infrared spectroscopy lokanathan arcot department of forest products technology school of chemical...

InfraredSpectroscopy

Lokanathan ArcotDepartment of Forest Products TechnologySchool of Chemical TechnologyAalto University

Dr. Lokanathan Arcot

2

Basis of Infrared Spectroscopy

Atoms Molecules

Bond(e– density transfer)

Dipole Moment𝛿0 𝛿0 𝛿+ 𝛿– + – Non-Polar

’0’Dipole

moment

Highly Polar’High’Dipole

moment


3

Vibrational Spectroscopy: Infrared

AsymmetricStretching

SymmetricStretching

In-plane Scissoring

Examples of Molecular Vibrations

Effect of Vibrations- Monopoles of dipole vibrate at a Freq.- Oscillating elec. Field of same Freq.

Others

Rocking

Wagging

Twisting

Absorption of Light of wavelength λ or FrequencyAbsorption occurs if the incident light wave has the same

frequency as oscillating electric field of a molecule vibrating in a ’non-zero’ dipole moment mode


4

Vibrational Properties of Bonds

k

2

1

21

111

mm

En is the energy of the nth vibrational leveln is an integerh is Planck’s constant is the frequency of the vibrationk is the force constant of the bondµ is the reduced massm1 and m2 the mass of the vibrating atoms 1&2

Frequency of Vibration

Reduced Mass

hnEn )2

1(

Energy of Vibration

Vibrational frequencies of a bond between two atoms (1 and 2)


5

Vibrational Properties of Bonds

k

2

1

21

111

mm

Frequency of Vibration Reduced Mass

hnEn )2

1(

Energy of Vibration

Vibrational frequencies of a bond between two atoms (1 and 2)

Main implications:

® Vibrational frequencies increase () with increasing bond strength (k)

® Vibrational frequencies increase with decreasing mass of the vibrating atoms


6

How a IR spectrum is recorded

Source of LightA range of λ

IntensityIO

SampleMoleculesAbsorption

TransmittedI Detector

IO - I

Course 3130, Dr. Lokanathan Arcot

7

55

60

65

70

75

80

85

90

95

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

0,20

0,22

0,24

0,26

0,28

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

IR spectrum of clay

• spectrum is plotted as a function of either absorbance or transmittance

Abs

orb

ance

% T

rans

mitt

ance

100 %

0

What an IR spectrum looks like

0I

IT

I =Intensity measured with a sample in the beam

)/1(log10 TA

Io= Intensity measured with no sample in the beam


8

A typical IR Spectrum

Unit Wavenumber (cm –1) instead of nm or Hz

Example: 2-pentanone


9

Why use Wavenumber (cm –1) instead of m or Hz

The electromagnetic spectrum

10-11(m)

Wave length increases 103(m)

-raysX-rays Ultra-violet

Visible Infrared Micro wave

Radiowave

Near Mid Far

0,8 µm 100 µm

10-11(m)

Wave length increases 103(m)

-raysX-rays Ultra-violet

Visible Infrared Micro wave

Radiowave

Near Mid Far

0,8 µm 100 µm2.5–25µm

Mid IR region is the most useful region for spectroscopyMicrons – 2.5µm to 25µmHertz – 120 THz to 12 THzWavenumber – 4000 cm –1 to 400 cm –1

c = λ* ∝ 1/ λ Example: 2.5µm = 2.5*10-4cmWavenumber = 1/Wavelength (cm)For 2.5µm we get 4000 cm –1

WhereC – velocity, λ – wavelength, – Frequency of light


10

• IR radiation induces vibrations in molecules and/or functional groups

• each vibration by a functional group is induced at a distinct wavelength (or wavenumber 1/)

Example: CH2 -group

Asymmetricalstretching(as CH2)

~ 2926 cm-1

Symmetricalstretching(s CH2)

~ 2853 cm-1

What causes the absorption?

