INTRODUCTION AND APPLICATIONS OF FT - INFRA – RED SPECTROSCOPY By

Mr. K.KANDASAMY,Assistant Professor ,Department of Chemistry,KSR College of Arts and Science for women,Tiruchengode – 215.

?????????????????Infra

red

Spec

tros

copy

Spectroscopy

“seeing the unseeable”

Using electromagnetic radiation as a Investigation to obtain information about atoms and molecules that are too small to see.

Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

 

INTRODUCTIONSpectrosc

opy

Absorption

spectroscopy

UV spectrosc

opy

IR spectrosc

opy

NMR spectrosc

opy

Emission spectrosc

opy

Fluorimetry

Flame photomet

ery

INFRARED SPECTROSCOPY

• Infrared spectroscopy • (IR spectroscopy) is

the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light

• Infrared Spectroscopy is the analysis of infrared light interacting with a molecule.

• It is based on absorption spectroscopy

IR Spectroscopy

I. IntroductionThe entire electromagnetic spectrum is used by chemists:

UVX-rays IRg-rays RadioMicrowave

Energy (kcal/mol)300-30 300-30 ~10-4> 300 ~10-6

Visible

Frequency, n in Hz~1015 ~1013 ~1010 ~105~1017~1019

Wavelength, l10 nm 1000 nm 0.01 cm 100 m~0.01 nm~.0001 nm

nuclear excitation (PET)

core electron excitation (X-ray cryst.)

electronic excitation (p to p*)

molecular vibration

molecular rotation

Nuclear Magnetic Resonance NMR (MRI)

INFRARED REGIONS RANGENear infrared region 0.8-2.5 µ(12,500-4000 cm-1)Main infrared region 2.5-15 µ(4000-667cm-1) Far infrared region 15-200 m µ(667-100 cm-1)

INFRARED REGIONS

PRINCIPLE• When infrared 'light' or radiation hits a

molecule, the bonds in the molecule absorb the energy of the infrared and respond by vibrating.

IR radiation

vanllin

Molecular vibrations

Theory of Infrared Absorption Spectroscopy

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.

Selection Rules ( Active and Forbidden Vibrations) (i) Infra-red light is absorbed only when a change in dipole movement character in the molecule takes place. (ii) If a molecule has a center of symmetry, then the vibrations are centrosymmetric and inactive in the Infra-red but are active in the Raman. (iii) The vibration which are not centrosymmetric are active in Infra-red but inactive in Raman.

DM = 0 IR Inactive Raman Active

What is Center of symmetry?

MOLECULAR VIBRATIONS

Fundamental Vibrations

Stretching

Vibration

Symmetric Asymmetric

Bendin

g Vibrati

onIn-planeBending

Scissoring

Rocking

Out Of Plane

Bending

Wagging

Twisting

Non-fundamental Vibrations

Over Tones,

Combination

Tones,Fermi

Resonance

“Molecular Vibrations”

What is a vibration in a molecule?“Any change in shape of the molecule-

stretching of bonds, bending of bonds, or internal rotation around single bonds”.

Why we study the molecular vibration?Because whenever the interaction b/w

electromagnetic waves & matter occur so change appears in these vibrations.

“MOLECULAR VIBRATIONS”

IR Spectroscopy

I. IntroductionC. The IR Spectroscopic Process

The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration

We perceive this vibration as heat

When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms

For a simple diatomic molecule, this model is easy to visualize:

FUNDAMENTAL VIBRATIONS

• Vibrations which appear as band in the spectra.

NON-FUNDAMENTAL

VIBRATIONS

• Vibrations which appears as a result of fundamental vibration.

Mol. vibration divided into 2 main types:

FUNDAMENTAL VIBRATIONS

1.Streching vibration Involves a continuous change in the inter atomic distance along the axis of the bond b/w 2 atoms.2.It requires more energy so appear at shorter wavelength.

