give the selection rules of u1

8/4/2019 Give the Selection Rules of U1

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TeAm DiGiT

SPECTROSCOPY

In this document there is Selection Rules , Lambert Beer Law , ElectronicTransition , Chromophere , Auxochrome , Chromogens , Bathochromic ,

Hypochromic , Hyperchromic , UV-Visible components , Conjugation and

Steric effects , Solvent effect & Wood Words Fiesher Rule are discussed in D

UV-VISIBLE

SPECTROSCOPY


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Aurthor: DIPESH R BARDOLIA 2 | P a g e

product of path length through the material and the initial radiant

intensity.

3. What is electronic transition? Explain different electronic

transition with examples

The absorption of UV or visible radiation corresponds to the

excitation of outer electrons. There are three types of electronic

transition which can be considered

i. * Transitionsii. * Transitions

iii. n * Transitionsiv. n* Transitions

* Transitions:The excitation between bonding sigma and anti-bonding sigma orbitals

(*

Transitions) requires large energies corresponding to

absorptions in the far UV region in the range from 120-200nm.thus,

these transitions in saturated hydrocarbons, containing just bonds,remain transparent in the near UV/Visible region.

Ex: methane, propane, cyclohexane etc. Display *

Transitions

and max for each of these compounds is below 140nm.

* Transitions:Transitions between bonding pi and antibonding pi are called

*

Transitions. These transitions occur with compounds containing

conventional double or triple bonds, aromatic rings and carbonyl or azogroups. Although ethylene absorbs strongly around 180nm in the far UVregion, conjugated electron systems are more readily excited.


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The molar absorptivity, for*

Transitions (max>1000) because the

and *

orbitals are situated in same plane and consequently the

probability of the jump of an electron from to *

orbital is very high.

Ex: Butadiene

n * TransitionsThe excitation of an electron in an unshared pair (non-bonding

electrons) on nitrogen, oxygen, sulphur or halogens to an antibonding

orbital is called n*

transition.

Ex: methanol, 1-Iodobutane, trimethylamine

n* Transitions:Transitions between non-bonding atomic orbitals holding unshared pair

of electrons and antibonding pi-orbitals are called n*

Transitions.

Non-bonding electrons are held more loosely than bonding electrons

and consequently undergo transitions at comparatively longer

wavelengths. These Transitions occurs with compounds containing

double bonds involving hetero-atoms bearing unshared pairs ofelectrons.

n*

Transitions is always less intense because the electrons in the n-

orbital are situated perpendicular to the plane of the -bond (and hence

to the plane of the * orbital) and consequently the probability of the

jump of an electron from n to *

orbital is very low and in fact zero

according to symmetry selection rules. However, vibrations of atoms

bring about a partial overlape between the perpendicular planes and son

*transition occurs, but only to a limited extend.

Ex: CO, CS, NO


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4. Define the terms: Chromophores, Auxochromes &

Chromogens.

Chromophores:

A chromophore is part of a molecule responsible for its color.

When a molecule absorbs certain wavelengths ofvisible light and

transmits or reflects others, the molecule has a color. A chromophore is

a region in a molecule where the energy difference between two

different molecular orbitals falls within the range of the visiblespectrum. Visible light that hits the chromophore can thus be absorbedby exciting an electron from its ground state into an excited state.

Typical examples are C=C,C C,C=N,C N,C=O,N=N, etc.; they all

absorb intensely at the short wavelength end of the spectrum but some of

them (e.g. Carbonyl) have less intense bands at higher wavelengthowing to the participation of n electrons.

Auxochromes:

An Auxochromes is a group that depends colour. Its presence causes a

shift in the UV or Visible absorption maximum to a longer wavelength.

The most conspicuous property of an auxochromic grouping is its ability

to provide additional opportunity for charge delocalization and thus to

provide smaller energy increments for transition to excited states.

