Dr. Soumendu Bisoi Assistant Professor Department of Chemistry, Narajole Raj College

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 λmax (nm) εmax

σ –bonded electrons

C-C σ → σ* ~150 - C-H

Lone pair electrons

n → σ* ~190 100-1000

π → π* ~190 500

n → π* ~300 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

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.

For example, benzene shows an absorption maximum at 255 mμ (εmax = 203) whereas aniline absorbs at

280 mμ (εmax = 1430). Hence, (-NH2) is an auxochrome.

Consider the following:

Aniline Anilinium ion

λmax = 280 nm λmax = 254 nm

εmax = 160 εmax = 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

λmax = 320 nm λmax = 385 nm

εmax = 2100 εmax = 4200

The presence of –OC2H5 group (auxochrome) increases the value of λmax and εmax .

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.

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.

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

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 Dr. Soumendu Bisoi Assistant Professor Department of Chemistry, Narajole Raj College 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 λmax (mμ) εmax

Ethylene 217 21,000

1,3- Butadiene 226 21,400

1,3,5-Hexatriene 254 21,400

β–Carotene (11-double - 465 125,000 bond)

Table 3. Effect of conjugated electronic transitions of carbonyl system

π → π* n → π* Compound Structure λ ε λ ε

Acetone 189 900 280 12

3-Buten-2-one 213 7,100 320 27

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.

In case of simple alkene (ethylene) we have only two molecular orbitals, one is ground state π bonding orbital and the other is excited state π* antibonding orbital. But in case of conjugated dienes, the π molecular orbitals of the two separate C=C groups combine to form two new bonding molecular orbitals designated as ψ1 and ψ2, and two new anti-bonding molecular orbitals designated as ψ3* and ψ4*. Form

Figure 10 it is clear that the transition of lowest energy π to π* transition in conjugated system is ψ2

(HOMO) to ψ3* (LUMO). Hence we can say that the conjugated dienes absorb at relatively longer wavelength than do isolated alkenes.

Figure 10. A comparison of the π → π*energy gap in a 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 system gradually shifts the absorption maxima (λmax) to longer wavelength.

If a compound has enough conjugated double bonds, it will absorb visible light and the compound will be colored. The β-carotene, a precursor of vitamin A, has eleven conjugated double bonds and its absorption

β-Carotene λmax = 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 bonded electrons become part of the π system of 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 → π* transitions. This transitions around 280 to 290 nm (ε = 15). Most n → π* transitions are forbidden and hence low intensity. Carbonyl compound also have a π → π* transitions at about 188 nm (ε = 900).

Carbonyl compounds have two principal UV transitions, the allowed π → π* transitions and the forbidden n → π* transitions.

The n → π* transitions, is weak (forbidden). Substituent on the carbonyl group by an auxochrome 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 → π* transition and lesser bathochromic effect on the π → π* transitions.

In carbonyl group is part of conjugated system or double bonds, both the n → π* and π → π* bands are shifted to longer wavelengths. However the energy of the n → π* transitions does not decrease rapidly as that of the π → π* band, which is more intense, If the conjugated chain becomes long enough, n → π* band buried under the more intense π → π* band. Figure 11 illustrates this effect for a series of polyene aldehydes.

Figure 11. The spectra of a series of polyene aldehydes.

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 cis isomer resulting in lesser π orbital overlap.

λmax = 283 mµ λmax = 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).

Table 4. Effect of solvent on the electronic transitions

π → π* n → π* Solvent λ ε λ ε

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.

π→π* Transitions

In case of π → π* transitions, the excited states are more polar than the ground state (Figure 13). If a polar solvent is used the dipole–dipole interaction reduces the energy of the excited state more than the ground state. Thus a polar solvent decreases the energy of π → π* transition and hence the absorption in a polar solvent such as ethanol will be at a longer wavelength (red shift) than in a non-polar solvent such as hexane.

Figure 13. Absorption shift (π → π* transitions) with change in polarity of the solvent n→π* transitions

In case of n → π* 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 state is decreased which further causes the increase in energy difference between the ground and excited energy levels. Therefore, absorption maxima resulting from n → π* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity.

Figure 14. Absorption shift (n → π* transitions) with change in polarity of the solvent

The absorption that results from transitions within the benzene Chromophore can be quite complex. The electronic transitions are basically of the π → π* type. Three electronic transitions takes place to these excited states (Figure 15). Primary bands at 184 and 202 nm and secondary (fine structure) band at 255 nm. Primary bands is allowed transitions (ε = 47000), but not observed under usual conditions, because wavelength are in the vacuum ultraviolet region of the spectrum. The second primary band is often shifted to longer wavelengths and can be observed under primary condition. This band is much less intense (ε = 7400) and it corresponds to a forbidden transition. The secondary band is the least intense of the benzene band (ε = 230) and corresponds to a symmetry forbidden electronic transition. Substitution on the benzene ring can cause bathochromic and hyperchromic shifts. These shifts are difficult to predict. We may gain qualitative understanding of the effects of substitution by classifying substituents into groups.

