Close

Education

Education

Education

Education

Education

Education

## Description

UV / VISIBLE SPECTROSCOPY

Mr. Santosh M. Damkondwar
January 21, 2013
Spectroscopy

* [tis the branch of science that deals with the
study of interaction of matter with light.

OR

* [tis the branch of science that deals with the

study of interaction of electromagnetic

* Electromagnetic radiation consist of discrete
packages of energy which are called as
photons.

* A photon consists of an oscillating electric field
(E) & an oscillating magnetic field (M) which
are perpendicular to each other.

Magnetic field

Source

* Frequency (v):

— It is defined as the number of times electrical field

— The unit for frequency is Hertz (Hz).
1 Hz = 1 cycle per second

* Wavelength (A):

— It is the distance between two nearest parts of the
wave in the same phase i.e. distance between two
nearest crest or troughs.

«Wavelength >

aaron

,

* The relationship between wavelength &
frequency can be written as:

c=VA
* As photon is subjected to energy, so
E=hv=hc/A

~The Electromagnetic Spectrum

Yisible
X-Rays [= Microwave
— I i
fre

meters | i 1 i i i i i i i i i i i

1083 10°11 10° 107 107° 103 101 10

cm [1 i i i i i [ i i i [1 i [ []

avelength < 101 107° 107 10° 107% 107! 10 10°

nm 1 1 1 1 1 1 L 1 1 L L 1 I J

Ek 104 102 10° 10? 104 10° 107 10°

Frequency Hz 1 i 1 1 i ] L J
102! 101%? 107 101% 1013 10! 10° 107

Energy kcal 1 1 1 1 1 1 1 j
10% 10% 104 102 10° 1072 1074 10°®

Dispersion
White Angle
Light

Red
Orange
Yellow
Green
Blue
Indigo
Violet

420 – 440 nm
440 – 490 nm

490-570 nm
V, rinciples of
7 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
(v) or wavelength (A).

Spectrometer is an instrument design to
measure the spectrum of a compound.

Principles of Spectroscopy

1. Absorption Spectroscopy:

* An analytical technique which concerns with
the measurement of absorption of

* e.g. UV (185 – 400 nm) / Visible (400 – 800 nm)
Spectroscopy, IR Spectroscopy (0.76 – 15 pm)

Principles of Spectroscopy

2. Emission Spectroscopy:

* An analytical technique in which emission
(of a particle or radiation) is dispersed
according to some property of the emission
& the amount of dispersion is measured.

* e.g. Mass Spectroscopy

Interaction of
EMR

Interaction of EMR with matter

1. Electronic Energy Levels:

* At room temperature the molecules are in the
lowest energy levels E..

* 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 E,, E,, …E, etc is called as

electronic transition and the difference is as:
AE=hv=E -E, where (n= 1,2; 3, ..: etc)
AE = 35 to 71 kcal/mole

Interaction of EMR with matter

2. Vibrational Energy Levels:

* 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.

Interaction of EMR with matter

3. Rotational Energy Levels:
* These energy levels are quantized & discrete.

* The spacing between energy levels are even
smaller than vibrational energy levels.

AE <AE <AE

rotational vibrational electronic

Lambert’s
Law

Lambert’s Law

When a monochromatic radiation is passed

through a solution, the decrease in the
intensity of radiation with thickness of the

solution is directly proportional to the
intensity of the incident light.

Let I be the intensity of incident radiation.
X be the thickness of the solution.
Then
Lambert’s Law

So, SE kan

Integrate equation between limit
[=Joatx=0 and
I=Tatx=l
We get,

In — Kl

Lambert’s Law

2.303 log = — K1/
17,
V4 K
log [+= /
rl 2.303
Is
Where, log — = A Absorbance
V4
K

= FE Absorption coefficient

A=FE.l Lambert’s Law

Beer’s Law

* When a monochromatic radiation is passed
through a solution, the decrease in the
intensity of radiation with thickness of the
solution is directly proportional to the
intensity of the incident light as well as
concentration of the solution.

