Uv visible spectroscopy by dr. pamod r. padole

LOGO
UV - Visible Spectroscopy
Dr. P. R. Padole
Associate Professor
Department of Chemistry
Shri Shivaji Science College,
Amravati

Spectroscopy
Defination:
 Spectroscopy is a branch of science which deals with
“interaction of electromagnetic radiation with matter.”
OR
 Spectroscopy may be defined as the study of
“interaction between the matter and electromagnetic
radiation.”
OR
 Spectroscopy involves the “interaction between
electromagnetic radiation and the substance under
investigation.”
OR
 Spectroscopy is the study of “interaction of light (or
electromagnetic radiation) with atoms and molecules.”

Study of spectroscopy:
Atomic Spectroscopy:
Atomic Spectroscopy deals with the
interaction of electromagnetic
radiation with atoms which are most
commonly in their lowest energy
state called as ground state.
Molecular Spectroscopy:
Molecular Spectroscopy deals with
the interaction of electromagnetic
radiation with molecules.

A molecule interacts with
electromagnetic radiation by
absorbing energy.
The absorption of energy can be
measured by instruments called
Spectrophotometers

We know that light is said to be dualistic in nature.
(wave as well as particle nature).
Where, X→ Axis of propagation of radiation
Y→ Axis represents direction of electric field
Z→ Axis represents direction of magnetic field.

Where, X→ Axis of propagation of radiation
Y→ Axis represents direction of electric field
Z→ Axis represents direction of magnetic field.

Wave length (λ): (W-14, ½ Mark)
The distance between the two adjacent (consecutive) crests or troughs
in a particular wave is called as wavelength (distance that the wave
moves during one cycle).
1 Ao (Angstrom) = 10-8 cm = 10-10 m = 0.1nm or 10-1 nm
1 nm (Nanometer) = 1 mµ (millimicrons) = 10-7 cm = 10-9 m =
10-6 µm (µ) = 10 Ao
1 µ (Micron) = 1 µm (micrometer) = 10-4 cm = 10-6 m = 10-3 nm

Frequency (ʋ): (W-15, ½ Mark)
The number of waves which can pass through a point
in one second is called as frequency.
C is the velocity of electromagnetic radiation.
(C= 3.0 x 108 m/s or 3.0 x 1010 cm/s)
Greater the wavelength, smaller is the frequency
or vice versa.
Frequency is expressed as ʋ (nu) in Hertz (Hz) or
Cycles per second (cps)
1Hz (Hertz) = 1 Cycle per second (cps)
1 MHz = 103 KHz (Kilo Hertz) = 106 Hertz

Wave number:
Q.1) _______________is the number of waves per centimeter. (W-19, ½ Mark)
The reciprocal of wavelength is called wave number (unit is cm-1).
In other words, it is defined as the total number of waves which can
pass through a space of one cm.
It is measured in term of number of waves/cm = cm-1 or Kaysers (K)
Where, 1 kayser = 1 cm-1
Energy (E): Energy of a wave of the particular radiation can
also be calculated by applying the relation:
E=h.v = h . c/T
Where, h = Plank’s constant (h= 6.626 x 10-27 erg.sec)
Energy of the light or electromagnetic radiation is measured in ergs or
joules or Kcal/mole
107 Ergs = 1 Joule (J)
Also, 1 Kcal = 4.184 KJ

Do you know?
Electromagnetic Radiation

Electromagnetic
Radiation
Q.1) What are electromagnetic radiation? How that is classified?
Q.2) What are electromagnetic spectrum?
Name the various electromagnetic regions?
Q.3) What is electromagnetic spectrum? (W-14, 1 Mark)

 Electromagnetic radiation is a form of energy
that is transmitted through space at an
enormous velocity
Defination:
The radiations associated with
electrical as well as magnetic
properties are called as
electromagnetic radiation.
The arrangement (or classification) of electromagnetic
radiation (EMR) in increasing or decreasing of their
wavelength or frequency is called as electromagnetic
spectrum.

 The relationship between wavelength &
frequency can be written as:
ν = c / λ
 As photon is subjected to energy, so
E = h ν = h c / λ

Violet 400 - 420 nm Yellow 570 - 585
nm
Indigo 420 - 440 nm Orange 585 - 620
nm
Blue 440 - 490 nm Red 620 - 780
nm
Green 490 - 570 nm

Principles of spectroscopy:
Absorption Spectroscopy:
The spectra produced by absorption of
some electromagnetic radiation
(EMR) of definite wavelength by the
matter are called as absorption
spectroscopy.
e.g. UV (185 - 400 nm) / Visible (400 - 800 nm) Spectroscopy, IR Spectroscopy
(0.76 - 15 μm)
Emission Spectroscopy:
The spectra produced by emission of some
electromagnetic radiation (EMR) of definite
wavelength by the matter are called as
emission spectroscopy.
e.g. Mass Spectroscopy

Why only absorption?
Q.1) Why is absorption and not emission
spectroscopy used to study the spectra of
organic compounds?

