Spectroscopy Ch201

8/7/2019 Spectroscopy Ch201

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Sp e c t r o s c o py


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I n t r o du c t i o n

It is the study of interaction .between matter and radiated energyLight interacting with matter as an

.analytical tool


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D if f e r e n t S p e c t r o sc o p i e s -UV visible :electronic states

/ -of valence e d orbital

transitions for solvatedtransition metals Fluorescence :emission of

/UV visible by certain

molecules -FT IR :vibrational transitions

of molecules

-FT NMR :nuclear spintransitions


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Ty p e s o f m a t e r i a l Spectroscopic

studies are

designed sothat theradiated

energyinteracts

withspecific

types of


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A t om s

Atomic absorption( )spectroscopy AAS involve

visible and ultravioletspectroscopies.

Atoms of different elementshave distinct spectra and

therefore atomicspectroscopy allows for the

identification and'


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Molecules

The combination ofatoms into

molecules leads

to the creationof unique typesof energetic

states andtherefore uniquespectra of the

transitions

between these.states Molecular


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Nuclei

Nuclei also havedistinct energy

states that are

widelyseparated and

lead to gamma

.ray spectraDistinctnuclear spin

states can have

their energyse arated b a


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- :X raycore electron

excitation

:UVvalanc

eelectr

onicexcita

tion

:IR

molecularvibrations

:Radio waves

Nuclear spin state(in a magnetic fie

E l e c t r on i c Exc i t a t i o n by UV/V i s Sp e c t r o s c opy :


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Spectroscopic Techniques andChemistry they Probe

UV-vis -UV visregion

Bondingelectrons

Atomic Absorption -UV visregion

AtomictransitionsFT-IR /IR Microw

ave

,Vibrations

rotationsRaman /IR UV VibrationsFT-NMR Radio

wavesNuclear spin

statesX-Ray Spectroscopy -X rays ,Inner electrons

elementalX-ray Crystallography -X rays -3 Dstructure


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S t i T h i d C U


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Spectroscopic Techniques and Common Uses

UV-vis -UV visregion

Quantitative

/ analysis Beer sLaw

Atomic Absorption -UV visregion QuantitativeanalysisBeer s Law

FT-IR /IR Microwave

Functional GroupAnalysisRaman /IR UV Functional Group

/Analysis quant

FT-NMR Radiowaves

StructuredeterminationX-Ray Spectroscopy -X rays Elemental

AnalysisX-ray Crystallography -X rays -3 D structure

Anaylysis


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UV and V i s ib l e S p e c t r o s c o p y

absorption spectroscopyor

reflectancespectroscopy in the

ultraviolet-visiblespectral

region
http://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Absorption_spectroscopyhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Visible_spectrumhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Absorption_spectroscopy


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The Sp e c t r o s co p i c P r o c e s s.1 ,In UV spectroscopy the sample is

irradiated with the broad spectrumof the UV radiation

.2 If a particular electronic transitionmatches the energy of a certain band,of UV it will be absorbed

.3 The remaining UV light passes throughthe sample and is observed

.4 From this residual radiation a spectrum is obtained with gaps at

these discrete energies this iscalled an absorption spectrum

5.6.


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Th e UV A b s o rp t i o np r o c e s s * * :and transitions -high

,energy accessible in vacuum UV ( max< ).150 nm Not usually observed- .in molecular UV Vis

* * :n and transitions

- ( ),non bonding electrons lone pairs(wavelength max) -in the 150 250 nm.region

* * :n and transitions

most common transitions observed in

- ,organic molecular UV Vis observed incompounds with lone pairs and

multiple bonds with max = -200 600.nm

Any of these require that incoming


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N a tu r e o ft h e s ea b s o r p t i o n s

: *Example transitions responsible for ~ethylene UV absorption at 170 nm calculated with

- -ZINDO semi empirical excited states methods(Gaussian 03 ):

HOMOu bonding molecular orbital LUMOg antibonding molecular orbital

h 170nm photon

Examplefor a

simple

enone

n

n

*

n**

**

** *

*

- *; max =218 =11,000 n- *; max =320

=100


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Th e Q uan t i t a t i v e P i c tu r eT r a n s m i t t a n c e :