* Let I be the intensity of incident radiation.
X be the thickness of the solution.

C be the concentration of the solution.
Then

Beer’s Law

dl

——aC.l]
dx
dl
So, [ “LL lgha 7
dx

Integrate equation between limit
[=Joatx=0 and
I=Tatx=l
We get,

1
In —=—-K’C.
I,

Beer’s Law

1,
2.303 log — = K.C.l
Il
I, K
log == C.l
1 2.303
V4
Where, 100 —2 = A Absorbance
v4
Kl – Molar extinction
2.303 coefficient

A=EC.] Beer’s Law

Beer’s Law

A=EICd

1 1
T=— OR —log7T =log —= A
/ l

0 0
From the equation it is seen that the absorbance
which is also called as optical density (OD) of a solution
in a container of fixed path length is directly
proportional to the concentration of a solution.
PRINCIPLES OF
UV – VISIBLE
SPECTROSCOPY

Principle

The UV radiation region extends from 10 nm
to 400 nm and the visible radiation region
extends from 400 nm to 800 nm.

Near UV Region: 200 nm to 400 nm
Far UV Region: below 200 nm

Far UV spectroscopy is studied under vacuum
condition.

The common solvent used for preparing
sample to be analyzed is either ethyl alcohol
or hexane.
Transitions

The possible electronic transitions can
graphically shown as:

3 3 i o (anti-bonding)
; – I x (anti-bonding)
. n— OG
e fa a + LA. i
TTT OpO
L n (non-bonding)
– opr
nt (bonding)

o (bonding)

The possible electronic transitions are

* 0 — oF transition

e T — T* transition

* n — o* transition

– * n — T* transition

« 0 — TF transition

[6 | * T — oF transition

* 0 — oF transition

* 0 electron from orbital is excited to
corresponding anti-bonding orbital o*.

* The energy required is large for this
transition.

* e.g. Methane (CH,) has C-H bond only and
can undergo o = o* transition and shows
absorbance maxima at 125 nm.

T — T* transition

* 1t electron in a bonding orbital is excited to
corresponding anti-bonding orbital rt*.

* Compounds containing multiple bonds like
alkenes, alkynes, carbonyl, nitriles, aromatic
compounds, etc undergo m = it* transitions.

* e.g. Alkenes generally absorb in the region
170 to 205 nm.

* n — o* transition

* Saturated compounds containing atoms with
lone pair of electrons like O, N, S and
halogens are capable of n = ¢* transition.

* These transitions usually requires less energy
than oc = o* transitions.

* The number of organic functional groups
with n = o* peaks in UV region is small (150
— 250 nm).

/ * n — TT* transition

* An electron from non-bonding orbital is
promoted to anti-bonding * orbital.

* Compounds containing double bond
involving hetero atoms (C=0, C=N, N=0)
undergo such transitions.

* n – 1t* transitions require minimum energy
and show absorption at longer wavelength
around 300 nm.

* 0 — T* transition

| & * T — o* transition [6

* These electronic transitions are forbidden
transitions & are only theoretically possible.

Thus, n 2 n* & nm = nt* electronic transitions
show absorption in region above 200 nm
which IS accessible to UV-visible
spectrophotometer.

*The UV spectrum is of only a few broad of
absorption.

Terms used
ou IN
UV / Visible
Spectroscopy

Chromophore

The part of a molecule responsible for imparting
color, are called as chromospheres.

OR

The functional groups containing multiple bonds
capable of absorbing radiations above 200 nm
due to n = nt* & mt = 1* transitions.

e.g. NO,, N=0O, C=0, C=N, C=N, C=C, C=§, etc

Chromophore

To interpretate UV — visible spectrum following
points should be noted:

1. Non-conjugated alkenes show an intense
absorption below 200 nm & are therefore
inaccessible to UV spectrophotometer.

2. Non-conjugated carbonyl group compound
give a weak absorption band in the 200 – 300
nm region.
Chromophore

e.g. || Acetone which has A, =279 nm

Zi

H,C CH, |
and that cyclohexane has A, = 291 nm. gE

When double bonds are conjugated in a
compound A _ is shifted to longer wavelength.

