06. UV Spectroscopy of Organic Compounds.pdf

10/18/2020
1
Ultra Violet Spectroscopy
Dr. Satish Dhirendra Mitragotri
(M.Sc., Ph.D., SET, NET, GATE and M.B.A. (Finance))
Department of Chemistry
Walchand College of Arts and Science, Solapur
Spectroscopy - Terminology
Spectroscopy - The study of interaction between
electromagnetic radiation and matter is spectroscopy.
Electromagnetic radiation – The oscillating electric
and magnetic fields which are perpendicular to each
other travel through medium and have definite energy
is known as electromagnetic radiation.
Different types of electromagnetic radiations are
o Sun light
oU V light
oVisible light
oX-ray
oMicrowave
oRadio frequency wave etc.
Electromagnetic spectrum – Entire range of wave-
length, energy, frequency over which electromagnetic
radiation present is known as electromagnetic
spectrum.
Different terms related to wave motion
1) Wavelength
2) Frequency
3) Wave number
4) Velocity
5) Energy
6) Particle properties of radiation.
Spectroscopy – Electromagnetic wave
Spectroscopy – Electromagnetic wave Spectroscopy - Terminology
1) Wavelength (λ) – The distance between two
particles which are in same phase.
•Wavelength is expressed by Lambda (λ)
•Measured in units of length starting from Km to
nm, more commonly used unit is Ao
1m = 1 X 102 cm = 1 X 106 µm = 1 X 109 nm = 1 X 1010 Ao
Crest
Trough

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2) Frequency (ν) – The number of waves passing
through a particular point in unit time (one second)
is known as frequency.
Unit of frequency is cycles per second or Hz
1 Hz = 1 cycle per second
It is direct measure of energy.
1 MHz = 1 X 103 KHz = 1 X 106 Hz
A fixed point
3)Wave number (ν-) – The number of waves per
centimeter is known as wave number.
Wave number is reciprocal of frequency, ν- = 1/ λ
Unit is kaiser or reciprocal centimeter = cm-1
A fixed Distance = 1 cm
4)Velocity (C) – The distance travelled by the wave in
one second is called as velocity of radiation.
•Velocity of radiation is fixed and it is
•3 X 108 m/s or 3 X 1010 cm/s
•Energy is directly proportional to velovity.
5)Energy (E) - Energy associated with radiation.
E = h X ν
Longer the wavelength lesser the energy
Shorter the wavelength more is the energy
6)Particle properties of radiation.
Spectroscopy – Electromagnetic Spectrum
Radiation Cosmic
Ray
Gamma
Rays
X-
Rays
UV Visible IR Microwave Radio
freq.
waves
Wavelength
cm
10-9 10-7 10-5 10-4 10-3 10-1 102
Energy
Kcal/mol
109 106 106 10 10-2 10-4 10-6
Frequency
Hz
Function Ioniza
tion
Electr
onic
transi
tion
Mole
cular
Vibra
tion
Rotational
motion
Nuclear
spin
Spectroscopy – Energy of molecule
Energy associated with any molecule can be resolved in four
different components
E Total = E Electronic + E Vibrational + E Rotational + E Spin
When radiation falls on any molecule one or more energy
parameters from molecule are changed and it is called
excitation.
Different type of energy type require different radiation for
this excitation process
Electronic- Electronic transition
Vibrational – IR Radiation
Rotational - Microwave
Spin- Radiofrequency wave

