uv - visible spectroscopy

Chapter 1

Ultraviolet/VisibleUltraviolet/Visible SpectroscopySpectroscopy

I t d ti f S tIntroduction of Spectroscopy• The structure of new compounds that are

isolated from natural sources or prepared in the p plab must be determined (and/or verified).– Chemical analysis (Classical methods)y ( )– Spectroscopy (Modern techniques)

• Spectroscopic techniques are non-destructive and generally require small amounts of sampleand generally require small amounts of sample.

Types of Energy in Each Region of the yp gy gElectromagnetic Spectrum

Region of Spectrum Energy Transitions

X-rays Bond Breaking

Ultraviolet/Visible Electronic

Infrared Vibrational

Microwave Rotational

Radiofrequencies Nuclear Spin (Nuclear Magnetic Resonance)

Electron Spin (Electron Spin Resonance) 3

μ μ

部分电磁波的波段分布示意图部分电磁波的波段分布示意图

Four common spectroscopic techniques used to determine structure:to determine structure:

Ult i l t S t– Ultraviolet Spectroscopy– Infrared Spectroscopy (IR)– Nuclear Magnetic Resonance Spectroscopy (NMR)– Mass Spectrometry (MS or Mass Spec)Mass Spectrometry (MS or Mass Spec)

Ult i l t S t• Ultraviolet Spectroscopy– Observes electronic transitions

• Provides information on the electronic bonding in a molecule

• Infrared spectroscopy:

Used to determine the– Used to determine the functional groupspresent in a molecule

• Mass spectrometryB k l l i t– Breaks molecule into fragments

• Analysis of the masses f th f t iof the fragments gives

MW and clues to the structure of the moleculemolecule

N l M ti R S t• Nuclear Magnetic Resonance Spectroscopy– Observes the chemical environment of the

hydrogen (or carbon) atoms in the molecule• Helps provide evidence for the structure of the carbon

k l t d/ th lk l tskeleton and/or the alkyl groups present

1H NMR of CH3CH2OHH NMR of CH3CH2OH 13C NMR of CH3CH2OH

1 1 Principle of UV Light Absorption1.1 Principle of UV Light Absorption

Ultraviolet light: wavelengths between 190 and 400 nm Visible light: wavelengths between 400 and 800 nmVisible light: wavelengths between 400 and 800 nm

Ultraviolet/visible spectroscopy involves theUltraviolet/visible spectroscopy involves the absorption of ultraviolet light by a molecule causing the promotion of an electron from a ground electronic state to an excited electronic state.

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Principle of UVp

10

There are several types of electronic transitions available to a molecule including:

σ to σ∗ (alkenes) to ∗ ( lk b l d lkπ to π∗ (alkenes, carbonyl compounds, alkynes, azo

compounds)to ∗ ( it lf d h ln to σ∗ (oxygen, nitrogen, sulfur, and halogen

compounds) to ∗ ( b l d )n to π∗ (carbonyl compounds)

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Instrument frameInstrument frame

12

SpectrometerSpectrometer

Multichannel photodiode array

Ultraviolet/Visible SpectroscopyHewlett-Packard 8452aDiode Array Spectrophotometer

1.2 Electronic Excitations

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Vibrational levels

Electronic excited stateElectronic excited state

Vibrational levels

Electronic ground state

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1 3 The Origin of UV Band Structure1.3 The Origin of UV Band Structure

A UV Absorption Band at Room Temperature (A)and at Lowered Temperature (B)

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1.4 the absorption law—the Beer-Lambert LawLaw

lIlog 0 ε clI

..log 0

10ε=

Io = the intensities of the incident lightI = the intensities of the transmitted lightl = the path length of the absorbing in centimetres

= the concentration in moles per litrelc

is called the absorbance or optical density;is known as the molar extinction cofficient

)/(log010

IIε

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1.5 Presentation of Spectra1.5 Presentation of Spectra

