1

Chem 325

UltraViolet-Visible

Spectroscopy

Electromagnetic Spectrum

λνλνλνλν = c

E = hνννν = hc/λλλλ

Organic Spectroscopy

Absorption of different EM radiation produces different

molecular energy changes

1. Radiowaves Nuclear Spin Transitions (NMR)

2. Microwaves Electron Spin Transitions (ESR)

3. Infrared Vibrational Transitions

4. Visible-NIR Raman Scattering (Vibrational)

5. Ultraviolet-Visible Electronic Transitions

UV-vis Absorption Spectroscopy

Light of wavelength 190 to ~800 nm is passed through a

sample. Amount of light that makes it through the sample is

compared to the amount when the sample is not present.

2

Transmittance and Absorbance

The transmittance is the ratio of the light that is detected

when the sample is present to the ratio when the sample is not

present.

Transmittance is measured in the following way:

0I

IT =

Beer-Lambert Law

Transmittance is related to concentration in a non-linear way

(exponential), so it is usually converted to the much more

useful quantity ‘absorbance’, where:

Absorbance is related to concentration in a linear way

according to the Beer-Lambert Law.

Where l is the pathlength in centimeters

And c is the concentration of the absorbing species in Molarity

(mol/L)

And εεεε is the molar absorptivity or molar extinction coefficient

I

IAAbsorbance

0log==

lcAAbsorbance ε==

Molar Absorptivities

Molar absorptivities may be very large for strongly absorbing compounds (ε >10,000) and very small if absorption is weak (ε = 10 to 100).

No absorption gives εεεε = 0!

Often given as logarithmic values

e.g. εεεε = 23,500 equivalent to log εεεε = 4.37

Units of εεεε

cl

A =ε L mol-1 cm-1 or M-1 cm-1

- very rarely stated explicitly!

Absorbance Spectra

The typical UV-vis spectrometer scans the

wavelength range of 190 nm to 800 nm, and the

absorption at each wavelength is plotted vs the

wavelength in nm.

0

0.5

1

1.5

2

255 280 305 330 355 380

Wavelength (nm)

Ab

so

rba

nc

e λλλλmaxλλλλmax λλλλmax λλλλmax

3

Electronic Transitions

UV and Visible light can be absorbed by organic

molecules causing an electronic transition. What

physically happens is an electron from the Highest

Occupied Molecular Orbital (HOMO) is promoted

to a higher energy orbital.

Chromophores

• Normal hydrocarbons: no UV-vis absorptions

• They do have absorption but in the FAR UV

(vacuum UV)

• Requires presence of a chromophore – a group with

easily promoted electrons

• Typically: ππππ-bond systems

Solvents

acetonitrile 190 nm n-hexane 201 nm

chloroform 240 methanol 205

cyclohexane 195 iso-octane 195

1,4-dioxane 215 water 190

95% ethanol 205 trimethylphosphate 210

Solvents generally required due to large absorptivities

Must be ‘transparent’, i.e. not absorb

Must be very pure

Solvent cutoffs (where they start to absorb!):

Solute-Solvent Interactions

OH

4

Electronic Energy Levels Electronic Transitions

Electronic Transitions

Organics that absorb in the UV and Visible region

(200 – 800 nm) generally contain one or more ππππ-

bonds.

Ethylene Orbital and Transitions

5

Molecular Orbital Diagrams

Conjugated molecules such as 1,3-butadiene absorb

at longer wavelengths.

Excited States

When a molecule absorbs light, this energy promotes an

electron from an occupied MO to an unoccupied MO. This

produces a ‘Singlet Excited State. A number of singlet

excited states are possible, and they are labelled S1, S2, …, Sn.

The ground state is always S0.