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Spectroscopy – Energy of molecule
Energy associated with any molecule can be resolved in
four different components
E Total = E Electronic + E Vibrational + E Rotational + E Spin
Electronic- Electronic transition
A B A+ B-
Vibrational – IR Radiation
A B A B
Rotational – Microwave
A B A
Spin- Radiofrequency wave
U V -Spectroscopy
Ultraviolet absorption spectrum arises from
transition of electrons from lower electronic
energy level to higher electronic energy level.
It is absorption spectroscopy where the amount
of energy absorbed by the molecule from
ultraviolet region is measured.
The wavelength of UV radiation is in between
200nm to 400 nm = 2000 A0 to 4000 A0
= 200 to 400 X 10-8 cm
The region from 100-200 nm is called as Vacuum
ultraviolet region
U V -Spectroscopy
The region from 200-400 nm is called as near or
quartz ultraviolet region
It comes prior to violet region of visible radiation
so it is called as ultra violet.
When there are two energy levels for e.g.
E2 and E1 where E2 E1
E2
E1
E2 – E1 = E
U V -Spectroscopy
When UV light falls on any molecule some part
of it may be absorbed, absorption depends on
two parameters
1. Energy of incident UV radiation
2. Structure of molecule
If the energy of radiation is kept same then
different molecules give different absorption
pattern i.e. UV spectrum.
Thus UV spectrum is a plot of absorbance (A)
verses wavelength.
What is absorbance ?
U V -Spectroscopy
When UV light falls on any sample it can interact
in three different ways
1. Reflection
2. Absorption
3. Transmission
Reflection
Absorption
Transmission
Incident
U V -Spectroscopy
What is absorbance ?
Io = Intensity of incident radiation
Ir = Intensity of reflected radiation
Ia = Intensity of absorbed radiation
It = Intensity of transmitted radiation
Transmittance (T)= It / Io
Opacity (O)= Io /It
Absorbance = log (Io /It)

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U V -Spectroscopy
Lambert’s Law - When a beam of
monochromatic radiation passes through the
absorbing medium the intensity of the light
radiation decreases exponentially as the length
of absorbing medium increases.
Beer’s Law – When a beam of monochromatic
radiation passes thorough the absorbing
medium the intensity of incident radiation
decreases with length of absorbing medium as
well as concentration of the soultion.
U V -Spectroscopy
Different types of electrons and electronic
transitions
1.Sigma (σ) electrons-
If compound contain only sigma electrons i.e.
electrons in present in sigma bond
e.g. CH3-H
2.Pi (π) electrons – Electrons present in double
or triple bond but that to C-C only
H2C=CH2
3.Non bonding (n) electrons – Electrons present
in hetero atoms bonded with carbon
CH3-HC=O, CH3-OH
U V –Spectroscopy- Types of Orbitals
σ
S
P
π
π*
n
σ*
S
P
Energy
U V –Spectroscopy – Absorption of energy by electrons
Energy
Bonding orbital
ΔE
Antibonding/Nonbonding orbital
ΔE
ER = hν = hc /λ
ΔE = ER
U V –Spectroscopy
Types of Electronic Transitions
σ to σ* n to σ* π to π* n to π*
Energy
σ* Antibonding
π* Antibonding
n Nonbonding
σ Bonding
π Bonding
U V –Spectroscopy – Types of electronic transitions
Energy
Bonding orbital
Antibonding orbital
Nonbonding orbital
Antibonding orbital
Bonding orbital
σ
σ*
π*
π
n
σ to σ*
π to π*
n to σ*
n to π*

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U V -Spectroscopy
Different types of electrons and electronic transitions
1.σ to σ* Transition – Transition of an electron from
bonding sigma orbital(σ) to antibonding sigma (σ*)
orbital is called as σ to σ* . This requires highest energy
and takes place by radiation of short wavelength ( 150
nm).
Stronger bond require shorter wavelength for transition.
e.g. CH3-H
2. n to σ* Transition - Transition of an electron from non
bonding (n) orbital to higher energy, antibonding σ*
orbital is called as n to σ* transition.
This type of transition occurs in compounds containing
hetero atoms like N,O,S,P and halogens
CH3-OH, CH3-SH, CH3 NH2
wavelength ( 180 nm – 200nm).
E-max is more for more probable transition.(R-I R-Cl)
U V -Spectroscopy
Different types of electrons and electronic transitions
3. π to π * Transition (K band) - Transition of an electron
from bonding (π) orbital to higher energy, antibonding
(π*) orbital is called as π to π * transition.
groups with multiple bonds.
C=C, C=O, N=N, C=N etc.
wavelength (200nm - 400 nm).
4. n to π * Transition (R band) - Transition of an electron
from nonbonding (n) orbital to higher energy,
antibonding (π*)orbital is called as π to π * transition.
groups with multiple bonds with heteroatons and
saturated carbon framework.
R-C=O, R-N=N, R-C=N etc.
wavelength (200nm - 300 nm).
U V -Spectroscopy
Wavelength of maximum absorption = λmax
Wavelength ( λ )
100 150 200 250 300 350 400
not a λmax
not a λmax
Intensity ε
6
5
4
3
2
1
0
U V -Spectroscopy
Maximum absorption intensity = εmax
Wavelength ( λ )
Intensity ε
100 150 200 250 300 350 400
Low absorption intensity = ε
6
5
4
3
2
1
0
U V -Spectroscopy
1.Bathochromic
shift (Red shift)
2.Hypsochromic
shift (Blue shift)
Wavelength ( λ )
Wavelength of maximum absorption λ max
6
5
4
3
2
1
0
Intensity ε
--------------------------------------------------------------
100 150 200 250 300 350 400
U V -Spectroscopy
Wavelength ( λ )
Intensity ε
Maximum absorption intensity = εmax
100 150 200 250 300 350 400
6
5
4
3
2
1
0
3.Hyperchromic shift
4.Hypochromic shift