2.4log;230max

== ελ nm272 3.1282 2.9

Ult i l t S t f B i A id i C l hUltraviolet Spectrum of Benzoic Acid in Cyclohexane19

C f SC f S1.6 Choice of Solvents1.6 Choice of SolventsSome solvents used in ultraviolet spectroscopy

Solvent Minimum wavelength for 1 ll

So e so ve s used u v o e spec oscopy

1 cm cell, nmAcetonitrile 190 Water 191 Cyclohexane 195Hexane 201 Methanol 203 Ethanol 204Ether 215 Methylene dichloride 220 Chloroform 237Carbon tetrachloride 257

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The first criterion for solventA good solvent should not absorb ultravioletA good solvent should not absorb ultravioletradiation in the same region as the substance whosespectrum is being determinedspectrum is being determined.Usually solvents which do not contain conjugated

t t it bl f thi lth hsystems are most suitable for this purpose, althoughthey vary as to the shortest wavelength at whichth i t t t lt i l t di tithey remain transparent to ultraviolet radiation.The solvents most commonly used are water, 95%ethanol, and n-hexane.

21

A second criterion ( the effect of a solvent on the fine (structure of an absorption band)

Ultraviolet Spectra of Phenol in Ethanol and in IsooctaneUltraviolet Spectra of Phenol in Ethanol and in Isooctane

22

Large energy lowering caused by g ysolvent interaction

23The effect of a polar solvent on a transition

A third property of a solvent

The ability of a solvent to influence the wavelength of ultraviolet light which will be absorbed.Polar solvents may not form hydrogen bonds as readily with excited states as with ground states of polar solvents. Transitions of the type are shifted to shorter wavelengths by polar*the type are shifted to shorter wavelengths by polar solvents. On the other hand, in some cases the excited states may form

*π→n

stronger hydrogen bonds than the corresponding ground state. In such cases, a polar solvent would shift an absorption to longer wavelength, since the energy of the electronic transition would be decreased. Transitions of the type are shifted to longer wavelengths by polar solvents.

*ππ →

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Solvent Shifts on the Transition in Acetone

*→ *π→n

S l t H O CH OH C H OH CHCl C HSolvent H2O CH3OH C2H5OH CHCl3 C6H14

nm,maxλ 264.5 270 272 277 279

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1.7 the Effect of Conjugation

Ultraviolet Spectra of Dimethylpolyenes, CH3(CH=CH)nCH3 ; A, n=3; B, n=4; C, n=5

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Conjugation of two chromophores not only results in abathochromic shift, but it also increases the intensity ofth b tithe absorption.

The Effect of Conjugation on Electronic Transitions

Alkenes λmax (nm) ε Ethylene 175 15 000

The Effect of Conjugation on Electronic Transitions

Ethylene 175 15,0001,3-Butadiene 217 21,000 1,3,5-Hexatriene 258 35,000 β-Carotene (11 double 465 125,000 β Carotene (11 double

bonds) ,

Ketones Acetone π→π* 189 900

n→π* 280 12 3-Buten-2-one π→π* 213 7,100

n→π* 320 27n π 320 27

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1.8 the Effect of Conjugation on Alkenes

Formation of the Molecular Orbitals for Ethylene

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Fi 1 13 A C i f thFig. 1-13 A Comparison of the Molecular Orbital Energy Levels and Energy of the π→π* Transitions in Eth lene and 1 3Transitions in Ethylene and 1,3-Butadiene

Conjugation makes the energy lowerj g gy

30

A Comparison of the π→π* Energy Gap

The longer the conjugated system, the longer the

A Comparison of the π→π Energy Gapin a Series of Polyenes of Increasing Chain Length

g j g y , gwavelength of the absorption maximum. 31

the Appearance of the Spectra of some Polyenesthe Appearance of the Spectra of some Polyenes32

An extension of the length of the conjugated systemconjugated system

B = OH OR X or NHB = -OH, -OR, -X, or –NH2

ΔΔEE1 1 ＞＞ΔΔEE22

Energy Relationships of the New Molecular Orbitals and the Interacting π System and its Auxochrome