Why only absorption?
Q.1) Why is absorption and not
emission spectroscopy used to study
the spectra of organic compounds?
Ans:
Emission spectroscopy cannot be used since
the emission of radiation from an organic
compound requires vey high temperature.
Organic compounds are low melting and thus,
generally decompose at high temperature.
It may result :
decomposition, dissociation, oxidation of
molecule or change in molecular structure.

Interaction of EMR
with
Matter

S.No. Molecular
spectra
Spectral range Type of molecular
transition
1.
Electronic spectra
Or UV-Visible
spectra
UV = 200 – 400 nm
Visible = 400 – 800 nm
Electronic state transition
(excitation)
Changes in electronic energy
levels within the molecule
2.
Vibrational spectra
Or IR spectra
IR = 4000 – 667 cm-1
Vibrational state transition
Changes in the vibrational &
rotational movement of the
molecule
3.
Rotational spectra
Or Microwave
spectra
Microwave radiation = 1-100 cm-1 Rotational state transition
4. NMR spectra
Radio frequency region
ʋ = 60 – 500 MHz
Nuclear spin state transition
in presence of magnetic field
with reversal spin.
5. Mass spectra
Electron beam impact
70 eV, 6000 KJ/mol
Ionization and fragmentation
of the molecule into a
spectrum of fragment ions.

Energy of molecule = Electronic energy + Vibrational energy + Rotational energy
Microwaves = Rotational excitation
Infrared = Vibrational excitation
UV & Visible = Electronic excitation

Interaction of EMR with matter
1.Electronic Energy Levels:
 At room temperature the molecules are in the lowest energy levels
E0.
Electronic energy:
This type of energy is associated with the motion of electrons
from one level to another energy level by considering the nuclei of
atom as fixed point.
The increase in electronic energy of molecule occurs due to increase in
kinetic energy.
 Electronic energy gives rise to absorption spectra in the UV-
Visible region of electromagnetic spectrum.
 When the molecules absorb UV-visible light from EMR, one of the
outermost bond / lone pair electron is promoted to higher energy
state such as E1, E2, …En, etc is called as electronic transition and the
difference is as:
∆E = h ν = En - E0 where (n = 1, 2, 3, … etc)
∆E = 35 to 71 kcal/mole

Moving Electron
Photon
Before
Photon wavelength
changes
After

Moving Electron
Photon
Before
Electron
velocity changes
Photon wavelength
changes
After

2.Vibrational Energy Levels:
 Vibrational energy: This type of energy is associated with
the vibrational motion of atom present in molecule.
 Vibrational energy gives rise to absorption spectra in the
IR region of electromagnetic spectrum.
 These are less energy level than electronic energy levels.
 The spacing between energy levels are relatively small i.e.
0.01 to 10 kcal/mole.
e.g. when IR radiation is absorbed, molecules are excited from
one vibrational level to another or it vibrates with higher
amplitude.

3. Rotational Energy Levels:
 Rotational energy: This type of energy is associated with
the rotational motion of molecule about its axis.
 Rotational energy gives rise to absorption spectra in the
Microwave region of electromagnetic spectrum.
 These energy levels are quantized & discrete.
 The spacing between energy levels are even smaller than
vibrational energy levels.
∆Erotational < ∆Evibrational < ∆Eelectronic
Energy of molecule = Electronic energy + Vibrational energy + Rotational energy

Microwaves = Rotational excitation
Infrared = Vibrational excitation (S-16, ½ Mark)
UV & Visible = Electronic excitation (S-15, ½ Mark)

Do you know?
Electronic Spectroscopy or Ultra-Violet
(UV)-Visible Spectroscopy

Electronic Spectroscopy or
Ultra-Violet (UV)-Visible Spectroscopy:
Defination:
The study of “interaction of UV-Visible
radiation (light) with an organic
molecules (compounds)is called as
“UV-Visible spectroscopy”.
In UV-Visible region, electronic transitions (excitations) are
caused from atoms and molecules from bonding levels to
antibonding levels and so it is also known as electronic spectroscopy.
The Ultra violet region, which extends from 200 – 400 nm and the
visible region from 400 – 800 nm are more useful to organic chemists.

Effect of ultra-violet or visible light:
Q.1) What is the effect of UV - Visible light on the
organic compound?
Ans: When the substance under investigation
is subjected to the action of UV or
visible radiation, then it causes
changes in the electronic energy levels
within the molecule.

Electronic Spectroscopy or
Ultra-Violet (UV)-Visible Spectroscopy
In UV-Visible region, electronic transitions (excitations) are
caused from atoms and molecules from bonding levels to
antibonding levels and so it is also known as electronic
spectroscopy.

