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Molecular SpectroscopyVisible and Ultraviolet Spectroscopy

UV/VI S SpectroscopyUV/VI S Spectrometer

Application for Quanti tative Analysis

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Ultraviolet: 190~400nmViolet: 400 - 420 nmIndigo: 420 - 440 nmBlue: 440 - 490 nm

Green: 490 - 570 nmYellow: 570 - 585 nmOrange: 585 - 620 nm

Red: 620 - 780 nm

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In tern al Energ y o f Mo lecu les

E total =E trans +E elec +E vib+E rot+E nucl E elec : electronic transitions (UV, X-ray)

E vib: vibrational transitions (Infrared) E rot : rotational transitions (Microwave) E

nucl : nucleus spin (nuclear magnetic

resonance) or (MRI: magnetic resonanceimaging)

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Electronic Spectroscopy

Ultraviolet (UV) and visible (VIS) spectroscopyThis is the earliest method of molecularspectroscopy.A phenomenon of interaction of molecules withultraviolet and visible lights.Absorption of photon results in electronictransition of a molecule, and electrons arepromoted from ground state to higherelectronic states.

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UV and Visible Spectroscopy

In structure determination : UV-VISspectroscopy is used to detect thepresence of chromophores like dienes,aromatics, polyenes, and conjugatedketones, etc.

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Electronic transitions

There are three types of electronic transitionwhich can be considered;

Transitions involving p , s , and n electronsTransitions involving charge-transferelectronsTransitions involving d and f electrons

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Absorbing species containingp , s , and n electrons

Absorption of ultraviolet and visibleradiation in organic molecules is restricted

to certain functional groups(chromophores ) that contain valenceelectrons of low excitation energy.

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UV/VIS

Vacuum UV or F ar UV(< 190 nm )

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s s Transitions

An electron in a bonding s orbital is excited tothe corresponding antibonding orbital. The

energy required is large. For example, methane(which has only C-H bonds, and can onlyundergo s s transitions) shows anabsorbance maximum at 125 nm. Absorptionmaxima due to s s transitions are not seenin typical UV-VIS spectra (200 - 700 nm)

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n s Transitions

Saturated compounds containing atoms withlone pairs (non-bonding electrons) are capableof n s transitions. These transitionsusually need less energy than s s transitions. They can be initiated by lightwhose wavelength is in the range 150 - 250 nm.The number of organic functional groups withn s peaks in the UV region is small.

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n p and p p Transitions

Most absorption spectroscopy of organiccompounds is based on transitions of n or p electrons to the p excited state. These transitions fall in an experimentallyconvenient region of the spectrum (200 - 700nm). These transitions need an unsaturatedgroup in the molecule to provide the p electrons.

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Chromophore Excitation l max , nm Solvent

C=C p p * 171 hexane

C=On p *

p p *

290180

hexanehexane

N=On p *p p *

275200

ethanolethanol

C-XX=Br, I

n s *n s *

205255

hexanehexane

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Terms describing UV absorptions

1. Chromophores : functional groups that giveelectronic transitions.

2. Bathochromic shift : shift to longer , also called

red shift.3. Hysochromic shift : shift to shorter , also called blue shift.

4. Hyperchromism : increase in of a band. 5. Hypochromism : decrease in of a band.

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MultiplicityTriplet State

For some of the excited states, there are stateswith a pair of electrons having their spins

parallel (in two orbitals), leading to total spin of1 and multiplicities of 3.

Total Spin Multiplicities

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Selection Rule

For triplet state: Under the influence ofexternal field, there are three values (i.e. 3energy states) of +1, 0, -1 times the angularmomentum. Such states are called tripletstates (T).According to the selection rule, SS, TT,

are allowed transitions, but ST, TS, areforbidden transitions.

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Selection Rules of electronictransitionElectronic transitions may be classed asintense or weak according to the magnitude ofmax that corresponds to allowed or forbidden

transition as governed by the followingselection rules of electronic transition:Spin selection rule : there should be no change

in spin orientation or no spin inversion duringthese transitions. Thus, SS, TT, areallowed, but ST, TS, are forbidden.

(

S=0 transition allowed )

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p p

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Components of a SpectrophotometerLigh t Sou rce

Deuterium Lamps a truly continuousspectrum in the ultraviolet region is

produced by electrical excitation ofdeuterium at low pressure.(160nm~375nm)

Tungsten Filament Lamps the mostcommon source of visible and nearinfrared radiation .

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Components of a SpectrophotometerMonochromator ( )

Used as a filter: the monochromator willselect a narrow portion of the spectrum(the bandpass) of a given source

Used in analysis: the monochromator willsequentially select for the detector to

record the different components (spectrum)of any source or sample emitting light.