An Auxochromes is attached to a carbon-carbon double bond a

bathochromic effect will be observed and if to the double bonds where nelectrons are available (e.g. C=O) a hypochromic effect results.


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Example:

uxochrome Unsubstituted Chromophore maxnm

substituted Chromophore

-CH3 H2C=CH-CH=CH2 217 H2C=CH-CH=CHCH3-NEt2 H3C-CH2CH2CH=CHCOCH3 228 Et2NCH=CHCOCH3-OR H3C- CH=CH-COOH 204 H3C- C(OCH3)=CH-COOH

-Cl H2C=CH2 175 H2C=CHCl

Chromogens:

A compound containing Chronopher is called chromogen

Ex:

5. Differentiate Bathochromic (Red Shift) & Hypsochromic (Blueshift) effect.

Bathochromic (Red Shift) Hypsochromic (Blue shift)

A shift of an absorption maximum

to longer wavelength is called

Bathochromic shift

A shift of an absorption maximum

to shorter wavelength is called

Hypsochromic shift.

It is produced by a change of

medium (*

Transitions

undergo Bathochromic effect withan increase in the polarity of

solvent)

It is produced by a change of

medium (n*

Transitions

undergo Hypsochromic effect withan increase in the polarity of

solvent)

Ex:Ethylene absorbs at 175nm in

comparison to 1-butene(max=185

nm)

Ex:Acetone absorbs at 279nm in

comparison to hexane(max=279

nm)


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6. Differentiate Hypochromic & Hyperchromic effect.

Hypochromic effect Hyperchromic effect

The absorption intensity is

decrease in this effect

The absorption intensity is Increase

in this effect

Ex: The intensity of primary &

secondary band of Benzoic acid are

decrease in Benzoic ion.

Ex:The intensity of primary &

secondary band of phenol are

increase in phenolet ion.

7. Give the different components in UV spectrometer. What is the

rule of monochromater? Explain

Components of UV spectrometer are

i. Source

ii. Monochromator or Wave selector

iii. Sample holder

iv. Detector

v. Recorder

Monochromator or Wave selector:

i. Virtually all UV spectra are recorded solution-phase

ii. Cells can be made of plastic, glass or quartz

iii. Only quartz is transparent in the full 200-700 nm range; plastic and

glass are only suitable for visible spectra

iv. A typical sample cell (commonly called a cuvet)

v. Solvents must be transparent in the region to be observed

8. Discuss the conjugation & steric effects in UV spectroscopy.

Conjugation:


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to * transitions, when occurring in isolated groups in a molecule,

give rise to absorptions of fairly low intensity. However, conjugation of

unsaturated groups in a molecule produces a remarkable effect upon the

absorption spectrum. The wavelength of maximum absorption moves toa longer wavelength and the absorption intensity may often increase.

Ex:

max=180-200nm

=10,000

max=250nm

max=217nm

=21,000

max=290nm

max=258nm

=35,000

max=360nm

The same effect occurs when groups containing n electrons are

conjugated with a p electron group

Ex: O O

CH3-C-CH3 , : CH3-C-CH2-CH3 max=290nm max=325nm

Aromatic systems, which contain p electrons, absorb strongly in the

ultraviolet

Ex:

N

N


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max=254nm

max=250nm

max=232nm

max=217nm

max=210nm

In general, the greater the length of a conjugated system in a molecule,

the nearer the max comes to the visible regionSteric Effects:

Electron systems conjugate best when the molecule is planar in

configuration. If the presence of an auxochrome prevents the molecule

from being planar then large effects will be noticed in the spectrum; e.g.,

m- and p-methyl groups in the diphenyls have predictable but slight

effects on the spectra compared with that of diphenyl itself. However,

methyl groups in the o-position alter the spectrum completely.

N

S

N

H

O


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Ex:

Meta Para Ortho

Cis and Trans isomers of linear polyenes also show differences in their

spectra. The all-trans isomer has the longer conjugated system. maxis at a longer wavelength and max (molar absorptivity or molar

extinction coefficient) is higher than for the all cis or mixed isomer.

