infrared and uv-visible spectroscopy

8/18/2019 Infrared and UV-Visible Spectroscopy

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An Introduction to Infrared andAn Introduction to Infrared and

UV-Visible SpectroscopyUV-Visible Spectroscopy


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LEARNING OBJE!IVES

Describe the principal regions of the electromagnetic

spectrum.

Describe the principles of infrared spectroscopy.

Describe the principles of UV-Vis spectroscopy. Describe and explain the principal factors that govern the

vibrational frequencies of bonds.

Describe and explain the principal factors that govern the

electronic absorption process in UV-Vis spectroscopy. Experimental and instrumental


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!"E ELE!RO#AGNE!I

S$E!RU#


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%"A! IS S$E!ROSO$&'

Atoms and molecules interact with electromagneticradiation E!"# in a wide variety of ways.

Atoms and molecules may absorb and$or emit E!". Absorption of E!" stimulates different types of

motion in atoms and$or molecules. %he patterns of absorption wavelengths absorbed and

to what extent# and$or emission wavelengths emittedand their respective intensities# are called &spectra'.

%he field of spectroscopy is concerned with theinterpretation of spectra in terms of atomic andmolecular structure and environment#.


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IN(RARE) S$E!ROSO$&

(nfrared radiation stimulates molecular

vibrations.

(nfrared spectra are traditionally displayed as)% percent transmittance# versus wave

number *+++-*++ cm-,#.

Useful in identifying presence or absence of

functional groups.


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IN(RARE) S$E!ROSO$&

OC

O

CH3

C

O

HO

OC

O

CH3

C

O

HO

avenumber ,$ wavelengthavenumber ,$ wavelength


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IN(RARE) S$E!ROSO$&

(n the (" region of(n the (" region of

the electromagneticthe electromagnetic

spectrum/ thespectrum/ the

absorption ofabsorption of

radiation by a sampleradiation by a sample

is due to changes inis due to changes in

the vibrationalthe vibrational

energy states of aenergy states of a

molecule.molecule.


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IN(RARE) S$E!ROSO$&

!ethane!ethane

"oc0ing or in

plane bending

11 11 11

2

11

1 1

1 1

1

1 1

2

1 1

2

1

1

2

11

2

11

2

1

Asymmetrical

stretching

3ymmetricalstretching

4ending or scissoring

%wisting or out-

of-plane

bending

agging or out-

of-plane

bending


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IN(RARE) S$E!ROSO$&

Only *ibrations t+at cause a c+an,e in polarity. ,i*e rise

to bands in IR spectra / 0+ic+ of t+e *ibrations for O1

are infrared acti*e'

O C O

O C O

O C O O C O

O C O

O C O

O C O O C O

3ymmetric stretch

Asymmetric stretch

4ending doublydegenerate#


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IN(RARE) S$E!ROSO$&

hat is a vibration in a molecule5 Any change in shape of the molecule- stretching of bonds/

bending of bonds/ or internal rotation around single bonds

hat vibrations change the dipole moment of amolecule5

Asymmetrical stretching$bending and internal rotation changethe dipole moment of a molecule. Asymmetricalstretching$bending are (" active.

3ymmetrical stretching$bending does not. 6ot (" active


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IN(RARE) S$E!ROSO$&

1uman 4reath1uman 4reath

0

20

40

60

80

100

5001000150020002500300035004000

wavenumber/cm-1

% T

O

H HO

H H

O C O

O C O

O C O


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IN(RARE) S$E!ROSO$&

1ow much movement occurs in the vibration of a 2-

2 bond5

10 pm

154 pmstretching vibration 7or a 2-2 bond with a

bond length of ,8*

pm/ the variation is

about ,+ pm.

bending vibration

4o 10 pm

7or 2-2-2 bond anglea change of *o is

typical. %his moves a

carbon atom about ,+

pm.


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IN(RARE) S$E!ROSO$&

hat wavelength of electromagnetic radiation is

involved in causing vibrations in molecules5

(nfrared ("# electromagnetic radiation causes vibrations in

molecules wavelengths of 98++-,8/+++ nm or 9.8 : ,8 mm#

1ow does the mass influence the vibration5 %he greater the mass - the lower the wavenumber

H2

I2


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IN(RARE) S$E!ROSO$&

Ethane 2hloroethane


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IN(RARE) S$E!ROSO$&

POSITIONPOSITION RE!CE #$SSRE!CE #$SS

%ON STREN&TH%ON STREN&TH'STI((NESS)'STI((NESS)

*I&HT $TO#S HI&H*I&HT $TO#S HI&H(RE+!ENC,(RE+!ENC,

STRON& %ONSSTRON& %ONSHI&H (RE+!ENC,HI&H (RE+!ENC,

STREN&THSTREN&TH CH$N&E INCH$N&E IN

PO*$RIT,.PO*$RIT,.