STRETCHING VIB. 1.Bending

vibrations are characterized by a change in the angle b/w two bonds.2.It requires less energy so appear at longer wavelength.

BENDING VIB.

Fundamental vibration is also divided into types:

Now, stretching vibration is further divided into :SYMMETRIC

VIB.

• Inter atomic distance b/w 2 atoms increases/decreases. (OR) The Movement of the atoms with respect to a particular atom in a molecule is in the same direction.

ASYMMETRIC VIB.

• Inter atomic distance b/w 2 atoms is alternate/opposite.

(OR) One of the atom approaches the central atom while the other departs from it. Symmetric Stretch Asymmetric Stretch

H H

C

H HC

asymmetricsymmetric

Bending vibration is divided into:

• If all the atoms are on same plane.

IN PLANE BENDIN

G

• If 2 atoms are on same plane while the 1 atom is on opposite plane.

OUT OF PLANE BENDIN

G

In-Plane bending and Out plane bending is further divided into:

Scissoring

Rocking

Wagging Twisting

scissor

H HCC

H HCC

H HCC

H HCC

rock

twistwag

in plane

out of plane

In- Plane BendingScissoring:

In this type, Two – atoms approach each other.Rocking:

In this type, The movement of the atoms takes place in the same direction.Out Plane BendingWagging:

Two atoms move ‘Up and down’ the plane with respect to the central atom.Twisting:

One the atom moves up the plane while the other moves down the plane with respect to the central atoms.

C.The IR Spectroscopic Process As a covalent bond oscillates – due to the oscillation of the dipole

of the molecule – a varying electromagnetic field is produced

The greater the dipole moment change through the vibration, the more intense the EM field that is generated

Infrared Spectroscopy

NON-FUNDAMENTAL VIBRATIONS

NON-FUNDAMENTA

L

OVER TONES:These are

observed at twice the

frequency of strong band.

Ex: carbonyl group.

COMBINATION TONES:

Weak bands that appear

occasionally at frequencies that

are sum/difference of 2 or more fundamental

bands.

FERMI RESONANCE:

Interaction b/w fundamental vibration &

overtones or combination

tones.Ex:CO2

C.The IR Spectroscopic ProcessWhen a wave of infrared light encounters this oscillating EM

field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed

The coupled wave now vibrates with twice the amplitude

Infrared Spectroscopy

IR beam from spectrometer

EM oscillating wavefrom bond vibration

“coupled” wave

Characteristic Vibrational Frequencies of BondsBonds are not rigid but behave like a spring with a mass at either end.

• Obey Hooke’s Law: F = -kx - sign is motion in negative site• This gives rise to a characteristic frequency for the vibration:

m1 and m2 = Mass of the Atoms in grams in particular bond k = Force constant of the bond. For Single bond, it is approximately 5x105 gm sec-2 It becomes double bonds and triple bonds respectively.C= velocity of the radiation = 2.998 x cm sec-1

massreducedk

_21p

21

21_mmmm

massreduced

The frequency is affected by• the masses of the atoms in the bond• the strength of the bond

The lower the mass, the higher the vibrational frequency.• Stretching frequencies for bonds to carbon: C-H > C-C > C-N > C-O

• The stronger the bond, the higher the vibrational frequency.• Stretching frequencies

• C≡C > C=C > C-C• C≡N > C=N > C-N• C≡O > C=O > C-O• spC-H > sp2C-H > sp3C-H

DEGREES OF FREEDOM Fundamental vibration of molecule depend on degree of freedom Each atom has 3 degree of freedom depend on x , y ,z For a molecule containing n number of atoms has 3n degree of freedom n = The Number of atom in a molecule. 3n Degree of freedom = Translational + Rotational + Vibrational. Molecule has always three translational degree of freedom. Rotational of a molecule about an axis (x,y,z) through the Center of gravity. So we calculate only number of vibrational degrees of freedom.