2,4 – hexadiene has A. =227 nm

CH CH
He Tr Pr THE TTR Re

Chromophore

3. Conjugation of C=C and carbonyl group shifts
the A, of both groups to longer wavelength.

e.g. Ethylene has A, =171 nm .

Acetone has A __ =279 nm ||

SN

HLG==CH] rac cH
Crotonaldehyde has A, = 290 nm

Auxochrome

The functional groups attached to a
chromophore which modifies the ability of the
chromophore to absorb light , altering the
wavelength or intensity of absorption.

OR

The functional group with non-bonding electrons
that does not absorb radiation in near UV region
but when attached to a chromophore alters the
wavelength & intensity of absorption.
Auxochrome

e.g. BenzeneA__ =255nm 88

OH

Phenol A, = 270 nm

NH,

Aniline A _, = 280 nm

Absorption
& Intensity

o Bathochromic Shift (Red Shift)

When absorption maxima (A…) of a
compound shifts to longer wavelength, it is
known as bathochromic shift or red shift.

The effect is due to presence of an auxochrome
or by the change of solvent.

e.g. An auxochrome group like —OH, -OCH,

causes absorption of compound at longer
wavelength.
1 Bathochromic Shift (Red Shift)

~ * In alkaline medium, p-nitrophenol shows red
shift. Because negatively charged oxygen
delocalizes more effectively than the unshared
pair of electron.

OH

Alkaline

medium :
OH oO

p-nitrophenol
Aa = 255 nm A. = 265 nm

28 Hypsochromic Shift (Blue Shift)

* When absorption maxima (A) of a
compound shifts to shorter wavelength, it is
known as hypsochromic shift or blue shift.

* The effect is due to presence of an group

causes removal of conjugation or by the
change of solvent.

© + Aniline shows blue shift in acidic medium, it
loses conjugation.

+
NH ; CI
Acidic
medium

Aniline
Aax = 280 nm Amax = 265 Nm

» Hyperchromic Effect

©» When absorption intensity (€) of a compound is
increased, it is known as hyperchromic shift.

e If auxochrome introduces to the
compound, the intensity of absorption

INCreases.™ Xx | EN
Ww |

Pyridine 2-methyl pyridine
A, .,=257 nm A. =260 nm

* Hypochromic Effect

~~ * When absorption intensity (g€) of a compound is
© decreased, it is known as hypochromic shift.

fr CH,

Naphthalene 2-methyl naphthalene
e = 19000 e = 10250

Shifts and Effects

Hyperchromic shift

Red
shift

Blue
shift

Absorbance (A)

Hypochromic shift
|

Wavelength (A)
APPLICATIONS OF
UV / VISIBLE
SPECTROSCOPY

Applications

* Qualitative & Quantitative Analysis:

— It is used for characterizing aromatic compounds
and conjugated olefins.

— It can be used to find out molar concentration of the
solute under study.

* Detection of impurities:

— It is one of the important method to detect
impurities in organic solvents.

* Detection of isomers are possible.

* Determination of molecular weight using Beer’s
aw.

REFERENCES

Reference Books

«Introduction to Spectroscopy
— Donald A. Pavia

* Elementary Organic Spectroscopy
—Y. R. Sharma

* Physical Chemistry
— Puri, Sharma & Pathaniya

Resources

e http://www?2.chemistry.msu.edu/faculty/reu
sch/VirtTxtJml/Spectrpy/UV- |

Vis/spectrum.htm

. http://en.wikipedia.org/wiki/Ultraviolet%E2 |
%80%93visible spectroscopy |

e http://teaching.shu.ac. uk/hwh/chemistry/tut
orials/molspec/uvvisab1l. htm |