33

Overlap of the electronsOverlap of the electrons

Hyperconjugation

Quasi-resonance structures

The net effect is an extension of the πsystem .34

1.9 the Effect of conformation and Geometry on Polyene SpectraGeometry on Polyene Spectra

• All-trans polyenes show only the Ψ2 → Ψ3* transition• In polyenes with one or more cis double bonds the Ψ → Ψ *• In polyenes with one or more cis double bonds, the Ψ2 → Ψ4*

transitions may become allowed — called “cis band”.• Any molecule which retains a center of symmetry, even when

i l i d bl b d ill t h th ipossessing several cis double bonds, will not show the cisband. 35

Spectra of isomeric β-carotenes, showing the development of the cis band.

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Empirical rules for caluclating UV/VisEmpirical rules for caluclating UV/Vis Absorptions

Woodward-Fieser Rules for Dienes

A. Woodward's Rules for Conjugated Carbonyl Compounds B. Mono-Substituted Benzene Derivatives C. Di-Substituted Benzene Derivatives D. Benzoyl Derivatives

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1.10 the Woodward-Fieser Rules for Dienes

j t d diconjugated dienesπ→π* transition ε=20,000 to 26,000 λ =217 to 245nm

B t di d i l j t d di i t i lButadiene and many simple conjugated dienes exist in a planar s-trans conformation.

Generally, alkyl substitution produces bathochromic shifts and y, y phyperchromic effects.

With certain patterns of alkyl substitution, the wavelength increases but the intensity decreasesincreases, but the intensity decreases.

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The central bond is a part of the ring system, the diene chromophoreis usually held rigidly in either the s trans or the s cis conformation

Cyclic dienes

is usually held rigidly in either the s-trans or the s-cis conformation

Homoannular DieneHomoannular Diene(s(s--cis)cis)less intense ε= 5 000less intense ε= 5 000 ––15 00015 000

Heteroannular DieneHeteroannular Diene(s(s--trans)trans)

less intense, ε= 5,000 less intense, ε= 5,000 ––15,00015,000λ longer (273nm)λ longer (273nm)

more intense, ε= 12,000 more intense, ε= 12,000 –– 28,00028,000λ shorter (234nm)λ shorter (234nm)

The actual rules for predicting the absorption of open chain and six-membered ring diene were first made by Woodward in 1941.

Since that time they have been modified by Fieser and Scott as aSince that time they have been modified by Fieser and Scott as a result of experience with a very large number of dienes and trienes.

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V l i d t t h t l di 214

Rules for diene and triene absorptionValue assigned to parent heteroannular or open diene 214 nmValue assigned to parent homoannular diene 253 nmIncrement for

(a) each alkyl substituent or ring residue 5 nm(a) each alkyl substituent or ring residue 5 nm (b) the exocyclic double bond 5 nm

(c) a double-bond extension 30 nm(d) auxochrome ―OCOCH3 0 nm

―OR 6 nm―SR 30 nm―Cl, ―Br 5 nm―NR2 60 nm

λcalc Total

H 3C H

C C

CH 3C C3

HC C

H

C

H

H C

H H

H

transoid: 214 nmalkyl groups: 3 × 5 = 15

229nm

H 3C H

transoid: 214 nmb d 217

H H

229nmobserved: 228 nm

observed: 217 nm

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CHCH3

a

aa A B

CH3

C C Oa

Exocyclic Double Bond

transoid: 214 nm

CH3CH2O

240 nmring residues: 3 × 5 = 15exocyclic double bond: 5

234 nm

observed: 241 nm

observed: 235 nm

CH3CH

CH3

CH3

CH3

cisoid: 253 nmring residues: 3 × 5 = 15exocyclic double bond: 5 278 nm273 nmobserved: 275 nm