= /T P P0

B(path through sample)

P0(power in)

P(power out)

:b s o r b a n c e= -A log 10 =T log 10P0/P

- ( . . . ): The Beer Lambert Law a k a Beer s Law

=A ebc , =Where the absorbance A has no units since Alog10P0 /P

eis the molar absorbtivity with units of L mol-1cm-1

b is the path length of the sample in cm


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BEERS LAWThe

absorption of;l ight by a

substance of

particularwavelength isproportional

to the numberof moleculesin the path

.of light

cuvette

sourceslit

detector


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Chromophores and AuxochromesChromophores and Auxochromes

:Chromophres- * *ted groups responsible for and n electron

. . . = , = , = = ( - ) ns e g C C C O N N and N O 200nm 800nm

) &3 Bathochromic Hypsochromic shift

It is the shift of maxto a longer wavelengthdue to substitution with certain functional( . . groups e g OH and NH 2), when two or more

,chromophores are present in conjugation change( ).in pH and effect of the medium solvent

( )ochromic shift or blue shiftIt is the shift of maxto a shorter

wavelength due to removal

- ( )Bathochromic shift or red hift

Are saturated groups posses unshared

,electrons and does

not absorb in near UV or visible radiations. . ,e g OH NH 2.

Auxochromes


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A u x o c h r o m e s when attached to chomophoric

,molecule increase both its

wave length and intensity of

.absorption maximumBecause auxochrome inters into resonance,interaction with the chromophore thus,increase the extent of conjugation shift

the absorption maximum to longer wave

e r c h r o m i c e f f e c tan increase in the intensity of

absorption usually due tointroduction of an auxochromep o c h r o m i c e f f e c tves a decrease in the intensity of absorptio


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Ef f e c t o f pH( - )The spectra of compounds containing acidic phenolic OH

or(-basic NH 2)groups are dependent on the pH of the

.medium .The U V spectrum of phenol in,acid medium benzenoid form

while in alkaline medium is the, phenate anion quinonoid form

The free pair of of electronsof O2increasing the

-elocalization of the ,electrons leading to the

.formation of conjugated system

,So electrons become more

energetic and need less energy,to be excited therefore absorb

;longer bathochromic shift

)red shift with hyperchromiceffect

Phenol


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App l i c a t i o n s /UV Vis spectroscopy is routinely

used in analytical chemistryforthe quantitativedetermination of

,different analytes such astransition metal ,ions highly

conjugatedorganic compounds, and.biological macromolecules

/UV Vis spectroscopy can be used to

determine the concentration of.the absorber in a solution The wavelengths of absorption peaks

can be correlated with the types

of bonds in a given molecule and
http://en.wikipedia.org/wiki/Analytical_chemistryhttp://en.wikipedia.org/wiki/Analytical_chemistryhttp://en.wikipedia.org/wiki/Quantitative_analysishttp://en.wikipedia.org/wiki/Transition_metalhttp://en.wikipedia.org/wiki/Transition_metalhttp://en.wikipedia.org/wiki/Conjugated_systemhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Conjugated_systemhttp://en.wikipedia.org/wiki/Transition_metalhttp://en.wikipedia.org/wiki/Quantitative_analysishttp://en.wikipedia.org/wiki/Analytical_chemistry


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Infrared Spectroscopy

n f r a r e ds p e c t r o s c o p y is thespectroscopythat deals

with theinfrared

region of theelectromagnetic spectrum

, that is

light with alon er
http://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Spectroscopy


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R E G I O N A V EL E N G T H( )mA V EN U M B E R(c m -1 )

R E Q U E N C YR A N G EH zN E A R .7 8- .5 -2 8 0 04 0 0 0 .8 x 1 014 -.2 x 1 0 14M I D D L E . -55 0 -0 0 0 2 0 0 .2 x 1 0 4- 6 x 1 12F A R -0