STRON&*, PO*$RSTRON&*, PO*$R%ONS &I/E%ONS &I/E

INTENSE %$NSINTENSE %$NS

ITHITH H,RO&EN %ONIN&H,RO&EN %ONIN& STRON& H,RO&ENSTRON& H,RO&EN

%ONIN& &I/ES%ONIN& &I/ES%RO$ %$NS%RO$ %$NS


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IN(RARE) S$E!ROSO$&

(n general(n general

BondBond

→→

C-HC-H C-DC-D C-OC-O C-ClC-Cl

ν ν/cm/cm-1-1

→→

30003000 22002200 11001100 700700

BondBond→→

CC≡≡OO C=OC=O C-OC-O

ν ν/cm/cm-1-1

→→

21432143 17151715 11001100


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IN(RARE) S$E!ROSO$&

4000-30004000-3000

cmcm-1-13000-20003000-2000

cmcm-1-12000-15002000-1500

cmcm-1-11500-10001500-1000

cmcm-1-1

O-HO-HN-HN-H

C-HC-H

CC CC

CC NN

CCCCCOCO

C-OC-OC-(C-(

C-CC-C

deformationsdeformations

(ncreasing energy

(ncreasing frequency


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IN!ER$RE!A!ION O( IN(RARE) S$E!RA An element of ;udgement is required in interpreting (" spectra but you

should find that it becomes relatively straightforward with practice. (t is often possible to assign the pea0s in the ,-1 and 6-1 stretching vibrations. %oo many bonds absorb in the region of


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IN!ER$RE!A!ION O( IN(RARE) S$E!RA

Ethanoic acid


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Infrared Instru2entation

3ample

compartment(" 3ourceDetector


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All modern instruments are 7ourier %ransformAll modern instruments are 7ourier %ransform

instruments.instruments.

(n all transmission experiments radiation from a(n all transmission experiments radiation from a

source is directed through the sample to a detector.source is directed through the sample to a detector. %he measurement of the type and amount of light%he measurement of the type and amount of light

transmitted by the sample gives information about thetransmitted by the sample gives information about the

structure of the molecules comprising the sample.structure of the molecules comprising the sample.


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Infrared E3peri2ental

!o obtain an IR spectru24 t+e sa2ple 2ust be!o obtain an IR spectru24 t+e sa2ple 2ust be

placed in a 5container6 or cell t+at is transparentplaced in a 5container6 or cell t+at is transparent

in t+e IR re,ion of t+e spectru27in t+e IR re,ion of t+e spectru27

Sodiu2 c+loride or salt plates are a co22onSodiu2 c+loride or salt plates are a co22on2eans of placin, t+e sa2ple in t+e li,+t bea2 of2eans of placin, t+e sa2ple in t+e li,+t bea2 of

t+e instru2ent7t+e instru2ent7


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InfraredE3peri2ental

IR transparent Salt $latesIR transparent Salt $lates


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%hese plates are made%hese plates are made

of salt and must beof salt and must be

stored in astored in a 0ater free0ater free

environment such as theenvironment such as thedessicator dessicator shown here.shown here.


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%he plates must also be%he plates must also be

handled with gloves tohandled with gloves to

avoid contact of theavoid contact of the

plate with moisture plate with moisturefrom one's hands.from one's hands.


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%o run an (" spectrum of a%o run an (" spectrum of a

liquid sample/ a drop or twoliquid sample/ a drop or two

of the liquid sample isof the liquid sample is

applied to a salt plate.applied to a salt plate.

A second salt plate is placedA second salt plate is placed

on top of the first one suchon top of the first one such

that the liquid forms a thinthat the liquid forms a thin

film BsandwichedC betweenfilm BsandwichedC between

the two plates.the two plates.


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%he cell holder is then%he cell holder is then

placed in the beam of the placed in the beam of the

instrument.instrument.

