This presentation is all about UV spectroscopy In this presentation I discussed principle of UV spectroscopy, Absorption law, Intensity shift ,effect of solvent on Absorption shift and all type of transition in UV spectroscopy. We all know that UV spectroscopy deals with the determination of structure of compounds with interaction of electromagnetic radiation (UV rays) with matter. And i mentioned about principle how UV spectroscopy work in which I discussed about excitation of matter electron and how we used absorption spectra in terms of absorbance. I also mentioned about effect of solvent on Absorption shift how the polar and non-polar compound are affected when we change the polarity of solvent.

1. “Ultra-Violet Spectroscopy”
• What is UV-Spectroscopy
• Principle of UV-Spectroscopy
• Absorption Law (Lambert’s and beer’s law)
• Type of Electronic transition
• Concept of Chromophore and Auxochrome
• Absorption and Intensity shift
• Effect of solvent on absorption shift
-By Gaurav Singh Bisht

2. “Ultra-Violet Spectroscopy”
• It deals with the determination of structure of a compound with
interaction of electromagnetic radiation with matter (Solid,Liquid,Gas)
• This spectroscopy is also known as absorption spectroscopy because In
this spectroscopy Substance can absorb energy from certain wavelength
of electromagnetic radiation.
• It is very useful to measure the number of conjugated double bonds and
also aromatic conjugation with the various molecule
• It is also distinguish between conjugated and non-conjugated system
• In UV-Spectroscopy, wavelength is usually expressed in nanometers. The
UV range normally extends from 100 to 400 nm

3. “Principle of UV Spectroscopy”
• When light is absorbed by matter, there will be an increase in the energy content of
molecules or atoms Which results the excitation of electrons from the ground state to a
higher energy state.
• The frequency of light absorbed by the material depends on the energy difference between
two energy states of molecules and it is always equal to the energy difference between the
ground state and excited state.
• The amount of radiation or photons absorbed results in the formation of the absorption
spectrum, which will be measured in terms of absorbance
• The absorbance of radiation by a compound produces a distinct spectrum, which is helpful
in serving as a marker or identifier of the compound.
• A spectrophotometer is able to record the amount of absorption by a sample at different
wavelengths in the UV and visible wavelength range.

4. “Absorption Law”
• There are two laws which govern the absorption of light by the molecule these are
• Lambert’s Law :-
• it states that when a beam of monochromatic radiation passes through a homogenous
absorbing medium the rate of decrease of intensity of radiation with thickness of absorbing
medium is proportional to the intensity of incident radiation
• Mathematically, the law is expressed as -
-dI/dx = kI
• I=intensity of radiation after passing through a thickness x, of the medium
• -dI/dx = rate of decrease of intensity of radiation with thickness of absorbing medium
• K = proportionality constant or absorption coefficient

5. • Beer’s Law :-
• This law state that when a beam of monochromatic radiation is passes through an
absorbing substance the rate of decrease of intensity of radiation with thickness of
the absorbing solution is proportional to the intensity of incident radiation as well
as the concentration of the solution
• Mathematically this law is stated as
-dI/dx = K’IC
• Where C is the concentration of the solution in mole/liter
• Alternative expression:
• on combining the two laws, the beer-Lambert law can be formulated as
log I./ I = Ecl = A
• I•= intensity of incident light, I= intensity of transmitted light
• C concentration of solution, L = path length of the sample
• A = absorbance , E= molar extension coefficient

6. “Electronic transition”
• Depending upon the types of bonding and antibonding
molecular orbital following type of transition may be possible
• σ – σ * transition:
• Molecules having only sigma bond are capable of showing this
type of transition. The higher amount of energy is required
for this transition and the value of absorption maximum is
below than 165 nm
• n - σ * transition:
• All the saturated molecule having hetro atom(N,O) or
halo(Cl,Br,I)atom are showing this type of transition the
energy required for this transition is always less than σ – σ *
transition the value of absorption maximum is always less
than one meter(less than 180mu).
• For example : water absorb at 167mu, CH3OH at 174mu,and
CH3Clat 169mu

7. • π→ π* transition:
• All the unsaturated molecules showing this type of transition (compound contain
double bond and triple bond and also in aromatic).The excitation of π electron
requires smaller energy and hence transition of this type occurs at longer
wavelength(190-204mu) .
• Example: ethane, butadiene ,benzene, propane
• n- π* transition:
• The unsaturated molecule having hetro or halo atoms showing this type of
transition. This type of transition requires least amount of energy out of all the
transitions discussed ever and hence occurs at longer wavelength (greater than
200mu) .