α

ground state (S0) S1 S2 S3

Electron Spin Selection Rule

S0 →→→→ S1 is spin-allowed

both states are ‘singlet’

spin multiplicity = 2S+1, S = ½ for each unpaired e-

Spin multiplicity = 3

A TRIPLET state

S0 →→→→ T1 is spin-forbidden

T1

6

State Diagrams

The possible excited states are often drawn as a

‘state diagram’ or ‘Jablonski diagram’. The

vibrational energy levels are shown for each

electronic level.

Intersystem

crossing

Electronic and Vibrational Transitions

Rigid Compounds

In rigid molecules, like polyaromatic hydrocarbons,

excitation to each vibrational level is resolved. These

narrow peaks appear in sets, called ‘bands’.

0

0.2

0.4

0.6

0.8

275 325 375 425

Wavelength (nm)

Ab

so

rba

nc

e

S0 �S1

ν = 0

ν = 1

ν = 2

ν = 3

Vibronic Structure

Non-Rigid Compounds

If the molecule is flexible, several conformations are

possible at any given time. The vibrational ‘fingers’

for each conformation are averaged to give a

rounded band.

0

0.4

0.8

1.2

1.6

2

200 250 300 350

Wavelength (nm)

Ab

so

rba

nc

e

S0 �S1

7

Types of Excitation

Only two types of excitation commonly encountered, depending

on the orbitals involved. These are:

A) ππππ* n (or n ����ππππ*)

Electron from an n orbital (non-

bonding or lone pair) is excited into a

ππππ* (antibonding) orbital. Commonly

the longest wavelength absorption for

ketones and aldehydes. ππππ* n

transitions are “forbidden”, so these

usually give weak bands (small εεεε)

B) ππππ* ππππ (or ππππ ����ππππ*)

Electron from a ππππ-orbital is excited into a ππππ*-orbital. This is

commonly the longest wavelength absorption for unsaturated

hydrocarbons.

What Factors Affect Absorption?

The major factor that affects absorption is the degree of

conjugation. The longer the conjugation, the lower the energy

required to excite (and therefore longer wavelength band).

Lengthening the conjugation

also increases the molar

absorptivity (more intense

absorption bands).

Adding substituents and

functional groups will have the

same effect but to a much

smaller degree.

Substituent Effects

Substituents may have any of four effects on a chromophore

1. Bathochromic shift (red shift) – a shift to longer λλλλ: lower E

2. Hypsochromic shift (blue shift) – shift to shorter λλλλ: higher E

3. Hyperchromic effect – an increase in intensity: higher εεεε

4. Hypochromic effect – a decrease in intensity: lower εεεε

200 nm 700 nm

εεεε

Hy

po

ch

rom

ic

Hypsochromic

Hy

perc

hro

mic

Bathochromic

Chromophores and Substituents

Empirical rules: help predict the λλλλmax of a specific compound

from the base chromophore and what substituents are

attached to it.

H2CCH2

ββββ-carotene

O

O

λλλλmax nm εεεε

175 15,000

217 21,000

258 35,000

n ���� ππππ* 280 27ππππ ���� ππππ* 213 7,100

465 125,000

n ���� ππππ* 280 12ππππ ���� ππππ* 189 900

8

Woodward-Fieser Rules for Dienes

For a compound to absorb above 200 nm, generally it will

contain some degree of conjugation. The simplest conjugated

molecule is a diene.

Woodward and Fieser developed a set of empirical rules to

help predict what the λλλλmax will be for a diene-based

compound.

There are separate rules for cyclic and acyclic dienes.

Butadiene is the simplest acyclic diene and has an absorption

maximum of 217 nm.

acyclic butadiene, λλλλmax = 217 nm

Woodward-Fieser Rules for Dienes

Next we add the contribution from any attached

groups (substitutents).Group Increment

Extended

conjugation

+30

Each exo-cyclic C=C +5

Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2

-Ph

+60

+60

Examples

Isoprene

acyclic butadiene = 217 nm

one alkyl subs. + 5 nm

222 nm

Experimental value 220 nm

Allylidenecyclohexane

acyclic butadiene = 217 nm

one exocyclic C=C + 5 nm

2 alkyl subs. +10 nm

232 nm

Experimental value 237 nm

Cyclic Dienes

There are two major types of cyclic dienes, with two different

base values.