%he sample is then scanned%he sample is then scanned

by the instrument utiliing by the instrument utiliing

predestinated parameters. predestinated parameters.

A satisfactory spectrum hasA satisfactory spectrum has

well defined pea0s-but notwell defined pea0s-but not

so intense as to causeso intense as to causeflattening on the bottom offlattening on the bottom of

the pea0s.the pea0s.


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%enc ac6


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%he salt plates are cleaned%he salt plates are cleaned by rinsing into a waste by rinsing into a wastecontainer with a suitablecontainer with a suitableorganic solvent-commonlyorganic solvent-commonly

cyclohexane -cyclohexane - 6EVE" 6EVE"A%E"A%E"

2loudy plates must be2loudy plates must be polished to return them to a polished to return them to atransparent condition.transparent condition.

%o polish cloudy windows/%o polish cloudy windows/rotate salt plate on polishingrotate salt plate on polishingcloth.cloth.


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UV-Visible Spectroscopy

Ultraviolet radiation stimulates molecular

vibrations and electronic transitions.

Absorption spectroscopy from ,


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CO

O

N C

H3C

O

H3CO

COO

N C

H3C

O

H3CO

ocaine


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Electronic transitions occur when the moleculeElectronic transitions occur when the molecule

absorbs energyabsorbs energy

Electronic !ransitions inElectronic !ransitions in

(or2alde+yde(or2alde+yde

Eec7rnc 7ran87n89 :; 8; an6 n eec7rn8 6 an6 < eec7rn8 C=ar>e 7ran8


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Electronic transitionsElectronic transitions !olecular >rbital %heory!olecular >rbital %heory


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d-d %ransitionsd-d %ransitions =d and *d ,st and 9nd=d and *d ,st and 9nd

transitions seriestransitions series

Fartially occupied dFartially occupied dorbitalsorbitals %ransitions from lower%ransitions from lower

to higher energy levelsto higher energy levels


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2harge %ransfer 2harge %ransfer

Electron donor and acceptor characteristicsElectron donor and acceptor characteristics Absorption involves e- transitions from donor to acceptor Absorption involves e- transitions from donor to acceptor

326326-- to 7e(((#to 7e(((# 7e((# and neutral 3267e((# and neutral 326

!etal is acceptor!etal is acceptor

"educed metals can be exception"educed metals can be exception


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!"E BEER-

LA#BER! LA%

7or a light absorbing

medium/ the light

intensity falls

exponentially with

sample depth.

7or a light absorbing

medium/ the light

intensity falls

exponentially with

increasing sample

concentration.

100xI

IT

I

IT

o

t

o

t

==

I I7

l

cuve77e 2 5 >

= 7 5 n 7 e n 8 5 7 ? ' I )

Sam:e 6e:7=

I

I7

l


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A b s o r b a n

c e

Cncen7ra7n

T Acl A ,+log−== λ ε T Acl A ,+log−== λ ε

T!e ne#ati$e lo#arit!m of T is calledt!e a%sor%ance & A' and t!is isdirectl( )ro)ortional to sam)le de)t!&called )at!len#t!* l ' and sam)leconcentration &c '+ T!e e,ation iscalled t!e Beer-.am%ert la+

ε is called t!e molara%sor)tion coefficient

and !as nits of dm3

mol-1 cm-1


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Beer-La2bert La0

li2itations Folychromatic GightFolychromatic Gight

Equilibrium shiftEquilibrium shift 3olvent3olvent

p1 p1


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UV-Visible Instru2entation

3everal types of spectrometer 3everal types of spectrometer


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UV-Visible Instru2entation

Gight sourceGight source Deuterium and hydrogen lampsDeuterium and hydrogen lamps

filament lamp filament lamp

He arc lampsHe arc lamps 3ample containers3ample containers

2uvettes2uvettes

FlasticFlastic

IlassIlass

JuartJuart


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Open-topped rectangularstandard cell (a) andapertured cell (b) for limitedsample volume

.CO.CO


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Ac0nowledgementsAc0nowledgements 4ette Kreu/ "uth Dusenbery at Department of 6atural4ette Kreu/ "uth Dusenbery at Department of 6atural

3ciences U!-Dearborn.3ciences U!-Dearborn.

Dr David L !cIarvey at Keele University 1ewlett Fac0ard

Andrew Lac0son at 3taffordshire University

infrared and uv-visible spectroscopy

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