1. UV - VISIBLE SPECTROSCOPY
2nd PART
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2. 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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3. 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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4. 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 molecules
eg . double or triple bonds
3) ‘n’ electrons : these are non bonded electrons
eg . S,O,N & Halogens
The various transition states are
1) n ∏*
2) ∏ ∏*
3) n σ*
4) σ σ*
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5. Different energy states associated with such transitions
can be given in this diagram:
σ*
∏*
n
∏
σ
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6. 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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7. 3. n → σ* :
This transition occurs in saturated compounds, with hetero atoms
Like S , N , O or halogens. It requires lesser energy when compared to
σ → 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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8. 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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9. 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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