For Linear Molecule, There are two degree of rotation (x,y Axis only) For linear (3n-5)degree of freedom represent fundamental vibrations Total degree of freedom = 3n Translational degree of freedom = 3 Rotational degree of freedom = 2 So Vibrational degree of freedom = 3n-3-2= 3n-5 For non linear molecule 3 degree of freedom represent rotational &

translational motion For non linear (3n-6)degree of freedom represent fundamental vibrations Total degree of freedom = 3n Translational degree of freedom = 3 Rotational degree of freedom = 3 (x,y,z) So Vibrational degree of freedom = 3n-3-3= 3n-6

For Example In linear molecule of carbon dioxide (CO2 ), The number of degrees of the freedomNumber of atoms (n) = 3Total degrees of freedom =3n = 3 x 3 = 9Translational = 3Rotational = 2Vibrational degree of freedom = 9-3-2=4For Linear CO2 Molecule, the theoretical number of fundamental bands should be equal to FOUR.

DM = 0 ѵ = 2350 cm-1 ѵ= 667cm-1

IR inactive IR active

For Non- Linear Molecule

In Non - linear molecule of H2O, The number of degrees of the freedomNumber of atoms (n) = 3Total degree of freedom = 3 x 3 = 9Translational = 3Rotational = 3 Vibrational degrees of freedom = 9-3-3= 3 So, theoretically there should be THREE Fundamental bands in the Infra-red spectrum of Water.

For Benzene molecule?It is Non-linear Molecule, Degrees of freedom 3n-6Number of atoms (n) = 12 Total degrees of freedom = 3 x 12 = 36Translational = 3Rotational = 3 Vibrational degrees of freedom = ?

30

Only those vibrational changes that result in

change in dipole movement appear as band

All vibrational changes don’t appear as band

Factors Influencing Vibrational Frequencies1. Coupled Vibrations and Fermi Resonance

Symmetric Asymmetric

Symmetric Asymmetric

Stretching Vibration of CH2 Methylene Group is Lower then the Stretching Vibration of –CH3

H H H H

CC

H H H H

CC

IR beam from spectrometer

EM oscillating wavefrom bond vibration

“coupled” wave

2. Electronic EffectInductive Effect, mesomeric Effect, Field Effect etc.

For Example, The Introduction of alkyl group to produces +I Effect.(i) Formaldehyde (HCHO) = 1750 cm-1 (ii) Acetaldehyde (CH3CHO) = 1745cm-1 (iii) Acetone (CH3COCH3) ) = 1717cm-1

Introduction of electronegative atom or group causes –I Effect (i) Acetone ( CH3COCH3) = 1715 cm-1 (ii) Chloroacetone ( CH3COCH2Cl) = 1725 cm-1 (iii) Tetrachloroacetone (Cl2CH-CO-CHCl2) = 1750 cm-1

1. Conjugation – by resonance, conjugation lowers the energy of a double or triple bond. The effect of this is readily observed in the IR spectrum:

• Conjugation will lower the observed IR band for a carbonyl from 20-40 cm-1 provided conjugation gives a strong resonance contributor

• Inductive effects are usually small, unless coupled with a resonance contributor (note –CH3 and –Cl above)

O

O

1684 cm-1 1715 cm-1

C=O C=O

CH3C

OX X = NH2 CH3 Cl NO2

1677 1687 1692 1700 cm-1

H2N C CH3

O

Strong resonance contributor

vs. NO

O

CCH3

O

Poor resonance contributor(cannot resonate with C=O)

Effects on IR bands2. Steric effects – usually not important in IR spectroscopy, unless they reduce the

strength of a bond (usually p) by interfering with proper orbital overlap:

• Here the methyl group in the structure at the right causes the carbonyl group to be slightly out of plane, interfering with resonance

3. Strain effects – changes in bond angle forced by the constraints of a ring will cause a slight change in hybridization, and therefore, bond strength