1 0 0 0-0 0 1 0 6 x 1 0 12 -

3 0 x 1 0 11O S TU S E D . -5 1 5 -0 0 0 6 7 0 .2 x 1 0 14 -2 x 1 0 13

I R -REG ION : 1 2 , 8 0 0 - 1 0 cm -1


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BA S IC PR INC I PLE

IR involves absorption phenomenon theabsorption of radiation depends onincreasing energy of vibration or rotationassociated with covalent bond in molecule

provide that such an increasing in energycauses a change in the dipole moment of

.molecule Hence in order to absorb IR radiation a

molecule must go a net change in dipole

moment due to its vibration or rotation.motion This means that nearly all molecules

containing covalent bond will show some

.degree of selective IR absorption


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n

n This makes absorption band appear as dips in

the curve than as maxima as in the case of- .UV Visn Each dip In the spectrum is called a band or

peak and represented absorption of IR

.radiation at that frequency by the samplen %The transmittance is 0 if all theradiation is absorbed and the transmittance

%is 100 for no absorption.


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R ad i a t i o n s o u r c e s

.1 Nernest glower

.2 Globar source

.3 Incandescent wire source

.4 Mercury arc

.5 Carbon dioxide laser source

.6 Tungsten filament lamp


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U s e of W av e n umb e r th a nwav e l e n g t h o f f e r s e v e r a la d v a n t a g e sn Wave number are directly proportional

to frequency and are expressed in(much more convenient numbers in

), -this region of the spectrum 5000500 cm-1

n Because the wave number is directly, proportional to frequency and energy

the use of wave numbers allowsspectra to be displayed linear in

,energy which is a distinct aid insorting out related vibrational

transitions


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IN FRARED THEORY-Molecular vibration can occur by 2 different mechanism- ,Firstly quanta of IR radiation can

excite.atoms to vibrate directly The

absorption of IR radiation give rise to the IR

spectrum-Secondly quanta of visible light

achieve the

same result indirectly


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fV i b r a t io nt r e t c hn g e n d i n g

S y m m e ti c

A s y m m t r i c

S c i s s r i n g

R o c k ig

W a g gi n g

T w i s ti n g

n p l a n eu t o f p l a


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Mo l e c u la r V ib r a t io n s Covalent bonds vibrate at only

.certain allowable frequencies


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S t r e tc h i n g F r e q u e n c i e s

Frequency decreases with increasing.atomic weight

Frequency increases with increasing

.bond energy


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V ib r a t i o n a l M od e s Nonlinear molecule with n atoms

-usually has 3n 6 fundamental.vibrational modes


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F i n g e r p r in t o f M o l e c u l e -Whole molecule vibrations and bending

.vibrations are also quantitized

No two molecules will give exactly(the same IR spectrum except).enantiomers

: -Simple stretching 1600 3500 cm-1.

: -Complex vibrations 600 1400 cm-1, .called the fingerprint region


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I R -A c t iv e a n d I n a c t i v e - .A polar bond is usually IR active A nonpolar bond in a symmetrical

molecule will absorb weakly or

.not at all


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n-pentane

CH3CH2CH2CH2CH3

3000 cm-1

1470 &1375 cm-1

2850-2960 cm-1

satd C-H


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CH3CH2CH2CH2CH2CH3

n-hexane


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cyclohexane

no 1375 cm-1

no CH3


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App l i c a t i o n s &Identification of all types of organic

many types of inorganic compounds Determination of functional groups Identification of chromatographic

effluents

Quantitative determination of compoundsin mixtures

&Determination of molecular composition

stereochemistry Determination of molecular orientation

( & )Polymers Solutions


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Identification of compounds by matchingunknown with reference( )spectrum Fingerprinting

. - .Detection of impurities 0 1 0 01

Analysis of formulations such as&insecticides copolymersAccu r a c y > %Favorable conditions 1 %Routine analysis 5


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S en s i t iv i t y & D e t e c t io nl im i t s %

Routine 2

&Most favorable conditions using . %special techniques 0 01

L im i t a t i o n s .Minimal element info is given for most

of samples Background solvent should be

transparent in

IR region Molecule must be active in IR region

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Spectroscopy Ch201