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UV - VISIBLE SPECTROSCOPY

2nd PART

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Introduction

Ultraviolet spectroscopy is concerned with the study of absorption of UV

radiation which ranges from 200nm to 400nm. Compounds which are colored

absorb radiation from 400nm to 800nm. But which are colorless absorb radiation from uv region. In case of uv as well as visible spectroscopy ,only the valance

electrons absorb the energy, there by the molecules undergoes transition from ground state to excited state. This

absorption is characteristic and depends on the nature of electrons present. The intensity of absorption depends on the

concentration and path length as given by BEER –LAMBERT’s law.

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THEORY OF ELECTRONIC SPECTROSCOPY

When any molecule absorbs ultraviolet or visible

light its electrons get promoted from ground state to the higher energy state .

In ground state ,the spins of electrons in each

molecular orbital are essentially paired.

In higher energy states , if the spins of electrons are paired ,then it is called as an excited singlet state.

according to the molecular orbital theory ,when a molecule is excited by the absorption of energy (uv or visible light),its electrons are promoted to from

BONDING TO ANTIBONDING ORBITAL

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TYPES OF ELECTRONIC TRANSITIONS

There are three types of electrons are generally present in organic molecules:

They are as followed:

1) ‘σ’ electrons : present in saturated molecules

2) ‘∏’ electrons : present in unsaturated moleculeseg . double or triple bonds

3) ‘n’ electrons : these are non bonded electronseg . S,O,N & Halogens

The various transition states are

1) n ∏*2) ∏ ∏* 3) n σ*4) σ σ*

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Different energy states associated with such transitions can be given in this diagram:

σ*

∏*

n

∏

σ

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ENERGY REQUIRED for excitation for different transitions are:

n → ∏ * < ∏ → ∏ * < n → σ* < σ → σ*

1.n → ∏ * : Of all the types of transition, n → ∏ * transition requires the lowest energy . This type of transitions are observed in the molecules having ‘n’ electrons.(present in S, N, O Or Halogens)

The presence of n → ∏ * transition can be

identified easily by comparing the UV spectrum of substance with the spectrum.

2. ∏ → ∏ * : The energy required of this transition is between

n → ∏ * and n → σ* .But extended conjugations and alkyl substitutions shifts the λmax towards longer λ (bathochromic shift).

Also trans isomer of olefin absorbs at longer λ with more intensity than cis isomer.

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3. n → σ* :

This transition occurs in saturated compounds, with hetero atoms Like S , N , O or halogens. It requires lesser energy when compared toσ → σ*transition. Normally the peaks due to this transition occurs from 180nm to 250nm. As these peaks are observed at lower end of the UV spectrum ,it can be called as end absorption. Some compounds with n → σ* transitions are: methyene chloride ,water , methanol, ethanol, ether etc.

4. σ → σ* :

This type of electronic transition requires the highest energy. This is observed with saturated compounds(especially hydrocarbons).the peaks do not observed in UV region, but occur in vacuum UV or for UV region . i.e. 125-135nm.

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Choice of solvents and solvent effects:

solvents play a important role in UV spectra. Since compound peak could be obscured by solvent peak. Hence the sample is selected in such a way that the solvent neither absorbs in the region of measurement nor affects the absorption of the sample. Some common solvents used and their absorption regions are:

water 191 nm

Cyclohexane 195 nm

Methanol 203 nm

Ethanol 204 nm

Ether 215 nm

chloroform 237 nm

Carbon tetrachloride 257 nm

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Effects of polar solvents on the sample :

1. Polar solvents shifts ∏→∏* transition from lower to higher wavelength.

Because alkyl substitutions occurs in case of olefins. This type of shift is called as bathochromic shift.

2. Polar solvents shift n→∏ * & n→∏* to shorter wavelength.

Because of formation of hydrogen bonding between the sample and solvent molecule. This type of shift is called as blue shift.

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