• As bond angle decreases, carbon becomes more electronegative, as well as less sp2 hybridized (bond angle < 120°)

O

C=O: 1686 cm-1

O

C=O: 1693 cm-1CH3

O O O O O

1815 cm-1 1775 cm-1 1750 cm-1 1715 cm-1 1705 cm-1

Effects on IR bands4. Hydrogen bonding

Two Types Hydrogen bonds are there 1. Intermolecular Hydrogen bonding ( Two B/w Atoms) 2. Intramolecular Hydrogen Bonding ( With in the Molecule)

Intermolecular Hydrogen bonds give rise to broad bands whereas bands arising from intramolecular hydrogen are sharp.

H-bonding can interact with other functional groups to lower frequencies

Intra molecular Steric hindrance to H-bondingin a di-tert-butylphenol

C=O; 1701 cm-1

OOH

OH

Functional Group Analysis

Structural/Functional Components

Infrared Absorption Frequencies

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

O—H (alcohols) 3200-3600

O—H (carboxylic acids) 3000-3100

First examine the absorption bands in the vicinity of 4000-3000 cm–1

1. Alkanes – combination of C-C and C-H bonds• C-C stretches and bends 1360-1470 cm-1

• CH2-CH2 bond 1450-1470 cm-1

• CH2-CH3 bond 1360-1390 cm-1

• sp3 C-H between 2800-3000 cm-1

Infrared SpectroscopyOctane

CH

HH

(w – s) (m)

2. Alkenes – addition of the C=C and vinyl C-H bonds• C=C stretch at 1620-1680 cm-1 weaker as

substitution increases

• vinyl C-H stretch occurs at 3000-3100 cm-1

• The difference between alkane, alkene or alkyne C-H is important! If the band is slightly above 3000 it is vinyl sp2 C-H or alkynyl sp C-H if it is below it is alkyl sp3 C-H

1-OcteneInfrared Spectroscopy

CC

H

HH

(w – m)

(w – m)

3. Alkynes – addition of the C=C and vinyl C-H bonds• C≡C stretch 2100-2260 cm-1; strength depends on

asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes

• C-H for terminal alkynes occurs at 3200-3300 cm-1

• Internal alkynes ( R-C≡C-R ) would not have this band!

1-OctyneInfrared Spectroscopy

C CH

(m – s)

(w-m)

4. Aromatics • Due to the delocalization of e- in the ring, C-C bond

order is 1.5, the stretching frequency for these bonds is slightly lower in energy than normal C=C

• These show up as a pair of sharp bands, 1500 & 1600 cm-1, (lower frequency band is stronger)

• C-H bonds off the ring show up similar to vinyl C-H at 3000-3100 cm-1

Ethyl benzeneInfrared Spectroscopy

H

(w – m) (w – m)

4. Aromatics• If the region between 1667-2000 cm-1 (w) is free of interference (C=O stretching

frequency is in this region) a weak grouping of peaks is observed for aromatic systems

• Analysis of this region, called the overtone of bending region, can lead to a determination of the substitution pattern on the aromatic ring

Monosubstituted

1,2 disubstituted (ortho or o-)

1,2 disubstituted (meta or m-)

1,4 disubstituted (para or p-)

G

G

G

G

G

G

G

Infrared Spectroscopy

5. Unsaturated Systems – substitution patterns• The substitution of aromatics and alkenes can also be discerned through the out-

of-plane bending vibration region • However, other peaks often are apparent in this region. These peaks should only

be used for reinforcement of what is known or for hypothesizing as to the functional pattern.