9. Discuss the solvent effect in UV visible spectroscopy?The effect on the absorption spectrum of a compound when diluted in a

solvent will vary depending on the chemical structures involved.

Generally speaking, non-polar solvents and non-polar molecules show

least effect. However, polar molecules exhibit quite dramatic differences

when interacted with a polar solvent. Interaction between solute and

solvent leads to absorption band broadening and a consequent reduction

in structural resolution and max.Ionic forms may also be created in

acidic or basic conditions. Thus care must be taken to avoid an

interaction between the solute and the solvent.

Figure 1(page no. 10) illustrates the effect of iso-octane and ethanol on

the spectrum of phenol, a change from hydrocarbon to hydroxylic

solvent. The loss of fine structure in the latter is due to broad band h-

bonded solvent-solute complexes replacing the fine structure present in

the iso-octane. The fine structure in the latter solvent illustrates the

principle that non-solvating or nonchelating solvents produce a spectrum

much closer to that obtained in the gaseous state.

Water and 0.1N solutions of hydrochloric acid and sodium hydroxide arecommonly used solvents for absorption spectrometry. Again care has to

be taken to avoid interaction. Where methodology requires buffering,

solutions have to be non-absorbing and generally both the composition

and pH will be specified. However, if this information is not available

lists can be found in the literature. For reactions in the 4.2 to 8.8 pH


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region, mixtures of 0.1N dihydrogen sodium phosphate and 0.1N

hydrogen disodium phosphate are generally used.

Figure 1 Spectra of Phenol in Iso-octane and in Ethanol

10. Give the different transitions in UV visible Spectroscopy?Arrange with their energy.

i. to *,

ii. n to *

iii. n to *

iv. to *.


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The energy required for various transition obeys following

order:

to *>n to *> n to *> to *

11. By using Wood-Words fisher rule calculate different values of

max.

Woodward-Fieser Rules for Dienes

The rules begin with a base value for max of the chromophore being

observed:

acyclic butadiene = 217 nm

The incremental contribution of substituents is added to this base value

from the group tables:

Group Increment

Extended conjugation (double bond) +30

Each exo-cyclic double bond +5

Alkyl/Ring residue +5

-OCOCH3

+0

-OR +6

-SR +30

-Cl, -Br +5

-NR2

+60

Woodward-Fieser Rules for Cyclic Dienes

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

values

Heteroannular (transoid): Homoannular (cisoid):


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

base max = 214 base max = 253

The increment table is the same as for acyclic butadienes with a couple

additions:

Group Increment

Additional homoannular +39

Where both types of diene

are present, the one with

the longer becomes thebase

Woodward-Fieser Rules for Enones/,-Unsaturated carbonyl

compound

Group Increment

6-membered ring or acyclic system Base 215 nm

5-membered ring Base 202 nm

Aldehyde Base 207 nm

Acidic/Ester Base 193 nm

Exocyclic 5

Extra Double Bond 30

Alkyl group or ring residue and higher 10, 12, 18-OH and higher 35, 30, 18

C C C

C C CC

C

O O


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-OR 35, 30, 17, 31-OCOCH

3 6-Cl 15, 12-Br 25, 30

-NR2 95

Homoanular diene component 39

Woodward-Fieser Rules for Aromatic system:

Parent Chromophore max

R = alkyl / acyclic or ring residue 246

R = H 250

R = OH or O-Alkyl 230

Substituent increment

G o m p

Alkyl or ring residue 3 3 10

-O-Alkyl, -OH, -O-Ring 7 7 25

-Cl 0 0 10

-Br 2 2 15

-NH2

13 13 58

-NHCOCH3

20 20 45

-NHCH3

73

-NR2

20 20 85

RO

G

give the selection rules of u1

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