Figure 15. Different energy states of benzene

SUBSTITUENT WITH UNSHARED ELECTRONS

Substituents that carry nonbonding electrons (n elctrons) can cause shifts in the primary and secondary absorption bands. The nonbonding electrons can increase the length of the π system through resonance. The more available these n electrons are for interaction with the π system of the aromatic ring, the greater the shifts will be. Example of groups with n electrons are the amino, hydroxyl, and methoxy groups are well as the halogens. In addition, the presence of non-bonding electrons introduces the possibility of n → π* transitions. If non-bonding electron is excited into the extended π*chromophore, the atom from which it is removed becomes electron-deficient and the π-system of aromatic ring becomes electron rich. This situation causes a separation of charge in the molecule and such excited state is called a charge-transfer or an electron-transfer excited state.

The pH of the sample solution can also have a significant effect on absorption spectra. The absorption spectra of certain aromatic compounds such as phenols and anilines change on changing the pH of the solution. Table 5 illustrates the effects of changing the pH of the solution on the absorption bands of various substituted benzenes.

Phenols and substituted phenols are acidic and display sudden changes in their absorptions maxima upon the addition of a base. After the removal of the phenolic proton, we get phenoxide ion. In the phenoxide ion lone pairs on the oxygen is delocalized over the π-system of the aromatic ring and increases the conjugation of the same. Extended conjugation leads to a decrease in the energy difference between the HOMO and LUMO orbitals, which results in red or bathochromic shift (to longer wavelength), along with an increase in the intensity of the absorption.

Neutral Basic

λmax = 210 (6200) 235 (9400)

270 (1450) 287 (2600)

Similarly, an aromatic amine gets protonated in an acidic medium which disturb the conjugation between the lone pair on nitrogen atom and the aromatic π-system. As a result, blue shift or hypsochromic shift (to shorter wavelength) is observed along with a decrease in intensity.

Neutral Acidic

λmax = 230 (8600) 203 (7500)

280 (1430) 254 (169)

Table 5. pH effects on absorption band

Primary Secondary Subtituent

λmax (nm) εmax λmax (nm) εmax

203 7400 254 204

-OH 210 6200 270 1450

-O- 235 9400 287 2600

-NH2 230 8600 280 1430

+ -NH3 203 7500 254 169

-COOH 230 11600 273 970

-COO- 224 8700 268 560

Substituents may have differing effects in the position of absorption maxima, depending upon whether they are electron releasing or electron withdrawing, any substituent, on the aromatic ring shifts the primary absorption band to longer wavelength. Electron-withdrawing groups have essentially no effect on the position of the secondary absorption band unless; of course the electron-withdrawing group is also capable of acting as chromophore. However, electron releasing groups increase both the wavelength and the intensity of the secondary absorption band. Table 6 summarizes theses effects, with electron releasing and electron-withdrawing susbtituents grouped together.

Table 6. Ultraviolet maxima for various aromatic compounds

Primary Secondary Subtituent

λmax (nm) εmax λmax (nm) εmax

203 7400 - -

-CH3 206 7000 261 225

-Cl 209 7400 263 190

Electron -Br 210 7900 261 192 releasing substituent -OH 210 6200 270 1450

-OCH3 217 6400 269 1480

-NH2 230 8600 280 1430

Electron -CN 224 1300 271 1000 donating substituent -COOH 230 11600 273 970

-NO2 268 7800 - -

DISUBSTITUENTS BENZENE DERIVATIVE

With disustituted benzene derivatives, it is necessary to consider the effect of each of the two susbtituents. For para-disubstitued benzenes, two possibilities exist. If both groups are electron releasing or if they are both electron with drawing, they exert similar to those observed with mono subtituted benzenes. The group with the stronger effect determines the extent of shifting of the primary absorption band. If one of the groups is electron releasing while the other is electron withdrawing, the magnitude of the shift of the primary band is greater than the sum of the shifts due to the individual groups. The enhanced shifting is due to resonance interactions of the following type.

References-

✓ William Kemp. (2002) Organic Spectroscopy. PAlGRAVE

✓ Y. R. SHARMA. (2013) Elementary Organic Spectroscopy ( Principles And Chemical Applications). S. Chand Limited.

✓ Pavia, D. L., Lampman, G. M., & Kriz, G. S. (1979). Introduction to spectroscopy: a guide for students of organic chemistry. Philadelphia, W.B. Saunders Co.

[The information, including the figures, are collected from the above references and will be used solely for academic purpose.]

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