Heteroannular (transoid): Homoannular (cisoid):

εεεε = 5,000 – 15,000 εεεε = 12,000-28,000

base λλλλmax = 214 base λλλλmax = 253

Increment table is the same as for acyclic butadienes with one

addition:

Additional Homoannular: +39

If two dienes are present in a molecule, the base with longer λλλλmax

is used.

9

Example

heteroannular diene = 214 nm

3 alkyl subs. (3 x 5) +15 nm

1 exo C=C + 5 nm

234 nm

Experimental value 235 nm

Structure Determination?

In the pre-NMR era of organic spectral determination, the

power of the method for discerning isomers is readily

apparent:

Consider abietic vs. levopimaric acid:

C

O

OHC

O

OH

levopimaric acidabietic acid

Types of Excitation

Only two types of excitation commonly encountered, depending

on the orbitals involved. These are:

A) ππππ* n (or n ����ππππ*)

Electron from an n orbital (non-

bonding or lone pair) is excited into a

ππππ* (antibonding) orbital. Commonly

the longest wavelength absorption for

ketones and aldehydes. ππππ* n

transitions are “forbidden”, so these

usually give weak bands (small εεεε)

B) ππππ* ππππ (or ππππ ����ππππ*)

Electron from a ππππ-orbital is excited into a ππππ*-orbital. This is

commonly the longest wavelength absorption for unsaturated

hydrocarbons.

Structure Determination!

C

O

OH

heteroannular diene = 214 nm

4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm

239 nm

homoannular diene = 253 nm

4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm

278 nmC

O

OH

abietic acid

levopimaric acid

10

WARNING !!

Three common errors:

R

This compound has three exocyclic double

bonds; the indicated bond is exocyclic to two

rings

This is not a heteroannular diene; you would use

the base value for an acyclic diene

Likewise, this is not a homooannular diene;

you would use the base value for an acyclic

diene

Woodward-Fieser Rules

See Pavia, Chapter 7, Section 10

for the Rules and worked examples!!

Product Analysis

Two possible enamines from this ketone. Can UV-vis tell

them apart?

O

CH3

CH3

NH2

CH3

NH2

Product Analysis

CH3

NH2

CH3

NH2

Base value 214

Homoannular 39

Alkyl substituents 3 ×××× 5 15

Exocyclic C=C 5

NH2 group 60

Predicted λλλλmax 333 nm

Base value 214

Alkyl substituents 3 ×××× 5 15

Exocyclic C=C 5

NH2 group 60

Predicted λλλλmax 294 nm

∗∗∗∗ ∗∗∗∗

11

The Carbonyl Group Absorptions

•Two possible absorptions

– Longer wavelength (lowest E) is n →→→→ ππππ*

– Symmetry ‘forbidden’, thus low εεεε

– Shorter wavelength (highest E) is ππππ →→→→ ππππ*

– Symmetry ‘allowed’, thus high εεεε

– Can conjugate with alkenes ππππ-systems

Woodward-Fieser Rules for Enones

Enones have a strong ππππ* ππππ band and

a longer wavelength ππππ* n that is

usually around 100 times less intense.

If the enones are conjugated enough,

the ππππ* ππππ band completely swamps out

the ππππ* n band.

Also, the positions of ππππ* ππππ bands are

much easier to predict with empirical

rules than ππππ* n bands.

For these reasons, the Woodward-

Fieser rules for enones ONLY APPLY

TO THE ππππ* ππππ TRANSITION!!