Asymmetrical stretching

(as CH2) ~ 2926 cm –1

Symmetrical stretching

(s CH2) ~ 2853 cm –1


11

What causes the absorption?Other fundamental vibrations in CH2 group, induced by IR radiation:

In-plane bending or scissoring

(δs CH2) ~ 1465 cm –1

Out-of-plane bending or wagging

(ωs CH2) ~ 2926 cm –1

Out-of-plane bending or twisting

(t CH2) ~ 1350-1150 cm –1

in-plane bending or rocking(r CH2)

~ 720 cm –1


12


4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

Tra

nsm

ittan

ce [

%]

Wavenumber [cm-1]

2926

2853

1450

as CH2

s CH2

, CH2

s CH2

Example:IR spectrum of cyclohexane(contains only CH2 groups)

Fundamental vibrations inCH2 induced by IR radiation

Asymmetrical stretching(as CH2)

~ 2926 cm –1

Symmetrical stretching(s CH2)

~ 2853 cm –1


13


4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

Tra

nsm

ittan

ce [

%]

Wavenumber [cm-1]

H2O

1596

3756

hydrogenbonding

Example 2: Water

Symmetricalstretching (s OH)

3652 cm-1

Asymmetricalstretching (as OH)

3756 cm-1

Scissoring(s OH)

1596 cm-1

IRinactive

Fundamental vibrations

• symmetrical stretching at 3652 cm-1 has no change in dipole moment® IR requires the vibration be such that it changes the dipole moment

• hydrogen bonding shifts the absorption to lower wavenumbers


14


4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

Tra

nsm

ittan

ce [

%]

Wavenumber [cm-1]

H2O

1596

3756

hydrogenbonding

Example 2: Water

Symmetricalstretching (s OH)

3652 cm-1

Asymmetricalstretching (as OH)

3756 cm-1

Scissoring(s OH)

1596 cm-1

IRinactive

Fundamental vibrations

• although water has only 2 IR bands in IR spectrum, they are very broad water usually disturbs the IR spectrum (samples are measured without water)

s – symmetricas - asymmetric

r – scissoringw - wagging


15

IR active vibrations


16

Carbon Dioxide

Bending

Asymmetric Stretching

Symmetric Stretching

Dipole moment change

Dipole moment change

No Dipole moment change


17

Carbon Dioxide – IR Spectrum


18

CHARACTERISTIC GROUPABSORPTIONS

OFORGANIC MOLECULES


19

Example: dodecane

CH3(CH2)10CH3

C-H stretch:as CH3: 2962 cm-1 s CH3: 2872 cm-1 as CH2: 2924 cm-1 s CH2: 2853 cm-1

C-H bend:s CH2: 1467 cm-1 asCH3: 1450 cm-1 s CH3: 1378 cm-1

CH2 rock: CH2: 721 cm-1

s – symmetricas - asymmetric

r – scissoringw - wagging


20

Example: tert-butyl alcohol

O-H stretch

C-H stretchC-H bend

1040 cm-1 C-O stretch

Neat sample


21

Example: phenol

O-H stretch

3008 cm-1 AromaticC-H stretch

Overtone orcombinationbands

C=C ringstretch

1360 cm-1

In-planeO-H bend

1224 cm-1

C-O stretch810 cm-1

752 cm-1

Out-of-planeC-H bend

690 cm-1

Out-of-plane C=C bend

Neat sample

Overtone – multiple of given frequency

Example: 2-pentanone

C-H stretch

1717 cm-1 C=O stretchfor ketones

1171 cm-1 C-CO-C stretch and bend

C-Hbend

1366 cm-1 s CH3 ofCH3O unit

C-H stretch

3300-2500 cm-1

Broad O-H stretch

Example: hexanoic acid

1711 cm-1

C=O stretch forcarboxylic acids

939 cm-1

O-H out-of-plane bend

1285 cm-1

C-O stretchwith C-O-H interaction

1413 cm-1

C-O-H in-plane bend

Neat sample

Example: octylamine

C-H stretch

1617 cm-1 N-H bend(scissoring) 1467 cm-1

CH2 of(scissoring)

1073 cm-1 C-N stretch

~780 cm-1 N-H wag

Diluted sample

Note: effect of hydrogen bondingIR spectra of alcohols, carboxylic acids, amines etc. are severelyaffected by their surrounding medium during the measurement.