Principles of Spectroscopy
 The principle is based on the measurement of spectrum
of a sample containing atoms / molecules.
 Spectrum is a graph of intensity of absorbed or emitted
radiation by sample verses frequency (ν) or wavelength
(λ).
 Spectrometer is an instrument design to measure the
spectrum of a compound.
Q.1) Discuss the principle or theory of electronic spectra or UV-Visible spectra?

Spectrum
In absorption spectrum is a graph of intensity of absorption
measured in terms of absorbance (A) or molar extinction coefficient
(molar absorptivity) or percentage (%) transmittance verse
wavelength in nm or frequency in cm-1.

Wavelength maximum (λmax)
What do you mean by λmax ? (W-15, 1 Mark)
λmax:
It is called as wavelength of maximum absorption (peak).
It is defined as the wavelength of radiation which is absorbed by
the molecule to maximum extent.
OR
The wavelength at which there is maximum absorption
observed is called as wavelength maximum (λmax).
OR
The wavelength at which there is maximum absorption is
denoted by wavelength maximum (λmax). (W-16, 1/2 Mark)
Absorbance (A) λmax
Or
Extinction coefficient

Wavelength (λ)
in nm

max

Molar extinction coefficient:
What do you mean by €max ?
€ max :
The intensity of absorption is maximum at
a particular wavelength is called €max .
Or
It is defined as the intensity of radiation which is
absorbed by the molecule to maximum extent.
Absorbance (A) λmax
Or
Extinction coefficient

Wavelength (λ)
in nm

max
What do you mean by €max ?

Beer-Lambert’s Law:
Statement: Beer’s law is extended (modified) Lambert’s
law for solution. This modified law is called as Beer-
Lambert’s law or simply Beers law for solution.
 It’s state that, ”When a monochromatic light is passing
through homogenous absorbing medium (solution);
the rate of decreases of intensity with thickness of
medium (solution) is directly proportional to intensity
of light passing through it and molar concentration of
solution.”
Or
 “With increase in thickness and molar concentration of
solution, the intensity of light (transmitted light)
decreases exponentially.”

Where, K’ is proportionaty constant. It is called as molar absorption coefficient.
It is defined as, “Rate of decrease of intensity of light with thickness
of solution per unit intensity of light per unit concentration of solution.”
Therefore,
……. Eqn (1)
Then, eqn (1) can be write as,
…… Eqn (2)
At l = 0 ; I = Io (Intensity of incident light)
At l = l ; I = It (Intensity of transmitted light)

After solving eqn (2) by taking integral on both side, we get final
equation as,
…. Eqn (3)
…. Eqn (4)
Where, € is called as molar extension co-efficient
or molar absorptivity.
or Absorbance (A)

Where,
O.D. is called as optical density or absorbance of solution.
l (or x ) is optical path length or cell thickness
Absorbance of absorbing species is measured at its wavelength of
maximum absorption, λmax, for various concentrations.

The UV spectra are usually recorded as absorbance(A) Vs
wavelength (λ). The intensity of peak is found out by
plotting graph: € or log € Vs wavelength (λ). .
The intensity of absorption ( €) depends upon following
two things:
 Size of the molecule &
 Change in dipole moment
The value of intensity of absorption (€ ) will be more if
change in dipole moment is more.
If there is no change in dipole moment, the value of
intensity of absorption (€ ) becomes zero.

Instrumentation:
Components of
UV-Visible spectrophotometer
Source
Filters & Monochromator
Sample compartment
Detector
Recorder

Five Basic Optical Instrument Components
 1) Source – A stable source of radiant energy at the
desired wavelength (or range).
 2) Wavelength Selector – A device that isolates a
restricted region of the EM spectrum used for
measurement (monochromators, prisms & filters).
 3) Sample Container – A transparent container used
to hold the sample (cells, cuvettes, etc).
 4) Detector/Photoelectric Transducer – Converts the
radiant energy into a useable signal (usually
electrical).
 5) Signal Processor & Readout – Amplifies or
attenuates the transduced signal and sends it to a
readout device as a meter, digital readout, chart
recorder, computer, etc.

Instrumentation (Spectrophotometer):
•Radiation source:
A deuterium or hydrogen lamps discharge of the range 180-400 nm (Ultra-violet radiation)
Or
Tungsten filament lamp of wavelength greater than 375 nm (Visible radiation) are used as
radiation source.
Q.1) Write at least one source of Ultra –violet radiation. (W-12, 1Mark)
Q.2) Write at least one source of Visible radiation. (W-12, 1Mark)

Electronic
Transitions
Q.1) Discuss the type of electronic transitions that occur in UV region with
suitable diagram. (S-16, 4 Mark)

The relative energies for these transitions are in the following order
σ → σ* > n → σ* > π → π* > n → π*
These are four major types of excitations taking place in the molecule.