Ketones

Group Increment

6-membered ring or acyclic enone Base 215 nm

5-membered ring parent enone Base 202 nm

Acyclic dienone Base 245 nm

Double bond extending conjugation 30

Alkyl group or ring residue α, β, γ α, β, γ α, β, γ α, β, γ and higher 10, 12, 18

-OH α, β, γ α, β, γ α, β, γ α, β, γ and higher 35, 30, 18

-OR α, β, γ, δα, β, γ, δα, β, γ, δα, β, γ, δ 35, 30, 17, 31

-O(C=O)R α, β, δα, β, δα, β, δα, β, δ 6

-Cl α, βα, βα, βα, β 15, 12

-Br α, βα, βα, βα, β 25, 30

-NR2 ββββ 95

Exocyclic double bond 5

Homocyclic diene component 39

C C CC

β α

C

γδ

δO

Aldehydes and Acids/Esters

Unsaturated system Base Value

Aldehyde 208

With αααα or ββββ alkyl groups 220

With α,βα,βα,βα,β or β,ββ,ββ,ββ,β alkyl groups 230

With α,β,βα,β,βα,β,βα,β,β alkyl groups 242

Acid or ester

With αααα or ββββ alkyl groups 208

With α,βα,βα,βα,β or β,ββ,ββ,ββ,β alkyl groups 217

Group value – exocyclic α,βα,βα,βα,β double bond +5

Group value – endocyclic α,βα,βα,βα,β bond in 5

or 7 membered ring

+5

12

Solvent Effects on Enones

For enones, the solvent will also affect the position of λλλλmax.

Solvent correction Increment

Water +8

Ethanol, methanol 0

Chloroform -1

Dioxane -5

Ether -7

Hydrocarbon -11

Examples

cyclic enone = 215 nm

2 x ββββ- alkyl subs.(2 x 12) +24 nm

239 nm

Experimental value 238 nm

cyclic enone = 215 nm

extended conj. +30 nm

ββββ-ring residue +12 nm

δδδδ-ring residue +18 nm

exocyclic double bond + 5 nm

280 nm

Experimental value 280 nm

O

R

O

αααα

ββββ

Product Analysis

Bromination a steroid can produce two possible products.

Dehydrobromination gives two enones. Can we tell them apart?

CH3

OH

CH3

OH

Br CH3

OH

Br

CH3

O

CH3

OH

Br2

-HBr -HBr

Product Analysis

CH3

OH

CH3

O

Base value 215

ββββ-alkyl 12

227 nm

Base value 215

ββββ-alkyl ××××2 24

Exocyclic C=C 5

244 nm

13

UV-Vis of Aromatics

Benzene has 6 ππππ-MOs which leads to a number of

transitions.

ππππ4444∗∗∗∗ ππππ5555∗∗∗∗

ππππ6666∗∗∗∗

ππππ2222

ππππ1111

ππππ3333

Aromatics

Benzene has three main bands, the E, K, and B bands.

The E band is also called the primary

band. It is strongly allowed (εεεε = 47,000)

but shows up below 200 nm.

The K band is called the second

primary band, can be observed above

200 nm if substituents cause a red shift.

Its molar absorptivity is 7400.

The longest wavelength B band (260

nm) is called the secondary band and is

forbidden and therefore weak (εεεε = 230)

Aromatics in General

Substitution with auxochromes lead to the same general

effects as observed for dienes and enones, but in a less

predictable way.

The formation of rules for predicting the position of the

bands is not very useful since there tend to be more

exceptions than there are rules. However, we can

certianly highlight qualitative trends.

1. Substituents with lone pairs will red-shift the primary and

secondary bands.

GGG G

Substituent effects

2. Protonating or deprotonating functional groups changes

how they affect the primary and secondary bands.

3. Functional groups that extend the conjugation red shift

the primary and secondary bands.