278 nmobserved: 275 nm41

CH3

CH3

CH3COOcisoid: 253 nmring residues: 5 × 5 = 25double bonds extending g

conjugation: 2 × 30 = 60exocyclic double bonds: 3 × 5 = 15 CH3COO–: 0CH3COO : 0

353 nmobserved: 355 nm

The Woodward-Fieser rules work well for polyenes with from one to four conjugated double bonds.j g

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1 11 the Fieser-Kuhn Rules for1.11 the Fieser-Kuhn Rules for polyenes

β Carotene 11 Double Bondsβ-Carotene, 11 Double Bondsλmax= 452 nm(hexane), ε=15.2 × 104

Lycopene, 13 Double Bonds (11 Conjugated)y p , ( j g )

λmax= 474 nm(hexane), ε=18.6 × 10443

E i i l R l f P lEmpirical Rules for PolyensRRnnMhexane

d105.16)7.10.48(5114)( −−−++=λ

nhexaneexoendo

4

max

max

1074.1)()()(

×=ε

where:

M = the number of alkyl substituentsM = the number of alkyl substituentsn = the number of conjugated double bonds Rendo = the number of rings with endocyclic double bonds Rexo = the number of rings with exocyclic double bondsexo g y

For example, β-Carotene:

M = 10 n = 11

Rendo = 2 Rexo = 0

Therefore:

λmax= 453.3 εmax= 19.1 ×104

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1 12 b l d1.12 carbonyl compounds; enones

T i i l i iTwo principal uv transition:forbidden280 290 nm

*π

n280-290 nm

allowed190 nm

Substitution by an auxochrome with a lone pair of

190 nm πSubstitution by an auxochrome with a lone pair of

electrons, such as –NR2, – OH, –OR, –NH2, or –X, as in amides, acids, esters, or acid chlorides, gives a pronounced hypsochromic effect on the n→π* transition and lesser bathochromic effect on the π→π* transition. (resonance interaction)(resonance interaction)

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the Hypsocromic Effect of Lone Pair Auxochromes on

the n→π* Transition of a Carbonyl Group

λ S l tλmax εmax Solvent

C

O

HCH3293 nm 12 Hexane

C

O

CH3CH3 279 15 Hexane

O

C ClCH3 235 53 Hexane

O 214 - Water

C NH2CH3

C

O

OCH2CH3CH3 204 60 Water

This shift is due primarily to the inductive effect of the oxygen, nitrogen, or halogen atoms. They withdraw electrons from the carbonyl carbon,g y ycausing the lone pair of electrons on oxygen to be held more firmly thanwould be the case in the absence of an inductive effect. 46

the Spectra of a Series of Polyene AldehydesIf the carbonyl group is a part of a conjugated system of double bonds, b h h * d h * b d hif d l l hboth the n→π* and the π→π* bands are shifted to longer wavelengths.

The energy of the n→π* transition does not decreased as rapidly as that of the π→π* band, which is more intense.If the conjugated chain becomes long enough, the n→π* band becomes “buried” under the more intense π→π* band. 47

the Orbitals of an Enone System Compared to those of the Non-interacting Chromophoresinteracting Chromophores

48

1.13 Woodward’s Rules for Enones

Conjugation of a double bond with a carbonyl group leads to absorption (ε = 8 000carbonyl group leads to absorption (ε = 8,000 to 20,000), π→π*, at 220 ~ 250 nm, predictablepredictable

n→π* at 310 ~ 330 nm much less intense (εn→π , at 310 ~ 330 nm, much less intense (ε = 50 to 100), not predictable

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OO

CHC

O

CH3CCCH3H3C

H3C CH3

OCOCH3

6-membered enone: 215nmacyclic enone: 215nmα －CH3: 10β CH 24

6 membered enone: 215nmdouble bond extending

conjugation: 30homocyclic diene 39β －CH3: 24

249nmobserved: 249nm

homocyclic diene 39δ-ring residue: 18

302nmb d 300observed: 300nm

CH3

OO

CH3

O

5-membered enone: 202nmBr

5-membered enone: 202nmB 25β-ring residue: 2× 12= 24

exocyclic double bond: 5231nm

α-Br: 25β-ring residue: 2× 12= 24exocyclic double bond: 5

observed: 226nm 256nmobserved: 251nm

1 14 Solvent Shifts – a more detailed1.14 Solvent Shifts a more detailed examination

