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Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 IMPORTANT TERMINOLOGIES IN UV-VISIBLE SPECTROSCOPY

CHROMOPHORE

When a molecule absorbs electromagnetic radiation in the ultraviolet/visible range, a transition

between different electronic energy levels occurs. The energy of transition and the wavelength of radiation

absorbed are properties of atoms not the electron themselves. The group of atoms due to which absorption

occurs is called chromophore (Table 1).

A chromophore is defined as an isolated covalently bonded group that shows a characteristic absorption

in UV/Visible region. For example C=C, C=C, C=O, C=N, N=N, R-NO2 etc. Table 1. Typical absorption of simple isolated chromophores

Chromophore Transition Ȝmax (nm) İmax

ıbonded electrons

C-C C-H ~150

Lone pair electrons

n ĺı ~190

100-1000

n ĺʌ ~190 ~300 500
15

AUXOCHROME

The substituents covalently attached to a chromophore which themselves do not absorb ultraviolet/

visible radiation, but their presence changes both the intensity as well as wavelength of the absorption

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 maximum are known as auxochromes. The substituents like methyl (-CH3), hydroxyl (-OH), methoxy (- OMe), halogen (-X), amino (-NH2) group etc. are some examples of auxochromes. These are also called colour enhancing groups. Auxochrome generally increases the value of absorption maxima by extending the conjugation through

resonance. The extended conjugation brings the lowest excited state (LUMO) closer to the highest ground

state (HOMO) and thus permits a lower energy (longer wavelength) transition. Actually, the combination

of chromophore and auxochrome behaves as a new chromophore having different value of absorption maxima.

ȝİmax = 203) whereas aniline absorbs at

ȝİmax = 1430). Hence, (-NH2) is an auxochrome.

Consider the following:

Aniline Anilinium ion

Ȝmax = 280 nm Ȝmax = 254 nm

İmax = 160 İmax = 1430

In aniline (-NH2) acts as a chromophore, But in anilinium ion, there is no lone pair on nitrogen atom.

Consider trans -azobenzene and trans -p-ethoxyazobenzene.

Trans -Azobenzene Trans -p-Ethoxyazobenzene

Ȝmax = 320 nm Ȝmax = 385 nm

İmax = 2100 İmax = 4200

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 The presence of OC2H5 group (auxochrome) increases the Ȝmax İmax . Mechanisim: All auxochrome groups contain non-bonding electrons. Due to this, there is extension of conjugation of the chromophore by sharing the non-bonding electrons.

Absorption and Intensity shifts

(a) Bathochromic Shift or Red shift

The shift of an absorption maximum towards longer wavelength or lower energy is called as

bathochromic shift. The red color has a longer wavelength than the other colors in the visible spectrum,

therefore this effect is also known as red shift. (b) Hypsochromic Shift or Blue Shift The shift of an absorption maximum towards the shorter wavelength or higher energy is called

hypsochromic shift. The blue color has a lower wavelength than the other colors in the visible spectrum

and hence this effect is also known as blue shift. (c) Hyperchromic Effect It is an effect that results in increased absorption intensity. The introduction of an auxochrome usually causes hyperchromic shift. (d) Hypochromic Effect

An effect that results in decreased absorption intensity is called hypochromic effect. This is caused

by a group which distorts the geometry of the molecule.

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4

Figure 8. Absorption and intensity shifts

FACTORS AFFECTING THE POSITION OF UV-VISIBLE BANDS

EFFECTS OF CONJUGATION

One of the best ways to bring about a bathochromic shift is to increase the extent of conjugation in a double-bonded system. In the presence of conjugated double bonds, the electronic energy levels of

Chromophore move closer together. When two or more chromophores are conjugated the absorption

maxima is shifted to a larger wavelength or shorter frequency. Conjugation increases the energy of the

HOMO and decreases the energy of LUMO. As a result less energy is required for an electronic transition

in a conjugated system than in a non-conjugated system. Figure 9 illustrates the bathochromic shift that is

observed in a series of conjugated polyenes as the length of the conjugated chain is increased. Figure 9. CH3-(CH=CH)n-CH3 UV spectra of dimethyl polyenes A) n = 3; B) n = 4; C) n = 5

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 Conjugation of the two chromophores not only results in a bathochromic shift but increase the

intensity of the absorption. The exact position and intensity of the absorption band of the conjugated

system can be correlated with the extent of conjugation in the system. Table 2 illustrates the effect of

conjugation on some typical electronic transitions. Table 2. Effect of conjugated electronic transitions of dienen system

Compound Structure Ȝmax ȝ İmax

Ethylene

217 21,000

1,3- Butadiene

226 21,400

1,3,5-Hexatriene

254 21,400

ȕCarotene (11-double

bond) - 465 125,000 Table 3. Effect of conjugated electronic transitions of carbonyl system

Compound Structure

ʌĺʌ* n ĺʌ

Acetone

189 900 280 12

3-Buten-2-one

213 7,100 320 27

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 The bathochromic shift that results from an increase in the length of a conjugated system implies that an increase in conjugation decreases the energy require for electronic excitation.