Types of Electronic Transitions:
Q.1) Explain the various types of electronic transition in UV-Visible spectra or electronic
spectra?
Q.2) Name and explain the possible types of electronic transition taking place due to
absorption of UV-Visible radiation by the organic compound?
Q.4) Discuss the various types of electronic transitions involved in UV-visible
spectroscopy. Draw molecular orbital energy diagram for electronic transitions.
(S-12, 4 Mark)
Q.6) Discuss the type of electronic transitions that occur in UV region with suitable
diagram. (S-16, 4 Mark)
Q.7) Explain the different types of electronic transitions that occur in ultraviolet region
with suitable diagram. (W-17, 4 M)
Q.8) Discuss the various types of electronic transitions with suitable example.
(W-19, 4 Mark)

• σ → σ* transition1
• π → π* transition2
• n → π* transition3
• n → σ* transition4
The possible electronic transitions are

Q.1) Explain with example of the σ → σ* transitions in electronic spectroscopy.
(W-15 & S-17, 2 Mark)
Q.2) Highest energy is required for σ-σ* transition in UV spectroscopy. (S-18, ½ Mark)
Q.3) CH4 molecule shows ____ electronic transition. (W-18, ½ Mark)
(a) σ → σ* (b) n → σ* (c) π → π* (d) n → π*

Defination: (S-12, W-15, S-16, S-17 & W-17, 1-2 Mark)
The transition of an electron from ground
state σ-BMO to higher energy vacant σ*-ABMO
is called as σ-σ* transition (transition of from
σ-BMO to σ*-ABMO).
It is resented diagrammatically as below:

All molecules contain σ-bond.
So they show σ-σ* transition.
Examples:
Saturated hydrocarbons like methane, ethane and other paraffins.
e.g. (i) CH4 shows σ → σ* transition, λmax= 125 nm. The σ-bond is very strong.
So, the σ-electron are tightly held.
So, σ → σ* transition requires high energy or short wavelength (below 200 nm).
As absorption in this region (below 200 nm) is beyond the range of ordinary
UV Spectrophotometer hence it is less informative.

Q.1) Explain with suitable example of the π → π* excitation (transition).
(S-11, S-13 & S-19, 2 Mark)
Q.2) Explain with example of the π → π* transitions in electronic
spectroscopy. (W-15 & S-17, 2 Mark)

Defination: (S-11, S-12, S-13, W-15,S-16, S-17 ,W-17 & S-19, 1-2 Mark)
The transition of an electron from ground state
π -BMO to higher energy vacant π*-ABMO is called as
π → π* transition.
It is resented in the energy level diagram as below:
For example: Alkenes, Alkynes, Carbonyl compounds (Aldehyde or Ketone), Cyanides,
Azo compounds, Aromatic compounds, etc.

Defination: (S-12, S-16 & W-17, 1-2 Mark)
non-bonding molecular orbital (n) to higher energy
vacant π*-ABMO is called as n → π* transition.
It is represented in the energy level diagram as below:
• n → π * transition3
n → π* transition requires least amount of energy hence occurs at longer wavelengths.
For example:
Compounds containing double bonds involving hetero atoms such as C=O, C=S, C=N,
N=O, etc, shows n → π* transition.
e.g. Aldehyde or ketone (>C=O) shows n → π* transition at 280 nm.

Defination: (S-11, S-12, S-13, S-16 & W-17, 1-2 Mark)
Q.1) Explain with suitable example of the n → σ* excitations (transition). (S-11, S-13 & S-19, 2 Mark)
non-bonding molecular orbital (n) to higher energy
vacant σ*-ABMO is called as n-σ* transition.
It is resented diagrammatically as below:
• n → σ * transition4
Note that:
n → σ* transition takes place in saturated compounds containing
one hetero atom with unshared pair of electrons (n electrons).
For example:
All organic compounds containing n electrons (nonbonding electrons or lone pairs) on
N, O, S or X atoms undergo n → σ* transition.
Saturated halides, alcohols, ethers, amines, etc.
e.g. Methyl alcohol CH3-O-H shows n → σ* transition

Their relative energies are in the following order
σ → σ* > n → σ* > π → π* > n → π*
The λmax value for the electronic transition increases
in the following order.
σ → σ* < n → σ* < π → π* < n → π*

Problems on Electronic Transitions:

Q.1) Predict the possible electronic transitions in each of the
following compounds:
Q.2) What types of electronic transitions are possible in
each of the following:
Q.3) List possible electronic transitions in:
Q.4) Write all possible electronic excitations in each of the
following compounds:
Q.5) What types of electronic transitions expected to occur
in each of the following:
Q.6) Give the types of electronic transitions in each of the
following:
Q.7) Identify the types of transition in each of the
following:

(i) Acetaldehyde (CH3CHO)
(S-14, W-14, W-15, W-16,S-19 & W-19, 1 Mark)
Ans:
Enegry level diagram for Acetaldehyde (CH3CHO) is given below:
σ → σ* > π → π* > n → σ* > n → π*
Highest Lowest energy
CH3 C H
O::