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U V -Spectroscopy
1.Bathochromic shift (Red shift)
2.Hypsochromic shift (Blue
shift)
Wavelength ( λ )
Intensity
E-max
Wavelength of maximum absorption
4.Hypochromic shift
100 150 200 250 300 350
U V -Spectroscopy
Different terms used in UV Spectroscopy
1.Chromophore – It is an isolated functional group
capable of absorbing visible and or ultraviolet
region.
2.It is unsaturated group responsible for electronic
absorption
Molecule which has chromospheres is called as
chromogen.
e.g. C=C, C=O, -NO2, -C=N-, -CN, -COOH, -CONH2 etc.
2. Auxochrome – It is functional group which do not
absorb color above 200nm but when attached to
chromophore causes shift in wavelength and or
intensity of absorption.
e.g. –OH, -X, -OR, -NH2
U V -Spectroscopy
Different terms used in UV Spectroscopy
1.Bathochromic shift (Red shift) – Shift of
absorption maxima to longer wavelength is
called as bathochromic shift or red shift. Here
λmax increases.
This is due to presence of auxochrome such as –
OH, -NH2, etc.
Decrease in solvent polarity causes red shift.
e.g. from alcohol to benzene
When there is extension in conjugation the red
shift is observed e.g. ethyene absorbs at 170nm
where as 1,3-butadiene absorbs at 217nm
U V -Spectroscopy
2.Hypsochromic shift (Blue shift) – Shift of
absorption maxima to shorter wavelength is called
as hypsochromic shift or Blue shift.
Here λmax decreases
Generally change in pH of medium causes this shift.
Aniline in absence of any solvent absorbs at 280 nm
but when HCl is added to it red shift is observed
and it absorbs at 200nm
Phenol in basic medium due to formation of
phenoxide ion has extension in conjugation and
absorbs at higher wavelength but when medium
terns acidic the conjugation is removed as ion is
converted to undissociated phenol and red shift in
absorption is observed.
4.Hypochromic shift
U V -Spectroscopy
3.Hyperchromic shift – Increase in the intensity
of absorption maxima is called as hyperchromic
shift. E-max increase Generally electron donating
effect causes this shift
4.Hypochromic shift - Decrease in the intensity
of absorption maxima is called as hyporchromic
shift. E-max Decrease. Disturbance in conjugation
causes this shift
U V -Spectroscopy
1.Bathochromic shift (Red shift)
2.Hypsochromic shift (Blue shift)
Wavelength ( λ )
Intensity E-max
4.Hypochromic shift