1. In most π→π* transitions(bathochromic shift, lower energy)( gy)

*π } Large energy loweringcaused by solvent

ENE

*π} caused by solvent

interaction

ERGY π π

Non-PolarSolvent Polar

Solvent

π

the Effect of a Polar Solvent on a Transition51

The ground state of the molecule is relatively non-polar, and the excitedstate is often more polar than the ground state. As a result, when a polar solvent is used, it interacts more strongly with the excited state than with the ground state, and the transition is shifted to longer wavelength.g g g

th d t tthe ground statethe π electron density equally distributed, the nuclei of C are shielded

the π* excited statethe π* excited statethe electron is promoted, the C becomes electron deficientThe excited state interacts more strongly with polarThe excited state interacts more strongly with polar or hydrogen bonding solvents

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2 In most n→π* transitions2. In most n→π transitions(hypsochromic shift, higher energy)

*π becomes electron

EN

π

*π

becomes electron deficient

C* OERG

Large solvent stabilizationdue to hydrogen bonding

n

n}

Yπ π

n C OO

HH

}

h Eff f P l S l * T i i

Non-PolarSolvent

PolarSolvent

•The ground state is more polar than the excited state. •Hydrogen bonding solvents interact more strongly with unshared

the Effect of a Polar Solvent on an n→π* Transition

•Hydrogen bonding solvents interact more strongly with unshared electron pairs in the ground state molecule 53

1 15 Aromatic Compounds1.15 Aromatic Compounds

M l l O bit l d E St t f BMolecular Orbitals and Energy States for Benzene

184 and 202 nm primary bands255nm secondary bands (fine-structure)

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The second primary band (184 nm,ε=47,000) in vacuum ultravioletregion

The 202 nm band is much lessintense (ε=7,400), it corresponds to aforbidden transition

The secondary band is the leastintense of the benzene bands (ε=230)intense of the benzene bands (ε 230).It corresponds to a symmetryforbidden electronic transition.

Ultraviolet Spectrum of Benzene

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A Substitutents with Unshared ElectronsA. Substitutents with Unshared Electrons

Y Y Y YY Y Y Y

• The non-bonding electrons can increase the length ofg gthe π system though resonance.

• The more available these n-electrons are forinteraction with the π system of the aromatic ring, thegreater the shifts will be. (-NH2, -OH, -OCH3, -X)

• Interactions of this type between the n and π• Interactions of this type between the n and πelectrons usually cause shifts in the primary andsecondary benzene absorption bands to longery p gwavelength.

• In addition, the presence of n-electrons in thesecompounds gives the possibility of n→π* transition.

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If an n-electron is excited into the extended π* chromophore,p ,the atom from which it was removed becomes electron-deficient, while the π system of the aromatic ring acquires anextra electron. This causes a separation of charge in theextra electron. This causes a separation of charge in themolecule.The extra electron in the ring is actually in a π* orbital.

Y Y Y Y

**

*Y Y Y Y

*

A charge transfer or an electron transfer excited state

With compounds which are acids or bases, pHchanges can have a very significant effect on the

A charge transfer or an electron transfer excited state

changes can have a very significant effect on thepositions of the primary and secondary bands.

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B S b i bl f j iB. Substituents capable of π-conjugation

C OR

C OR

C OR

C OR

Interaction of the benzene ring electrons and the πelectrons of the substituent can also produce a new

R

electron transfer band. This new band may be so intenseas to obscure the secondary band of the benzenesystem.system.

Notice that the opposite polarity is induced; the ringbecomes electron deficient.