(ii) Cyclopropane (W-09 & W-11, 2 Mark)
(iii) CH3-CH2-CH2-NH2 (S-10 & W-14, 2 Mark)
(iv) CH2=CH-CH=CH2 (S-10 & W-14, 2 Mark)
(v) Methyl bromide (W-11, 1 Mark)
(vi) Cyclohexanol (S-12, W-12 & W-14, 2 Mark)
(vii) Benzene (S-12, S-16 & W-16, 2 Mark)
(viii)Thiourea (S-12, 2 Mark)
(ix) Acetophenone (W-12, 2 Mark)
(x) Aniline (S-13 & W-14, 2 Mark)
(xi) Cyclohexane (S-13, W-14 & S-16, 2 Mark)
(xii) Furan (S-13 & W-14, 2 Mark)
(xiii) Methane (CH4) (W-13, S-14 & S-19, 1 Mark)
(xiv) CH3-Cl (S-14, W-16, S-17, S-19 & W-19, 1 Mark)
(xv) CH3-CHO (S-14, W-14, W-15, W-16,S-19 & W-19, 1 Mark)

(xvi) CH2=CH2 (S-14, W-14, W-15 & S-19, 1 Mark)
(xvii) CH3-NH2 (W-14, S-18 & W-18, 1 Mark)
(xviii) CH3-CH3 (W-14, W-15, S-17, W-18 & W-19, 1 Mark)
(xxi) CH3-O-CH3 (W-15, 1 Mark)
(xxii) CH3-CH2-NH2 (S-16 & S-17, 1 Mark)
(xxiii) CH3COCH2CH3 (S-16, 1 Mark)
(xxiv) CH3CH=CH2 (S-16, 1 Mark)
(xxv) CH3CH2CH3 (W-16, W-17 & S-18, 1 Mark)
(xxvi) CH3CH2CH=CH2 (S-17 & W-19, 1 Mark)
(xxvii) CH3CH2CH2OH (W-17,1 Mark)
(xxviii) CH3-CN (W-17, 1 Mark)
(xxix) CH3CH2Br (W-17, 1 Mark)
(xxx) CH2=CH-Cl (S-18, 1 Mark)
(xxxi) CH≡CH (S-18, 1 Mark)
(xxxii) CH3CH2Cl (W-18, 1 Mark)

Transition probability
(Allowed and Forbidden transitions):
The extinction coefficient εmax = 0.87 x 1020 P.a.
Where, P = Transition probability with values from 0 to 1
a = Target area of the absorbing system,
i.e., a chromophore
There is a direct relationship between the area of the chromophore
and the absorption intensity (εmax).
Allowed transition:
The transition with the values of Extinction coefficient (εmax) more than 104
are usually called allowed transitions.
For ex. π → π* transitions in 1, 3 butadiene (217 nm, εmax 21,000).
Forbidden transitions:
The forbidden transition is a result of the excitation of one electron from the
loan pair present on the hetero atom to an antibonding π* orbital. The
values of Extinction coefficient (εmax) for forbidden transitions are generally
below 104 (10,000).
For ex. n → π* transition in carbonyl compounds (εmax 10-100)
Q.1) What are allowed and forbidden transitions in electronic (UV & Visible) spectroscopy? (S-15, 4 Mark)
Explain each with example.

Terms used
in
UV / Visible
Spectroscopy

Chromophore:
Q.1) Define and explains: (i) Chromophore with suitable examples.
(W-09, S-10, W-11, W-13, S-14, W-14, S-16, W-16, S-17, W-18 & W-19, 1-2 Mark)
Chromophore: [Greek: Chromo= colour, Phores= bearer]
Defination:
The unsaturated group which gives colour to the
compound (substance) by absorbing UV-Visible
radiation (light) is called as chromophore.
It contains π-electron.
Examples: Some important chromophores are ;
>C=C<, -CΞC-, ─N═O (nitroso), ─NO2,
─N═N- (azo), >C═O, >C═S (Thio),
─C≡N, etc.
The compounds bearing chromophores are known as chromogens.

Chromophore:
The compounds bearing chromophores are known as chromogens.
There are two types of chromophores:
• Chromophores in which the group contains π electrons and
they undergo π → π* transitions.
For ex. Ethylenes, acetylenes, etc.
• Chromophores which contain both π electrons and n (non
bonding) electrons and they undergo two types of transitions
i.e. π → π* and n → π*.
For ex. Carbonyls, nitriles and azo compounds.

Auxochrome:
Q.1) Define & explain the term: Auxochrome with suitable examples.
(W-09, S-10, W-13, W-14, S-16, W-16, W-17 & S-19, 1-2 Mark)
Auxochrome:
(increases intensity of colour & λmax of chromophore)
[Greek: Auxanein= to increase; Chroma= colour]
Defination:
The saturated group containing lone pair of
electrons which increases intensity of colour and
wavelength of maximum absorption (λmax) of
chromophore is called as auxochrome.
It contains non-bonding electron (lone-pair electrons).
e.g. -OH, -NH2, -X (X=Cl,Br, I), etc. are
auxochrome.