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U V –Spectroscopy – Effect of conjugation
Effect of conjugation on position of UV and
visible absorption bands.
n to π * Transition (R band) is the most easy to
detect in normal UV spectrum
Presence of conjugated chromospheres (C=C)
(alternate double and single bond) causes
bathochromic or red shift.
If two chromophores are separated by more
than two single bonds then conjugation is
disturbed and no red shift is observed.
e.g. CH2=CH2 - λmax is 175 nm
CH2=CH2-CH2-CH2=CH2 λmax is 175 nm
Energy
P
P
P
π*
π
P ΔE1
ΔE2
ΔE1 ΔE2
π1
π1*
π2
π2*
π3
π3*
a)Ethene CH2=CH2 has - λmax is 175 nm also 1,4-
pentadiene also has CH2=CH2-CH2-CH2=CH2 λmax
is 175 nm as the conjugation is disturbed and
thus there is no effect of presence of two
chromophores.
b)If the chromophores are present in
conjugation the red shift is observed.
Ethene - CH2=CH2 has - λmax is 175 nm
1,3-butadiene
CH2=CH-CH=CH2 - λmax is 217 nm
1,3,5-hexatriene
CH2=CH-CH=CH-CH=CH2 - λmax is 256 nm
More number of conjugated double bonds lead
to more overlap of HOMO and LUMO and thus
energy difference between the orbitals
decreases leading to absorption at higher
wevelength. Decreasing in energy leads to
increase in wavelength.
c) It two different chromophores are present
e.g. C=C and C=O, e.g.
CH2=CH2 has - λmax is 175 nm
CH2=O has - λmax is 290 nm
Where as CH2=CH-CH2=O has λmax is 320 nm
Thus conjugation increases λmax to higher value.
Applications of U V -Spectroscopy
1.Determination of structure - UV spectroscopy
provides information mainly about the
extension in conjugation.
Though not useful for complete structure
elucidation it provides key information about
different types of electron present in molecule.
If UV spectrum of molecule is available then it
can be compared with unknown spectrum of
identification.
2.Determination of stereochemistry of
compounds – UV spectrum is useful in
determination of stereochemistry of molecules
specially compounds exhibiting cis-trans
isomerism.
The trans conformation has planar structure and
hence the conjugation is not disturbed. In these
molecules thus λmax – Has higher value. Where
as in case of cis isomer the planar nature is
disturbed and thus orbital overlap is not
completely effective resulting in lowering of λmax

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This can be confirmed by two examples where
two isomers cis and trans isomers have different
λmax values.
Cis -stilbene has λmax value at 280 nm and ἐ-max
at 13500 where as trans -stilbene has λmax value
at 295 nm and ἐ-max at 27000.
Cis –cinnamic acid has λmax value at 264 nm and
ἐ-max at 10700 where as trans –cinnamic acid
has λmax value at 273 nm and ἐ-max at 15900.
H H H
H
Cis -stilbene
labda max = 280 nm
e-max = 13500
Trans -stilbene
labda max = 295 nm
e-max = 27000
H H
COOH
H
H
COOH
Cis -Cinnamic acid
labda max = 264 nm
e-max = 10700
Trans -Cinnamic acid
labda max = 273 nm
e-max = 15900
U V –Spectroscopy-Woodward-Fieser Rules for
calculations of λmax
Woodward-Fieser rules for calculation of λ max value(nm)
for dienes and trienes
Sr.No. Name of the
Motif
Skeletal structure λ max
value(nm)
1 a Basic value for
Un-substituted
diene
214
1 b Basic value for
Conjugated diene
214
C C C C
C C
C C
C C
Sr.No. Name of the
Motif
value(nm)
1 c Basic value for
Hetero-
annular diene
214
1 d Basic value for
semi cyclic
diene
214
Sr.No. Name of the
Motif
value(nm)
1 e Basic value for
homo-annular
diene
253
2 a Addition for
extended
conjugation
[for every –C=C-]
+ 30 nm
Diene - C C
Sr.No. Name of the Motif Skeletal structure λ max
value(nm)
2 b Addition for
exocyclic double
bond
+5
2 c Addition for
exocyclic double for
two ring systems at
the same time
+10

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value(nm)
2 c Addition for
two ring systems at
the same time
+10
3 a Addition for
substitution (ring
residue Alkyl group
-R)
+ 5 nm
CH3
, C2
H5
, C3
H7
etc.
R =
C C C C
R
value(nm)
3 b Addition for
substitution –OR
alkoxy group
+ 5 nm
3 c Addition for
substitution –
acyloxy + O nm
CH3, C2H5, C3H7 etc.
R =
C C C C
OR
CH3, C2H5, C3H7 etc.
R =
OCOR
C C C C
value(nm)
3 d Addition for
substitution –
Halogens
+ 5 nm
3 e Addition for
substitution – N-R
(N-Alkyl) + 60 nm
X
X = Cl, Br
C C C C
NR2
C C C C
value(nm)
3 f Addition for
substitution – S-R
(S-Alkyl)
+ 30 nm
SR
C C C C
Woodward-Fieser rules for calculation of λ max value(nm) of α-β
unsaturated ketones and aldehydes
value(nm)
1 a Basic value for
acyclic ketone
215
1 b Basic value for six
member ring
ketone 215
O
C C
C O
value(nm)
1 c Basic value for five
member ring
ketone
202
1 d Basic value for α-β
unsaturated
aldehydes 207
O
C C
C O
H