5602688,700224-C(O)O-

97027311,600230-C(O)OH

1692547,500203-NH3+

1,4302808,600230-NH2

2,6002879,400235-O-

1,4502706,200211-OH

2042547,400203.5-H

ελλλλmaxελλλλmaxSubstituent

SecondaryPrimary

5602688,700224-C(O)O-

97027311,600230-C(O)OH

1692547,500203-NH3+

1,4302808,600230-NH2

2,6002879,400235-O-

1,4502706,200211-OH

2042547,400203.5-H

ελλλλmaxελλλλmaxSubstituent

SecondaryPrimary

14

Electronic Effects

Electron withdrawing groups (EWG) red-shift the primary

band and electron donating groups (EDG) red-shift both

bands.

7,800269-NO2

1922617,900210-Br

1902647,400210-Cl

1,4302808,600230-NH2

1,4802696,400217-OCH3

1,4502706,200211-OH

9,800224-C(O)CH3

11,400250-C(O)H

97027311,600230C(O)OH

1,00027113,000224-CN

2252617,000207-CH3

2042547,400203.5-H

ελλλλmaxελλλλmaxSubstituent

SecondaryPrimary

7,800269-NO2

1922617,900210-Br

1902647,400210-Cl

1,4302808,600230-NH2

1,4802696,400217-OCH3

1,4502706,200211-OH

9,800224-C(O)CH3

11,400250-C(O)H

97027311,600230C(O)OH

1,00027113,000224-CN

2252617,000207-CH3

2042547,400203.5-H

ελλλλmaxελλλλmaxSubstituent

SecondaryPrimary

Electron donating

Electron w

ithdrawing

Di-Substituted Aromatics

1) If both are EWG or both are EDG, the effect is equal to the

stronger of the two.

2) If one is an EWG and the other is an EDG, the overall effect

is additive if they are ortho or meta to one another.

3) If one is an EWG and the other is an EDG, the overall effect

is greater than additive if they are para to one another.

NOO

OH OH

NOO+

+

__ _

Woodward and Fieser Again…

Woodward and Fieser were able to come up with some rules

for predicting the λλλλmax of a subset of aromatic compounds –

those containing a carbonyl attached to the ring.

The rules are for R = H, R, OH, and OR.

The rules are not as accurate as for dienes and enones, but

are generally within 5 nm. The following are the possible

base structures.RO

G

230R = OH or O-Alkyl

250R = H

246R = alkyl or ring residue

λλλλmaxParent Chromophore

230R = OH or O-Alkyl

250R = H

246R = alkyl or ring residue

λλλλmaxParent Chromophore

Woodward-Fieser Rules for Aromatics

A substituent will have a different effect depending on

whether it is ortho, meta, or para substituted. Note that the

strongest effects are observed for para substituents.

85

73

45

58

15

10

78

25

10

p

-NHCH3

1313-NH2

22-Br

2011-O-

33Alkyl or ring residue

Substituent increment

00-Cl

77-O-Alkyl, -OH, -O-Ring

moG

2020-N(CH3)2

2020-NHC(O)CH3

85

73

45

58

15

10

78

25

10

p

-NHCH3

1313-NH2

22-Br

2011-O-

33Alkyl or ring residue

Substituent increment

00-Cl

77-O-Alkyl, -OH, -O-Ring

moG

2020-N(CH3)2

2020-NHC(O)CH3

15

Visible Spectrum

Molecules absorbing between 400 and 800 nm can be detected

by the human eye. The common colours and their associated

wavelengths are given below.

UV Vision

Many insects have vision that extends into the UV region,

which is useful for them since many flowers have UV

colourings that are invisible to humans.

Appearance of Coloured Compounds

The appearance of coloured compounds can be determined

by looking at the colour wheel below.

Select the λλλλmax of the compound of interest, and the

appearance of that compound will be the colour opposite it.

What we ‘see’ is the opposite of the colour absorbed.

ββββ-carotene

Consider ββββ-carotene

ββββ-carotene, λλλλmax = 455 nm

This molecule absorbs at 455 nm

(in the blue), therefore the

compound will appear orange.