Auxochrome:
Dye must be attached to fibres by means of stable chemical bonds.
Those bonds are formed by some acidic or basic groups.
Such groups are known as auxochromes.
Auxochrome:
─OH, ─COOH, ─SO3H (acidic)
─NH2, ─NHR, ─NR2 (basic)
N=N N=N OH
Azobenzne
(Red)
p-hydroxyazobenzene
(Bright red)
Chromophore
Auxochrome

Direct dyes or Substantive dyes:
NH2Fibre HO Dye NH3Fibre O Dye
wool or silk
Basic Acidic
How Dyes Attach to Fibers
How a reactive dye binds to fibers

Auxochrome
e.g. Benzene λmax = 255 nm
Phenol λmax = 270 nm
Aniline λmax = 280 nm
OH
NH2

Q.1) Identify various shifts (effects) denoted by the letters A, B, C & D in the following plot.
(S-11, 2 Mark)
Q.2) Identify various shifts (effects) denoted by the letters P, Q, R & S in the following plot.
(W-14, 2 Mark)

Wavelength ( λ )
Absorbance(A)
Shifts and Effects
Hyperchromic shift
Hypochromic shift
Red
shift
Blue
shift
λmax

• Bathochromic Shift (Red Shift)1
Q.1) Define & explain the term: Bathochromic effect with
suitable example.
(W-11, S-13, W-13, S-14, S-16, W-16, S-17 & W-18, 1-2 Mark)
Q.2)The shift of absorption to longer wavelength due to
substitution or solvent effect is called bathochromic shift or
Red shift. (S-14, ½ Mark)
Q.3)The shift of absorption due to auxochrome or change in
solvent towards longer wavelength is known as
bathochromic shift or Red shift. (W-14, 1/2 Mark)
Q.4) What is meant by Bathochromic shift?
(S-16, 1 Mark)

Bathochromic Effect (Shift) or Red Shift:
(increase in λmax)
Defination:
The increase in λmax (wavelength of maximum absorption)
towards longer wavelength (Red region) is called as
bathochromic effect or Red shift.
Or
The shift of absorption to longer wavelength due to
substitution or solvent effect is called bathochromic shift or
Red shift.
Or
The shift of absorption due to auxochrome or change in solvent
towards longer wavelength is known as bathochromic shift
or Red shift.

When absorption maxima (λmax) of a compound shifts to longer
wavelength, it is known as bathochromic shift or red shift.
For ex. p-nitrophenol shows Red shift in alkaline medium.

• In alkaline medium, p-nitrophenol shows
red shift.
• As compared to unshared electron pair present on (-OH)
the negatively charged oxygen (II) delocalizes more
effectively and cause red shift.
p-nitrophenol
λmax = 255 nm λmax = 265 nm
OH
N
+ O
-
O
OH
-
Alkaline
medium
O
-
N
+ O
-
O

The bathochromic effect or Red shift is produced due to:
1) Presence of auxochrome (increase in lone pairs)
2) Increase in conjugation
3) Increase in polarity of solvent
4) Any effect which produces additional lone pair and
negative charge on hetero donor atom causing
greater resonance delocalization of lone pair.
[Note: The n → π* transition for carbonyl compounds shows Bathochromic shift
when the polarity of solvent decreases.]

The polar solvent increases π → π* λmax (red shift) and
decreases n → π* and n → σ* λmax (blue shift).
[Note: The n → π* transition for carbonyl
compounds shows Bathochromic shift when the
polarity of solvent decreases.]

Hypsochromic shift (Blue shift):
(decrease in λmax)
Defination: The decrease in λmax (wavelength of
maximum absorption) towards shorter
wavelength (Blue region) is called as
hypsochromic effect or Blue shift.
Or
The shift of absorption wavelength towards the
shorter wavelength side is called as
Hypsochromic shift. (S-16, ½ Mark)
• Hypsochromic Shift (Blue Shift)2
Q.1) Define & explain the term: Hypsochromic shift.
(S-12, S-13, S-14, S-16, W-16, W-17, S-18, W-18 & S-19, 1-2 Mark)

The hypsochromic effect or Blue shift is
produced due to:
1) Presence of electron attracting group
(electron withdrawing group) or removal of
electron donating group.
2) Removal of conjugation
3) Decrease in polarity of solvent
4) Any effect which remove lone pair of
electrons on hetero atom.