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Monochromator Czerny-Turner design

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Grating

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Photomultiplier Detector

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Principle of PhotomultiplierDetector

The type is commonly used.The detector consists of a photoemissive

cathode coupled with a series of electron-multiplying dynode stages, and usually calleda photomultiplier.

The primary electrons ejected from the photo-cathode are accelerated by an electric field soas to strike a small area on the first dynode.

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Principle of PhotomultiplierDetector

The impinging electrons strike with enoughenergy to eject two to five secondary electrons,which are accelerated to the second dynode toeject still more electrons.A photomultiplier may have 9 to 16 stages,and overall gain of 10 6~10 9 electrons perincident photon.

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Quantitative AnalysisBeers Law

A= e bce : the molar absorptivity (L mol -1 cm -1)b: the path length of the sample

c :the concentration of the compound in

solution, expressed in mol L -1

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Transmittance

I0 I

b

303.2

log)log(

)log(303.2)ln(

0

00

00

00

0

k

bc AT I

I

I

I kbc

I

I

dbkc I

dI

kcdb I

dI

I

I T

I

I

b

e

e

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External Standard and theCalibration Curve

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Fluorescence Spectroscopy

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Excitation of Molecules

Excitation of molecules can be broughtabout by absorption of two bands of

radiation, one centered about thewavelength l 1(S0S 1) and the secondaround the shorter wavelength l 2(S0S 2).

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Vibrational Relaxation (VR)

The molecule can rapidly dissipate excessvibrational energy as heat by collision withsolvent molecules.Average lifetime of vibrational relaxationexcited molecule is 10 -12sec or less.Fluorescence always involves a transition from

the lowest vibrational level of an excited stateto any one of the vibrational level of theground state.

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Internal Conversion (IC)

A molecule passes to a lower energy electronicstate.The molecule can pass from a low vibrationallevel of S 2 to an equally energetic vibrationallevel of the first excited singlet S 1.The mechanism of internal conversion process

S2 S1 is not well understood.Average lifetime of internal conversion

process is 10 -12sec or less.

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Fluorescence

This process of emitting a photon fordeexcitation of S 1 to S 0.

Average lifetime of fluorescence is 10-9

~10-7

sec.

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Intersystem Crossing (ISC)

This process of non-radiative transfer from thesinglet to the triplet state.

Intersystem crossing are most common inmolecules that contain heavy atoms, such asiodine or bromine.Interaction between the spin and orbitalmotions becomes large in the presence of suchatoms.

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Phosphorescence

From T 1, the molecule can return to S 0 byemission of photon.

A triplet-singlet transition is much less probability than a singlet-singlet conversion.The excited triplet state with respect to emission

ranges from 10-4

to 10sec.

f

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Rate of DeactivationProcesses

Absorption 10 -14~10 -15secVibrational Relaxation and Internal Conversion

10-12

sec or lessFluorescence 10 -9~10 -7secPhosphorescence 10 -3~10sec

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Quantum Efficiency Comparisonfor p p * and n p * transition

The majority radiation of fluorescentcompounds is produced by a transitioninvolving either the n p * or the p p *excited state.The molar absorptivity of a p p * transition isordinarily 100 to 1000 times greater than foran n p * process.

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The inherent lifetime associated with ap p * transition is shorter (10 -7 to 10 -9 sec

compared with 10-5

to 10-7

sec for annp * process) and k f is larger. The rateconstant for intersystem crossing k i issmaller for p p * excited states.Fluorescence is more commonlyassociated with p p * states than withnp * states because the deactivation

processes that compete with fluorescenceare less likely to occur.

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Fluorescence and Structure

The most intense and most useful fluorescent behavior is found in compounds containingaromatic functional groups with low energyp p * transition levels.Compounds containing aliphatic and alicycliccarbonyl structures or highly conjugateddouble bond structure may also exhibitfluorescence.

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H 3 C CH 3

CH 3

CH 2 OH

CH 3 CH 3 CH 3

Organic Compounds for

Fluorescence

Vitamin A has a blue fluorescence with an emissionmaximum at approximately 500nm in ethanol.

highly conjugated compound

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Fluorescence Instrumentation

Source xenon-arc lamp for higherwavelengths and mercury-arc lamp for shorterwavelengthsL ensing system quartz lenses can be used inthe ultraviolet wavelength range (200-380nm),and glass lenses can be used in the visiblewavelength range (380nm-700nm)

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Fluorescence Instrumentation

Emission wavelength selector : this system isgenerally placed at an angle of 90 o with respectto the excitation axis to minimize interferencesfrom transmitted and scattered exciting light.Beam Spli tter : A beam splitter is a mirror of

partial reflectance. A proportion of light would be reflected from the beam splitter while therest would be transmitted through it unaffected.