ββββ-carotene is the primary

pigment that colours carrots.

16

Lycopene and Indigo

Similarly, lycopene absorbs at 474 nm and appears red. This

compound is the main pigment found in tomatoes.

λλλλmax for indigo is at 602 nm – in the orange region of the

spectrum – this is absorbed, the compliment is now indigo.

lycopene, λλλλmax = 474 nm

NH

HN

O

O

indigo

Azo Dyes

One of the most common classes of organic dyes are the azo

dyes. They have the following general structure:

These dyes have an EDG on one ring and an EWG on the

other, similar to disubstituted benzenes.

N N

EDGsEWGs

Azo Dyes

Azo Dyes are used to colour a variety of materials.

OH

N

N

NO2

Para Red

NN

NH2

H2N

Fast Brown

NNO3S

HO

SO3

Sunset Yellow (Food Yellow 3)

Azo Dyes

Many azo dyes are pH sensitive, which makes them useful pH

indicators.

NNO3S N

CH3

CH3

NHNO3S N

CH3

CH3

Yellow, pH > 4.4 Red, pH < 3.2

Methyl Orange

17

Final Notes

UV-Vis not especially useful as a primary tool for determining

the structures of organic molecules. However, it is useful in

certain instances to distinguish between isomers.

Main contribution that UV-Vis makes to structure

determination is that it can readily identify the presence of

conjugated ππππ-systems and unique chromophores.

Final Notes

• UV-vis most widely used instrumental technique in

chemistry and science/medicine!

• Detection in chromatography.

• Monitoring reaction kinetics (chem, biol, medicine)

• Materials science, synthesis, analytical, inorganic, organic,

physical chemistry, biochemistry, biology, medical

applications, food industry.

(R), (S) Nomenclature• Different molecules (enantiomers) must have different

names.

• Configuration around the

chiral carbon is specified

with (R) and (S).

• Cahn – Ingold - Prelog

Chiral Molecules and Optical Activity Polarimetry

• Use monochromatic light, usually sodium D-line, 589 nm

• Movable polarizing filter to measure angle

• Clockwise = dextrorotatory = d or (+)

• Counterclockwise = levorotatory = l or (-)

• Optical activity

• Not related to (R) and (S)

=>

18

Specific Rotation

Observed rotation depends on the length of the cell and

concentration, as well as the strength of optical activity,

temperature, and wavelength of light.

[αααα] = αααα (observed)

c •••• l

c is concentration in g/mL

l is length of path in decimeters.

Racemic Mixtures

• Equal quantities of d- and l-enantiomers (or R and S forms).

• Notation: (d,l), (±±±±), (R,S)

• No optical activity.

• The mixture may have different b.p. and m.p. from the pure

enantiomers!

Normal UV-vis spectrum

Isotropic radiation

(unpolarized)

Same spectrum for both

enantiomers

It is possible to measure the optical rotation (of an

enantiomer solution) at different wavelengths by

using a monochromator.

The variation of optical rotation with wavelength

of the (plane) polarized light is called OPTICAL

ROTATORY DISPERSION, a.k.a. the ORD

spectrum

19

• Difference in molar absorptivity εεεε is called CIRCULAR DICHROISM

• Optically active compounds rotate the plane of linearly

polarized light

• Different εεεε for left and right circularly polarized light,

∆ε∆ε∆ε∆ε = εεεεL - εεεεR ≠≠≠≠ 0

• Related to the difference in refractive index for left and right

circularly polarized light

Normal UV-vis

CD Spectrum

Chiroptical Spectroscopy

• Optical rotatory dispersion (ORD) and circular

dichroism (CD) known as CHIROPTICAL methods

USES

• Deconvolution of overlapping UV-vis bands

• Relative chiralities of isomers

• Absolute configurations of molecules

• Biomolecules: amino acids, proteins, enzymes, DNA

• Macromolecules, polymers with ‘handedness’