cm-1

- C ≡ C - H def 700-610 cm-1

-C ≡ C str 2260-2100 cm-1

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Eg: 1-Hexyne

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Aromatic Compounds

General structure : Ar-RWave number

C -H str 3100-3000cm-1

C -C str (in ring) 1600-1585cm-1

1500-1400cm-1

C-H bend 900-675cm-1

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Eg: Toluene

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Alkyl HalidesGeneral structure : R-X

Wave number

C-H str 3000-2850 cm-1

C-H def 1300-1150 cm-1

C-X str 850-515 cm-1

eg: C-Cl str C-Br str

850-550 cm-1

690-515 cm-1

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Eg: 1-Bromopropane

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Alcohols

General structure : R-OH

Wave number

O-H str 3500-3200 cm-1

C-O str 1260-1050 cm-1

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Eg: Ethanol

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KETONESGeneral structure : R-CO-R

Wave number

C=O str

aliphatic ketones 1715 cm-1

α,β-unsaturated ketones 1685-1666 cm-1

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Eg: 2-Butanone

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AldehydesGeneral structure : R-CHO

Wave number

H-C=Ostr 2830-2695 cm-1

C=O str

aliphatic aldehyde 1740-1720 cm-1

α,β-unsaturated aldehydes 1710-1685 cm-1

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Eg: Butaraldehyde

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CARBOXYLIC ACIDSGeneral structure : R-COOH

Wave number

O-H str 3300-2500 cm-1

C=O str 1760-1690 cm-1

C-O str 1320-1210 cm-1

O-H bend 1440-1395 cm-1

950-910 cm-1

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Eg: Hexanoic acid

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ESTERSGeneral structure : R-COOR

Wave number

C=O str

aliphatic esters 1750-1735cm-1

α,β-unsaturated 1730-1715cm-1

C-O str 1300-1100cm-1

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Eg: Ethyl acetate

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AminesGeneral structure : R- NH2

Wave number N-H str 3400-3250cm-1

1O amines 3400-3300cm-1

3330-3250cm-1

2Oamines 3350-3310cm-1

3O amines no bands

N-H bend (1O only) 1650-1580cm-1

N-H wag 910-665cm-1

C-N str

aromatic amines 1335-1250cm-1

aliphatic amines 1250-1020cm-1

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Eg: Aniline

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NITRO GROUP

General structure: R-NO2

Wave number

N-O str (asymmetric) 1550-1475cm-1

N-O str(symmetric) 1360-1290cm-1

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Eg: Nitromethane

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Recent Advancements

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Attenuated Total Reflectance Spectroscopy( ATR Spectroscopy)

•Utilizes the phenomenon of Total Internal Reflection

•Angle of incidents > critical angle

•The beam penetrates a fraction of a wavelength beyond the reflecting surface.

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• when a material that selectively absorbs radiation, the beam loses energy at the wavelength where the material absorbs.

• The resultant attenuated radiation is measured.

• plotted as a function of wavelength and gives rise to the absorption spectral characteristics of the sample.

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Applications

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Quantitative analysis• Based on the determination of the

concentration of one of the functional group of the compound being estimated.

• Concentration is determined using

Beer- lamberts law

A = I1/I0 = abc

A∞ c ; ‘a’ and ‘b’ are constant.

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Qualitative analysis• By comparison with reference spectra.

• Group frequency region: (3700-1500cm-1)▫Hydrogen bonding region

By str vibration between hydrogen and other atom (O-H)▫Triple bond region

Triple bond produce a peak in the region 2700-1850 cm-1▫Double bond region

Compound having double bond- peak at 1850- 1500cm-1

• Finger print region: (1500-700cm-1)▫Identity of a compound▫Due to bending vibrations

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Miscellaneous applications•Study of progress of reaction.

•Detection of impurities.

•Tautomerism.

•Functional group identification.

•Presence of water in sample.

•Industrial applications.

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Conclusion Infrared spectroscopy is certainly one of the most important analytical techniques

available to today’s scientists for the determination of identity and molecular

structure of organic compounds

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References

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• William Kemp; Organic Spectroscopy; 3rd edition; page no:20-94.

• Robert M Silverstein; Spectrometric Identification of Organic Compounds; 6th edition; page no:71-109.

• Skoog, Holler, Crouch; Instrumental Analysis; 5th edition; page no:477- 528.

• John R Dyer; Application of Absorption Spectroscopy; page no:23-33.

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•H.Kaur; Instrumental Method of Chemical Analysis; 6th edition; page no:181-184,192-195.

•P.S Kalsi Spectroscopy of organic compound; 6th edition, page no 67,74.

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Thank you