8. “Concept of Chromophore and Auxochrome”
• Concept of Chromophore:
• Originally , a Chromophore was considered any system which is
responsible for imparting colour to the compound
• It is defined as any isolated covalently bonded group that shows a
characteristics absorption in the ultraviolet or the visible region.
• There are mainly two types of Chromophore-
• Chromophore in which the group contain π electron and they undergo
n- π* transition such Chromophore are ethylene acetylene.
• Chromophore which contain both pi and nonbinding electrons. Such
Chromophore undergo two type of transition (π→ π* and n- π*
transition). Example of this type are carbonyls, nitriles, azo
compounds

9. • Concept of Auxochrome:
• Auxochrome group can be define as any group-which does not itself act as a
Auxochrome but whose presence brings about a shift of the absorption band
towards the red and of the spectrum(longer wavelength).
• The effect of Auxochrome is due to its ability to extend the conjugation of a
Chromophore by sharing of nonbonding electron
• These are also known as colour enhancing group .Thus, a new Chromophore results
has a different value of absorption maximum.
• Example: -OH,-OR,-NH2,-NR2,-SH etc.

10. “Absorption and Intensity Shift”
• Bathochromic Effect:
• It is an effect by which the absorption maximum is shifted towards
longer wavelength due to the presence of an auxochrome or by the
change of solvent. Such and absorption shift towards longer
wavelength is called Red shift or bathochromic shift.
• Example: n-π transition for carbonyl compounds Experience
bathochromic shift when the polarity of solvent is decreased.
• Hypochromic Shift :
• It is an effective by which the absorption maximum is shifted
towards shorter wavelength the absorption shift towards lower
wavelength is called Blue ship or hypochromic shift it may be
caused by the removal of conjugation and also by changing the
polarity of the solvent

11. • Hyperchromic effect:
• It is an effect Due to which the intensity of
absorption maximum increases.
• Example: The band for Pyridine At 257mu,
Emax-2750 is shifted to 262mu to Emax-3560
for 2- methyl Pyridine. The introduction of an
Auxochrome usually increase the intensity of
absorption.
• Hypochromic shift :
• it is an effect Due to which intensity of absorption maximum is decreased. The introduction
of group which distorts the geometry of the molecule causes Hypochromic shift.

13. “Effect of Solvent”
• A most suitable solvent is one which does not itself absorb in the region
under investigation. A dilute Solution of the sample is always prepare for
the spectral analysis. Most commonly used solvent is 95% ethanol.
• The position and the intensity of absorption maximum for a particular
Chromophore by changing the polarity of Solvent.
• The absorption maximum for nonpolar compounds is same in
ethanol(polar) as well as in hexane (nonpolar),diene and conjugated
hydrocarbon does not experience any appreciable change In absorption
shift.
• In case polar compounds the absorption maximum is usually shifted in two
different way when we change the polarity of solvent –

14. • π→ π* transition(intense):
• If the carbonyl group is more polar in the excited state than
absorption is shifted to longer wavelength with increase in the
polarity of solvent which helps in stabilizing the non bonding
electrons in the excited state.
• n-π* transition(less intense):
• If the carbonyl group Is more polar in the ground state
than the excited state, then increasing polarity of
solvent stabilizes the non-bonding electron in the ground
state due to hydrogen bonding. Thus, absorption is
shifted to lower wavelength.
• Example: absorption maximum of acetone is at 279mu as
compared to 264mu in water.