For ex.: Aniline shows blue shift in acidic medium.
Protonation of aniline in acidic medium; removes lone pair with π-system
of benzene ring (it loses conjugation).
So, it causes blue shift.
Aniline
In acidic medium, an unshared pair (lone pair) on
nitrogen atom of aniline is not available for
delocalization in cation (II).
NH2
H
+
Acidic
medium
NH3
+
Cl
-

[Note that:
(i) In most π → π* transitions, the excited state are
more polar than the ground state.
As a result, in π → π* transition, absorption will shift
toward longer wavelength, λmax (red shift), is referred
to as Bathochromic effect.
σ → σ* > π → π* > n → σ* > n → π*

[Note that:
(ii) Molecules with non-bonded electrons (n) are able
to interact with hydrogen bonding solvents to a greater
extent in the ground state than in their excited state.
As a result the n → π* transition absorption will shift
toward shorter wavelength as the hydrogen bonding
ability of the solvent increases. A shift toward shorter
wavelength is called a hypsochromic effect.]

• Hyperchromic Effect3
Q.1) Define & explain the term: Hyperchromic shift with suitable example.
(S-12, W-13, S-14, W-18 & S-19, 1-2 Mark)
Q.2) Increase in the intensity of absorption in UV-Visible spectrum is called as
hyperchromic effect. (S-13 & W-17, ½ Mark)
Hyperchromic effect or shift: (increase in εmax )
Defination:
The increase in intensity (greater intensity) of
UV-visible absorption maxima (εmax) is called as
hyperchromic effect or shift.
Or
Increase in the intensity of absorption in UV-Visible
spectrum is called as hyperchromic effect.

Hyperchromic effect or shift occurs due to:
1) Presence of auxochrome (increase in lone pairs)
2) Increase in conjugation
For ex.
p-nitrophenol shows in alkaline medium.

• When absorption intensity (ε) of a
compound is increased, it is known as
hyperchromic shift.
• If auxochrome introduces to the compound,
the intensity of absorption increases.
Pyridine 2-methylpyridine
ε = 2750 ε = 3560
N N CH3

• Hypochromic Effect4
Q.1) Define & explain the term: Hypochromic shift. (W-13 & S-17, 2 Mark)
Hyochromic effect or shift: (decrease in εmax )
Defination:
The decrease in intensity (smaller or
lesser intensity) of UV-visible absorption
maxima ( εmax) is called as hypochromic effect
or shift.
Or
Decrease in the intensity of absorption in UV-
Visible spectrum is called as hypochromic
effect.

Hypochromic effect or shift occurs due to:
1) Presence of electron attracting group or
removal of electron donating group.
2) Removal of conjugation
3) Decrease in polarity of solvent
4) Any effect which remove lone pair of
electrons on hetero atom.

For ex. Biphenyls shows
λmax= 252 nm, εmax= 19000,
whereas
2,2’-dimethyl-biphenyl shows
λmax= 270 nm, εmax= 800.
Biphenyls 2,2’-dimethyl-biphenyl
λmax= 252 nm, max= 19000 λmax= 270 nm, max= 800

Biphenyls 2,2’-dimethyl-biphenyl
λmax= 252 nm, ε max= 19000 λmax= 270 nm, ε max= 800
[Note that:
The decrease of 18,200 in the value εmax of 2,2’-
dimethyl-biphenyl is due to the hypochromic
effect of the methyl groups which distort the
chromophore by forcing the rings out of
co-planarity resulting in the loss of conjugation.]

Cyclo-decapentaene is cyclic with conjugated system
and have 10 electrons so follows Hucke’s rule but
the planar structure is strained and transforms
into stable non planar structure, therefore, it is
non aromatic.

Effect of conjugation on UV-Visible spectra:
Q.1) Explain the effect of conjugation on position and intensity of UV-Visible absorption band?
Q.2) What is the effect of conjugation on UV-Visible spectra?
Q.3) λmax for ethylene is 185 nm while λ max for 1,3-butadiene is 217 nm. Why? (W-09, 2 Mark)
Q.4) λmax for CH3.(CH=CH)3.CH3 is 263 nm but that for CH3.(CH=CH)6 .CH3 is 352 nm.
Explain with reason. (S-10, 2 Mark)
Q.5) Justify the following observations on the basis of Ultraviolet spectroscopy: (S-10, 2 Mark)
CH2=CH-CH=CH2 λmax= 217 nm
CH2=CH-CH2-CH=CH2 λmax= 175 nm
Q.6) Why compounds containing extended conjugation have higher λmax? Explain with suitable
example. (S-11 & S-12, 2 Mark)
Q.7) Explain: why 1,3-butadiene absorbs at higher wavelength than ethene. (S-13, 2 Mark)
Q.8) Distinguish on the basis of UV spectroscopy. (S-13, 2 Mark)
Q.9) λmax of CH2=CH2 is at 165 nm & λmax of CH2=CH-CH=CH2 is at 217nm. Justify.
(W-13 & S-14, 2-4 Mark)
Q.10) Why ethylene absorbs at 170 nm while 1,3-butadiene absorbs at 217 nm in UV-Visible
spectroscopy? Give reason. (S-2015, 4 Mark)
Q.11) Arrange the following compounds in the increasing order of their λmax values. Give
reasons. (W-17, 4 Mark)
(i) Cyclohexatriene (Benzene, π → π* λmax= 255 nm)
(ii) Cyclohexane (σ → σ* λmax= 125 nm)
(iii) 1,3-cyclohexadiene (π → π* λmax= 217 nm)
Q.12) Distinguish the following molecule on the basis of UV spectroscopy: (S-18, 4 Mark)
Ethene and 1,3-butadiene.