In gas phase ordiluted in a solvent

Neat sample Broad O-H stretch

Narrow O-H stretch


26

Absorption regions of some organic functional groups

O-H, N-H

C-H (unsaturated)

C-H (aliphatic)

X=Y, X=Y=Z stretch

C=O

C=C (olefinic)

C=C (aromatic)

4000-3200

3000 2800 2500-2000 1800 1600 1400 cm-1


27

Example - Cellulose

O-Hstretch

C-Hstretch

water

O-H C-H

bending

frompyranose

ring structures

O*O

OHOH

OH

*n

• although a relatively simple molecule, cellulose is more complex than cyclohexane or water® IR spectrum of cellulose comprises of ~ 60 different bands® qualitative analysis based only on IR is difficult® often IR is used as complementary technique (especially with NMR)


28

Example - Cellulose

O-Hstretch

C-Hstretch

water

O-H C-H

bending

frompyranose

ring structures

O*O

OHOH

OH

*n

• IR is reliable with pure compounds when spectral libraries are used

• IR is also a handy tool for quick detection of certain functional groups


29

Example – Modified Cellulosecellulose vs. trimethylsilyl cellulose (TMSC)

O-Hstretch

C-Hstretch

water

O-H C-H

bending

frompyranose

ring structures

O*O

OHOH

OH

*n

O*O

(CH3)3SiOOSi(CH3)3

OSi(CH3)3

*n

O-H

C-H (fromCH and CH3)

pyranosering

Si-C


30

Example of Nanoparticle Characterization using IR Spectroscopy

How do we follow this reaction ?

STEP 1: Look at all the bonds in reactantsSTEP 2: Reactant specific bonds

Difference between Cellulose (CNC) and cationic molecule (C18)CH2 groups – 1 in each glucose molecule

16 in each cationic moleculeC-N group – only in cationic

Cationic

From Thesis: http://www.diva-portal.org/smash/get/diva2:506963/FULLTEXT02


31

How do we follow a reaction ?

STEP 1: Look at all the bonds in reactants

STEP 2: Reactant specific bonds (CH2 , C-N)

STEP 3: Check if the bonds are IR or Raman active

Raman and IR comparisonhttp://www.horiba.com/fileadmin/uploads/Scientific/Documents/Raman/bands.pdf

IR bandshttp://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html

We need a IR/Raman database


32

Example of Nanoparticle Characterization using IR Spectroscopy

SymCH2 assymCH2

C=O


33

Example of Polymer Characterization using IR Spectroscopy

Alternating Co-polymer

Random Co-polymer

Ethylene

Propene

What is the difference between the two polymers?


34

Polyethylene-propylene co-polymer

Ethylene(C 2)

Propene(C 3)

+

Random Copolymer


35

IR spectroscopy of mixture PE, PP

X- axis on top is in wavenumber units and blow it is microns

PE- Polyethylene, PP- Polypropylene

85% PE15% PP

55% PE45% PP

PP


36

Pyrolysis IR spectroscopy of mixture PE, PP

PE

PP

450 °C

PyrolysisVinyl groups

450 °C

Pyrolysis

Vinylidene groups

Out of plane C-H olefinic (C=C) bending

909 cm-1

889 cm-1

PE- Polyethylene, PP- Polypropylene


37

Note: Polyethylene (PE) - Vinyl group – 909 cm-1

Polypropylene (C3) Vinylidene – 889 cm-1

Increasing PP %


Different IR spectra after pyrolysis of

mixtures of PP and PE


38

Note: Polyethylene (PE) - Vinyl group – 909 cm-1

Polypropylene (C3) Vinylidene – 889 cm-1

Increasing PP %

Peak ratio at 909cm-1 and 889 cm-1 gives a quantitative estimate of relative amount of Propylene-Ethylene copolymer ratio



39

Summary of Part I• Basics of IR spectroscopy

Conditions for IR absorptionFrequency of vibration – bond characteristics

• A typical IR spectrum• Vibrational modes

IR active- inactive (Water and Carbon dioxide)• Simple examples – Alkane, OH, COOH, NH, H-bond• General molecular bond-IR absorption bands• Cellulose and silylated cellulose• Example I - Nanoparticle

Cellulose Nanocrystal• Example II – Polymer – Pyrolysis IR spectroscopy

Polyethylene-porpylene copolymer


40

Short Break


41

INSTRUMENTATION


42

reducedenergy atdistinct

Radiationsource

Spectralapparatus

IRbeam

E=h

SAMPLEenergy

absorbedat distinct

c

IRbeam

Detector

Computer

absorbanceof energy

plotted as afunction of 1/

Radiation source:• Globar or • Nernst rod

• thermal radiators• provide high intensity in IR region

IR instrument


43


Radiationsource

Spectralapparatus

IRbeam

E=h

SAMPLEenergy

absorbedat distinct

c

IRbeam

Detector

Computer

absorbanceof energy


IR instrument

Detectors

Thermal detectors:• based on the changes in material upon thermal energy of radiation