The effect of acidity or basicity of the solution on such achromophoric substituent group

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C. Disubstituted benzene DerivativesC. Disubstituted benzene Derivatives

A. For para-, two possiblities:. o pa a , wo poss b es:1. Both groups are electron-releasing or electron-

withdrawing, similar to monosubstituted benzenes;2. One is electron-releasing while the other is electron-

withdrawing, enhanced shifting (the magnitude of theshift of the primary band is greater than the sum ofshift of the primary band is greater than the sum ofthe shifts due to the individual group.)

N+O

H2N N+O

H2NNO

H2NO

2

resonance interactionsresonance interactions59

Empirical Rules for Benzoyl DerivativesO

Parent chromophore: C

O

R

R = Alkyl or ring residue 246y gR = H 250R = OH or OAlkyl 230

Increment for each substituent: -Alkyl or ring residue o- m- 3 Alkyl or ring residue o-,m- 3

p- 10 -OH,-OCH3,or –OAlkyl o-,m- 7 p- 25

O- o 11 -O o- 11 m- 20 p- 78 -Cl o-,m- 0 p- 10 -Br o-,m- 2 p- 15 -NH2 o-,m- 132 , p- 58 -NHCOCH3 o-,m- 20 p- 45

-NHCH p- 73 NHCH3 p- 73 -N(CH3)2 o-,m- 20 p- 85

60

Br

O

O

COH

HO

HOHO

OHparent chromophore: 246nmi id 3o-ring residue: 3

m-Br: 2251nm

parent chromophore: 230nmm-OH: 2× 7 = 14251nm

observed: 253nmm-OH: 2× 7 = 14p-OH: 25

269nmobserved: 270nm

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Application of UV1.To obtain the information of conjugation, ε

2 Qualification analysis2.Qualification analysis

3.Quantitative analysis: A=εbc

Example spectra

Example spectra

A practical guide: Exercises

What do you look for in an ultraviolet spectrum?  spectrum?  

A single band of low intensity (10 to 100) in the region 250­360nm with no major absorption at shorter wavelengths360nm, with no major absorption at shorter wavelengths(200­250nm) n→π*

A i l j d h h ( i O N S)A simple, or unconjugated, chromophore (contains an O, N,or S): C=O, C=N, N=N, ‐NO2,‐COOR, ‐COOH, or –CONH2

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Two bands of medium intensity (ε=1,000 to 10,000), both withTwo bands of medium intensity (ε 1,000 to 10,000), both withλmax above 200 nm an aromatic system

A d d l f fi t t i th l l th b d (i l•A good deal of fine structure in the longer wavelength band (in non-polar solvents only).•Substituent on the aromatic rings will increase the molar absorptivity above 10 000above 10,000.•In polynuclear aromatic substances, a third band will appear near 200 nm.

Bands of high intensity (ε=10,000 to 20,000), above 210 nm an α,β-unsaturated ketone or a diene or polyene

•The longer the length of the conjugated system, the longer the observed wavelength will be.•For dienes using the Woodwood fieser rules•For dienes, using the Woodwood-fieser rules•More than four double bonds, using Fiese-Kuhn rules

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Simple ketones, acids, esters, amides, and other compounds p , , , , pcontaining both π system and unshared electron pairs, will show two absorptions:n→π* at longer wavelength (>300nm low intensity)n→π* at longer wavelength (>300nm, low intensity)π→π* at shorter wavelength (<250nm, high intensity)

•With conjugation (enones), the λmax of the π→π* band moves to lingerwavelengths , Woodward’s rulesε above 10,000, bury the weaker n→π* transitiony•α,β-unsaturated esters and acids, Nielsen’s rules

Compounds which are highly colored are likely to contain a long-chain conjugated system (4~5) or a polycyclic aromatic chromophore.chromophore.

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Homework and CourseHomework and Course Requirements

• Comprehend the rationale Ultraviolet and visible tspectra

• Master the fundamental type of electronic transition

• Master the major component parts and the demand of every part of visible ultraviolet spectrophotometer

• Finish Homework