1,3-butadiene absorbs at longer wavelength :
1,3-butadiene absorbs at longer wavelength than ethylene (ethene):
With increase in conjugation:-
1) Resonance stabilization (delocalization of π-electrons) increases.
2) Number of π-Molecular orbitals increases.

CH2=CH2 CH2=CH-CH=CH2 CH2=CH-CH2-CH=CH2
Ethylene 1,3-butadiene 1,4-pentadiene
λmax= 165-185 nm λmax= 217 nm λmax= 175 nm
εmax= 15,000 ε max= 20,000 εmax= 15,000

CH2=CH2 CH2=CH-CH=CH2 CH2=CH2
So, the energy difference between ground state and excited state decreases.

Consequently,
As the conjugated system increases in length,
the energy required for a π → π* transition becomes
less and absorption will occur at longer wavelength.
So, the energy difference between ground state and excited state decreases.

Effect of conjugation in Carbonyl Compounds
π→ π* & n→ π* transition occurs-
>C=C-C=O

Choice of solvent in UV-Visible spectra:
The condition or properties of good solvent in
UV-Visible spectra are given below:
1. The solvent should be transferent in UV-Visible
region, i.e., It should not absorb in UV-Visible
region.
2. It should not interact with solute.
3. It should be less polar.
4. It should be pure.
Examples:
Cyclohexane (195 nm), water (190 nm),
95% ethyl alcohol (200 nm) are good solvent in
UV-Visible spectra.

Choice of solvent in UV-Visible spectra:
Examples:
Cyclohexane (195 nm), water (190 nm),
95% ethyl alcohol (200 nm) are good solvent in
UV-Visible spectra.
Why absolute ethyl alcohol
can not be used as solvent
in UV-Visible spectra?

Use of UV spectra in Structural determination:
Value assigned to Parent diene system (>C=C-C=C<) = 217 nm
Increments added for:
Diene system within a ring (homoannular diene) = 36 nm
Each Alkyl substituent or ring residue (-R) = 5 nm
The exocyclic nature of each double bond = 5 nm
A double bond extension = 30 nm
CH3
CH3
A B

C) Auxochrome:
(i) -OAcyl (-OCOCH3) = 0 nm
(ii) -OAlkyl (-OR) = 6 nm
(iii) -SAlkyl (-SR) = 30 nm
(iv) -Cl, -Br = 5 nm
(v) –NAlkyl2 (-NR2) = 60 nm

CH2 CH CH CH2
A B A B
Example no. 1 Example no 2
Example no. 7
Example no 3 Example no. 4 Example no. 5
Example no. 6 Example no. 8 Example no. 9 Example no. 10
CH3
Example no. 11
H3C

Problems:
CH3
Q.1) How many substitutents are attached to double bond
carbon’s molecule below?
Has the basic conjugated system been extended?
How many exocyclic double bonds are in the molecule?

Problems:
Q.2) For the following compound, how many
substituents are attached to double bond carbon’s
of the conjugated system?
Are these any exocyclic double bond?
CH3
A B

Why absolute ethyl alcohol can not be used as solvent in UV-
Visible spectra?
Benzene λmax = 255 nm

Why absolute ethyl alcohol can not be used as solvent in UV-
Visible spectra?
Absolute ethyl alcohol can not be used
as a solvent because it contains traces
of benzene as impurity.
This benzene shows absorption maxima.
It lies in UV-Visible region.
So, it interact with absorption maxima solvent.

Happiness
Unhappy
Unstable
HIGH energy
Happy
Stable
LOW energy

Reactants Products
(unstable) (stable)
Reactants Products
(stable) (unstable)

Reference Books:
Introduction to Spectroscopy
Donald A. Pavia
Elementary Organic Spectroscopy
Y. R. Sharma
Practical Pharmaceutical Chemistry
A.H. Beckett, J.B. Stenlake

RESOURCES:
 http://www2.chemistry.msu.edu/faculty/reusch/VirtT
xtJml/Spectrpy/UV-Vis/spectrum.htm
 http://en.wikipedia.org/wiki/Ultraviolet%E2%80%93v
isible_spectroscopy
 http://teaching.shu.ac.uk/hwb/chemistry/tutorials/m
olspec/uvvisab1.htm

Uv visible spectroscopy by dr. pamod r. padole

Uv visible spectroscopy by dr. pamod r. padole

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