Photoelectric detectors:• based on changes in electrical conductivity caused by radiation in semiconductors


44


Radiationsource

Spectralapparatus

IRbeam

E=h

SAMPLEenergy

absorbedat distinct

c

IRbeam

Detector

Computer

absorbanceof energy


Spectra ApparatusDispersive Vs Fourier Transform

Dispersive : One wavelength of IR radiation at a time

FTIR : Simultaneously measuring several wavelengths


45

FT-IR instrument• all modern commercial IR instruments are Fourier Transform Infrared (FT-IR) Spectroscopes

• two beams: one fixed length, the other variable length

• the beam length is varied by moving mirror

• occasionally the difference in wavelengths hits an integer constructive interference

• occasionally, the difference in wavelengths hits an odd integer of one quarter of the wavelength destructive interference


46

FT-IR instrument

Result:INTERFEROGRAM

Fouriertransformation

IR SPECTRUM


47

FT-IR instrument

• FT-IR allows the detection of all the wavenumbers simultaneously

• before FT-IR, each wavenumber had to be measured separately measuring one spectrum took several hours, maybe days

• introduction of FT-IR in 1960s revolutionised the IR technique

• advances in computer technology in 1980s (PC) made FT-IR a common instrument in all chemical laboratories


48

Types of Samples (Physical)

SOLIDS

LIQUIDSPowder

Solid SurfaceRough Smooth


49

Sample PreparationRemember: Water interferes with IR spectroscopy

Samples should be dry

Compare this with Sample Preparation for Raman Spectroscopy

I. Dispersion of sample inside a IR transparent matrixPelletMullNeat

II. Direct measurement (no sample preparation required)Attenuation Total ReflectanceDiffuse ReflectancePhotoacoustic Spectroscopy


50

Dispersion of sample inside a IR transparent matrix

Pellet - for powdersAlkali Halides become IR transparent pellets upon applying high pressure

Example: KBr

Mix KBr with sample

Load it up in the pressing machine

5000-10000 psi

The formed Pellet can be used for IR absorption

spectroscopy

Absorbance measurement


51

Sample between two IR transparent plates

Mull - for powders: Sample is mixed with a liquid and placed in-between two NaCl plates for IR absorbance meaurement. (Why not KBr? )

Nujol is brand of mineral oil most commonly used to make mull ( Nujol Mull)The oil or liquid used must be transparent in the IR region of interest, non-volatile

Mix sample with liquid

(mineral oil)

Spread it over one of the pair of NaCl plates

Make a sandwitch of sample between two plates


52

Sample inbetween two IR transparent plates

Neat - for liquid sample: Liquid sample is placed in-between two NaCl plates for IR absorbance measurement.

Place liquid on one of the pair of

NaCl plates

Make a sandwitch of sample between two

plates

Absorbance measurement

IR Transmittance after Sample preparation

sample mixedwith KBr

Io

Detector

Disadvantages:• pressing KBr pellets is laborius• KBr is hygroscopic water interferes the analysis

IKBr Pellet /NaCl plates

What about IR spectroscopy without sample preparation?

Direct measurement (no sample preparation required)Attenuation Total ReflectanceDiffuse ReflectancePhotoacoustic Spectroscopy


54

Attenuated total reflectance (ATR-IR)

What is Total Internal Reflection ?


55


Total Internal Reflection creates an Evanescent Wave

Upon internal reflection the electric and magnetic field of incident light partially propogate into the upper lower refractive index medium


56


Total Internal Reflection creates an Evanescent Wave

The intensity of field decays exponentially as a function of distance Evanescent means vanishing

Z – distance from surfaceI – intensity of fieldd – arbitrary distance


57


InternalReflectionElement (IRE)

IR beam detector

n2

n1

evanescent wave

~1m

sample

sample

IRE

• a beam of radiation is reflected on the interface of two materials with different refractive indices (n2n1) an evanescent wave appears in the material with lower refractive index (n2)

Evanescent wave penetratation depth:

221

2

1

sin2 nd p

l is the wavelength of IR radiationq is the incident angle of IR radiationn21 is the ratio of refractive indices (n2/n1)


58


IR beam detector

n2

n1

evanescent wave

~1m

sample

sample

IRE

• penetration depth usually in the order of 1-5 µm

• evanescent wave is absorbed selectively by the sample IR spectrum of the surface of the sample ATR-IR

• ATR crystal (internal reflection element) is very small easy to select a location on the sample mapping

2.5 mm

2.5 mm

Mapped area 10 x 10 mm2

Analysis depth 1-2 µm

2.5 mm

2.5 mm

Mapped area 10 x 10 mm2

Analysis depth 1-2 µm



59


IR beam detector

n2

n1

evanescent wave

~1m

sample

sample

IRE

Advantages:• minimal sample preparation• fast analysis• selecting locations enable mapping

Disadvantages:• problems in reproducibility: the contact between the sample and ATR-IR crystal (internal reflection element) is not always reproducible

Sample typesPowderSolids (Pulp, paper)



60

Sample viewing with visible light

surface spectra with ATR-objective

Photo and instrumentation: VTT

IR microscope

Photo and instrumentation: VTT

IR microscope

• IR microscope enables analysis of visually intriguing spots on the sample

• for instance, a dirt speckle on paper can be selected by the microscope and subjected to ATR-IR analysis


62

SAMPLEDETECTOR

inert gas

IR-radiation

microphone

• absorption of IR radiation generates heat in the sample heat waves reach the sample surface heat is released to the inert gas above the sample pressure changes in the gas pressure changes are detected with a sensitive microphone

Photoacoustic detection or photoacoustic spectroscopy (PAS)

Sample typesPowderSolids (Pulp, paper)


63

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)

Specular reflection Diffuse reflection

When incident light penetrating a surface is scattered in all directions,the phenomenon is called diffuse reflectance.

Applicable to: - powders - rough surfaces


64


Transmission-reflectance:• When light hits a particle, it can pass through or reflect• When passing through, the particle absorbs IR radiation• Transmission-reflectance event can occur many times• Finally, the outcoming IR beam is collected by a spherical mirror


65


Three main ways to prepare a sample for DRIFTS measurement:

(1) Powderize the sample

(2) Scratch the sample surface and collect the detached pieces on an abrasive paper

(3) Disperse particles (like colloids) in a volatile solvent and allow the solvent to evaporate by placing a few drops on a substrate rough surface


66

Reflection absorption infrared spectroscopy (RAIRS)

• IR beam is projected on a reflective surface (substrate) which supports an ultrathin film at grazing (very small) angle• Only those vibrations, which are perpendicular (P-polarized) to the surface, are IR active and give rise to an observable absorption band


67

Reflection absorption infrared spectroscopy (RAIRS)

• Film roughness and thickness affect the spectrum• Measurements require an ultrahigh vacuum (UHV)• Restricted to ultrathin films on solid supports which reflect IR light


68

Alkane thiol

30°10°

CH2-stretching2855 cm-1 sym 2925 cm-1 asym

(HS)

Example – Alkane thiol self assembly on Au and Silver surface Reflection absorption infrared spectroscopy (RAIRS)

Au Silver

J. Phys. Chem. B, 1998, 102 (2), 426-436

Good handbooks on IR spectroscopy

Silvestein, Bassler, Morrill Spectrometric identification of organic compounds, Wiley

Williams, Fleming Spectroscopic methods in organic chemistry, McGraw-Hill

Koenig Spectroscopy of polymers, Elsevier

(Several editions available from all titles)


70

Summary of Part II• Instrumentation

IR sources, Detectors• Dispersive Vs FITR• Sample preparation

KBr PelletMull NujolNeat

• Direct MeasurementAttenuated Total Internal ReflectionPhotoacoustic SpectroscopyDiffuse ReflectanceReflection Absorption


71

Have a nice weekend

Next week

Monday – Surface Plasmon Resonance

Wednesday – Quarz Crystal Microbalance

Friday – Atomic Force Microscopy

infrared spectroscopy lokanathan arcot department of forest products technology school of chemical...

Documents

frequency of light slide

beam slide

pentanone slide

vibrational spectroscopy

frequency absorption

twisting ch

rocking ch

ir radiation