RC

HC

RC

HCH2

RC

HC

RC

RCH2

RC

RC

R

H

R

H

R

H

985-997905-915

cm-1

960-980

665-730

885-895

790-840

R

R

R

R

R

RR

cm-1

730-770690-710

735-770

860-900750-810680-725

800-860

Infrared Spectroscopy

6. Ethers – addition of the C-O-C asymmetric band and vinyl C-H bonds• Show a strong band for the antisymmetric C-O-C

stretch at 1050-1150 cm-1

• Otherwise, dominated by the hydrocarbon component of the rest of the molecule

Diisopropyl etherInfrared Spectroscopy

O

(s)

7. Alcohols• Strong, broad O-H stretch from 3200-3400 cm-1

• Like ethers, C-O stretch from 1050-1260 cm-1

• Band position changes depending on the alcohols substitution: 1° 1075-1000; 2° 1075-1150; 3° 1100-1200; phenol 1180-1260

• The shape is due to the presence of hydrogen bonding

1-butanolInfrared Spectroscopy

OH

(m– s)br

(s)

8. Amines - Primary• Shows the –N-H stretch for NH2 as a doublet

between 3200-3500 cm-1 (symmetric and anti-symmetric modes)

• -NH2 has deformation band from 1590-1650 cm-1

• Additionally there is a “wag” band at 780-820 cm-1 that is not diagnostic

2-aminopentaneInfrared Spectroscopy

NH H

(w) (w)

9. Amines – Secondary• N-H band for R2N-H occurs at 3200-3500 cm-1

as a single sharp peak weaker than –O-H

• Tertiary amines (R3N) have no N-H bond and will not have a band in this region

pyrrolidineInfrared Spectroscopy

N H

(w – m)

10. Aldehydes • C=O (carbonyl) stretch from 1720-1740 cm-1

• Band is sensitive to conjugation, as are all carbonyls (upcoming slide)

• A highly unique sp2 C-H stretch appears as a doublet, 2720 & 2820 cm-1 called a “Fermi doublet”

Cyclohexyl carboxaldehydeInfrared Spectroscopy

CH

O

(w-m)

(s)

APPLICATIONS OF INFRA - RED SPECTROSCOPY1. Identifications of an Organic Compounds: Most Organic Compounds is conformed in Finger print region 2. Structure Determination: This technique helps to establish the structure of an unknown compounds.3. Qualitative analysis of functional groups: The Presence or absence of absorption bands help in predicting the presence of certain functional group in the compounds. Presence of Oxygen may be –OH, C=O, COOR, -COOH etc. But an absorption band between 3600-3200 cm-1

4. Distinction between two types of hydrogen bonding: To find the Inter Or Intra molecular H- Bonding.5. Quantitative analysis: It help to make a quantitative estimation of an organic mixture.For Example

Xylene commercial is mixture of Ortho, Meta, Para Compound. The separate of the mixture can not be easily done. But percentage composition of the mixture can be determine.6. Conformational Analysis: Chair or Boat Form 7. Geometrical Isomerism: Cis or Trans , Syn or Anti8. Study the Keto – enol tautomerism:

Disadvantages of IRSample Constraint:

Infrared spectroscopy is not applicable to the sample that contains water since this solvent strongly absorb IR light.Spectrum Complication:

The IR spectrum is very complicated and the interpretation depends on lots of experience. Sometimes, we cannot definitely clarify the structure of the compound just based on one single IR spectrum. Other spectroscopy methods, such as ( Mass Spectrometry) MS and ( Nuclear Magnetic Resonance) NMR, are still needed to further interpret the specific structure. Quantification:

Infrared spectroscopy works well for the qualitative analysis of a large variety of samples, but quantitative analysis may be limited under certain conditions such as very high and low concentrations.

Reference books1. William Kemp, Organic Spectroscopy – ELBS.2. Sharma.Y.R, Elementary Organic Spectroscopy, Principles and applications- S. Chand & Co.,3. Banwell, Fundamentals of Molecular spectroscopy Tata McGraw Hill.4.

E- Source Reference:1. SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

2. http://www.chemcenter/org http://3. www.sciencemag.org4. http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec/.htm5. http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html6. http://hiq.linde-gas.com/international/web/lg/spg/likelgspg.nsf/DocByAlias/anal_infra

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