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10
value(nm)
2 a Addition for
extended
conjugation
[for every –C=C-]
+ 30 nm
2 b Addition for
exocyclic double
bond
+ 5 nm
Diene - C C
value(nm)
2 c Addition for
two ring systems at
the same time
+ 10 nm
2 d Addition for
two ring systems at
the same time
+ 10 nm
value(nm)
2 d Addition for homo
annular component
+ 39 nm
Different positions δ ϒ β α
-C=C-C=C-C=O
OR
Substituent at position α β ϒ δ
3 a Addition for substitution –R,
Alkyl group
10 12 18 18
3 b Addition for substitution –OR,
alkoxy group
35 30 17 31
3 c Addition for substitution –
OCOR, acyloxy group
6 6 - 6
3 d Addition for substitution –-OH,
hydroxyl
35 30
-
50
3 e Addition for substitution –
Halogens -Cl (chloro)
15 12 - -
3 f Addition for substitution –
Halogens -Br (Bromo)
25 30 - -
3 g Addition for substitution – N-R
(N-Alkyl)
- 95 -
-
3 h Addition for substitution – S-R
(S-Alkyl) -
85 - -
Calculations of λmax of Dienes and Enones
Diene
C C C C
H
H
H H
H
H
H R
C C C C
H H
H
R R
R
R
R
C C C C
R R
R
R
R
R
R
R
R
R
Enone
Conjugated aldehyde
C C
C O
R
H
R
H O
R
R
C C
C O
H
R
H
R
O
R
R

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11
Structure Component/s Value
in nm
1 Basic value for acyclic diene = 214 nm
Alkyl substituents (2 X 5) = 10 nm
Total = 224 nm
Extended conjugation (1 X 30) = 30 nm
Total = 244 nm
Total = 229 nm
EC
in nm
Total = 259 nm
Exhocyclic double bond (1 X 5) = 5 nm
Total = 234 nm
EC
EDB
in nm
6 Basic value for acyclic diene =214 nm
Alkoxy substituent (1 X 6) = 6 nm
Total =235 nm
7 Basic value for homoammular
diene
=253 nm
Total =303 nm
OMe Alkoxy
EDB
EDB
EC
in nm
8 Basic value for homoammular diene = 253 nm
Hylogen substituents (1 X 5) = 5 nm
Total = 308 nm
S-Alkyl substituents (1 X 5) = 30 nm
Total = 348 nm
Cl
Halogen
EC
SCH3
EC
EDB
EDB
S-Alkyl
in nm
Halogen substituents (1X 5) = 5 nm
Total = 338 nm
Br
EC
EC
EDB
Halogen
Calculations of λmax of Dienes and aldehydes
in nm
1 Basic value for acyclic enone = 215 nm
Alkyl substituent on α-C (1 X 10) = 10 nm
Total = 225 nm
Alkyl substituent on β-C (1 X 10) = 12 nm
Total = 257 nm
O
O
EC

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12
Structure Component/s Value in
nm
3 Basic value for α-β unsaturated
aldehyde
= 207 nm
Total = 241 nm
4 Basic value for α-β unsaturated
aldehyde
= 207 nm
Alkyl substituent on ϒ-C (1 X 18) = 18 nm
Cl on ϒ-C (1 X 0) = 0 nm
Total = 270 nm
O
O
Cl
EC
in nm
Alkyl substituent on δ-C (1 X 18) = 18 nm
Alkoxyl substituent on δ-C (1 X 31) = 18 nm
Total = 304 nm
6 Basic value for five member enone = 202 nm
Br - substituent on β-C (1 X 18) = 30 nm
Total = 242 nm
O
OMe
EC
Alkoxy
EDB
O
Br
nm
7 Basic value for six member enone = 215 nm
Homo annular component (1 X 39) = 39 nm
HO - substituent on β-C (1 X 30) = 30 nm
Total = 350 nm
Total = 349 nm
O
OH
O
EC
HAD
nm
9 Basic value for five member enone = 202 nm
Alkyl substituent on β -C (1 X 12) = 12 nm
Total = 311 nm
Total = 273 nm
O
EC
O
EDB
EC

06. UV Spectroscopy of Organic Compounds.pdf

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