ʌugated dienes, the ʌ

molecular orbitals of the two separate C=C groups combine to form two new bonding molecular orbitals

ȥ1 ȥ2, and two new anti-ȥ3ȥ4*. Form

Figure 10 it is clear that the transition of lowest ʌʌȥ2 ȥ3* (LUMO). Hence we can say that the conjugated dienes absorb at relatively longer wavelength than do isolated alkenes. ʌĺʌa series of polyenes of increasing chain length As the number of conjugated double bonds is increased, the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is progressively lowered. Therefore, the increase in size of the conjugated sysȜmax) to longer wavelength.

If a compound has enough conjugated double bonds, it will absorb visible light and the compound will be

ȕ-carotene, a precursor of vitamin A, has eleven conjugated double bonds and its absorption

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 maximum gets shifted from ultraviolet to the blue region of the visible spectrum giving it an orange colour.

ȕ-Ȝmax = 452 (nm), orange

In a qualitatively similar fashion, many auxochromes exert their bathochromic shifts by means of

an extension of the length of the conjugated system. The strongest auxochormes invariably possess a pair

of unshared electrons on the atom attached the double bond system. Resonance interaction of this lone pair

with the double bonds increases the length of the conjugated system. As a result of this interaction, as just shown, the non ʌ molecular orbitals, increasing its length by one extra orbital.

Carbonyl compounds; ENONES

Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n ĺʌ transitions. This transiİn ĺʌ

ʌĺʌions and the forbidden

n ĺʌ

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4

The n ĺʌ

with a alone pair of electrons, such as NR2, -OH, -NH2, as in amides, acids, esters, or acid chlorides, give

a pronounced hypsochromic effect on the n ĺʌʌĺʌ transitions. In carbonyl group is part of conjugated system or double bonds, both the n ĺʌʌĺʌ shifted to longer wavelengths. However the energy of the n ĺʌ

ʌĺʌn ĺʌ

ʌĺʌFigure 11 illustrates this effect for a series of polyene aldehydes. Figure 11. The spectra of a series of polyene aldehydes.

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 F Figure 12. The orbitals of an enone system compared to those of the noninteracting chromophores.

UV/vis spectroscopy can also used be used to study geometric isomerism of molecules. The trans isomer

absorbs at longer wavelength with a larger molar extinction constant than cis isomer. This can be

explained by the steric strain introduced in the cʌ

Ȝmax = 283 mµ Ȝmax = 320 mµ

EFFECT OF SOLVENT ON ABSORPTION:

The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species.

The choice of solvent can shift peaks to shorter or longer wavelengths.

For example for Mesityl oxide, following shifts are observed for the two electronic transitions on moving

from low polarity solvent hexane to water, which has higher polarity (Table 4).

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 Table 4. Effect of solvent on the electronic transitions

Solvent

ʌĺʌ* n ĺʌ

Hexane 230 12,600 327 98

Ethanol 237 12,600 315 78

Water 245 10,000 305 60

It is usually observed polar solutions (ethanol) give absorption maxima at longer wavelength than

non polar (hexane) solutions. Water and alcohols can form hydrogen bonds which results the shifting of

the bands of polar substances. Since polarities of the ground and excited state of a chromophore are

different, hence a change in the solvent polarity will stabilize the ground and excited states to different

extent causing change in the energy gap between these electronic states. Highly pure, non-polar solvents

such as saturated hydrocarbons do not interact with solute molecules either in the ground or excited state

and the absorption spectrum of a compound in these solvents is similar to the one in a pure gaseous state.

ʌĺʌ Transitions

ʌĺʌFigure 13). If

a polar solvent is used the dipoledipole interaction reduces the energy of the excited state more than the

polar solvent such as ethanol will be at a longer wavelength (red shift) than in a non-polar solvent such as

hexane.

Dr. Soumendu Bisoi

Assistant Professor

Department of Chemistry, Narajole Raj College

Compiled and circulated e-learning materials of Application of Spectroscopy to Simple Organic

Molecules

Course: DSE2A; B Sc (General), Sem V, Title: Section B: Organic Chemistry - 4 Figure 13. Absorption shift (ʌĺʌ) with change in polarity of the solvent

ĺʌ transitions

In case of n ĺʌ transitions (Figure 14), the polar solvents form hydrogen bonds with the ground

state of polar molecules more readily than with their excited states. Consequently the energy of ground

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