Electronic transitions

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UV-Vis spectroscopy
Electronic absorption spectroscopy
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spectroscopy

• The interactions of radiation and matter are
  the subject of the science called spectroscopy.

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Properties of Electromagnetic Radiation

• Electromagnetic radiation is a form of energy
  that is transmitted through space at enormous
  velocities.
• Electromagnetic radiation have properties of
  wavelength, frequency, velocity, and amplitude.
• In contrast to sound waves, light requires no
  supporting medium for its transmission thus, it
  readily passes through a vacuum.

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ELECTROMAGNETIC SPECTRUM
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Absorption and Emission
Absorption
Emission
Absorption A transition from a lower level to a
higher level with transfer of energy from the
radiation field to an absorber, atom, molecule,
or solid. Emission A transition from a higher
level to a lower level with transfer of energy
from the emitter to the radiation field.
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UV and Visible Spectroscopy

• Ultraviolet Spectroscopy involves the
  measurement of absorption of light in the visible
  and ultraviolet regions (visible region 400-800
  nmUV region 200-400 nm) by the substances under
  investigation.
• Since the absorption of light involves the
  transition from one electronic energy level to
  another with in a molecule, UV spectroscopy is
  also known as electronic spectroscopy.

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PRINCIPLE OF UV SPECTROSCOPY

• Absorption of visible and ultraviolet light
  produces changes in the electronic states of
  molecules associated with the excitation of an
  electron from a lower to a higher energy level.
• But it must be noted that each el electronic
  level in a molecule is associated with a no. of
  vibrational sub-levels and each vibrational
  energy level in turn is associated with a no. of
  rotational sub-levels.

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Appearance of broad bands and not sharp peaks in
the spectrum.

• Due to the mixing of vibrational and rotational
  changes with electronic changes in the molecules,
  there will be a large no. of possible transitions
  requiring only slightly different energies.
• As a result the absorption spectrum contains a
  large no. of lines which are too close together
  to be distinguished separately and are recorded
  in the form of broad bands in the spectrum
  obtained.

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Electronic excitation from the ground state to
the excited state accompanied By changes in
vibrational and rotational sub-levels.
Here E0Ground state

EExcited state.
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Beer-Lamberts Law

• B eer Law When a monochromatic light is
  passed through a substance dispersed in a
  non-absorbing solvent, the absorption of light is
  directly proportional to the molar concentration
  of the substance.
• Lambert s Law When a of monochromatic light is
  passed through a substance dispersed in a
  non-absorbing solvent, the absorption of light is
  directly proportional to the path length of the
  sample.
• B eer- Lambert s Law B y combining above two
  Laws," The absorption of light by a substance at
  a particular wavelength is directly proportional
  to the no. of molecules of the substance in the
  path of light.

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Thus log I0/I a c (Beer's
Law) log I0/I a l
(Lambert'Law)
log I0/I a cl (Beer Lamberts
Law)
log I0/I cl log
I0/I Optical Density or absorbance (A)


Therefore,
(A)log I0/I cl

Where- A Absorbance (optical density) I0
Intensity of light on the sample cell I
Intensity of light leaving the sample cell c
molar concentration of solute l length of
sample cell (cm)
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Limitation of Beer Lamberts Law

• High concentrations
• Solute and solvent form complexes
• Thermal equilibrium exist between the ground
  state and the excited state
• Fluorescent compounds are present in solution

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Components of instrumentation

• 1) Sources Argon, Xenon, Deuterium, or Tungsten
  lamps
• Deuterium Lamps-a truly continuous spectrum in
  the ultraviolet region is produced by electrical
  excitation of deuterium at low pressure.
  (160nm375nm)
• Tungsten Filament Lamps-the most common source of
  visible and near infrared radiation.
• 2) Sample Containers Quartz, Borosilicate,
  Plastic.

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3) Monochromators Quartz prisms and all
gratings, Used as a filter the monochromator
will select a narrow portion of the spectrum (the
band pass) of a given source Used in analysis
the monochromator will sequentially select for
the detector to record the different components
(spectrum) of any source or sample emitting
light. 4) Detectors which continuously measures
the intensity ratio of the beams transmitted
through the sample and the solvent respectively.
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Instrumentation
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General Instrument Designs Double Beam Space
resolved
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Presentation of the spectrum
p?p
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Origin of electronic spectra

• Absorptions of UV-vis photons by molecule results
  in electronic excitation of molecule with
  chromophore.
• The electronic transition involves promotion of
  electron from a electronic ground state to higher
  energy state, usually from a molecular orbital
  called HOMO to LUMO.

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UV-Vis light causes electrons in lower energy
molecular orbitals to be promoted to higher
energy molecular orbitals. HOMO LUMO
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Electronic transitions

• There are following types of electronic
  transition takes place in UV and visible region
• s s Transitions
• n s Transitions
• n p transitions
• p p transitions

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Vacuum UV or Far UV (? lt190 nm )
(Important in organic chemist)
UV/VIS
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Electronic transition

• s s Transitions Transitions in which a s
  bonding electron is excited to an antibonding s
  orbital are called s s transitions. These
  transitions are shown by only saturated
  hydrocarbons.
• For example, methane (which has only C-H bonds,
  and can only undergo s s transitions) shows an
  absorbance maximum at 125 nm. Absorption maxima
  due to s s transitions
• These wavelengths are lesser than 200 nm and
  fall in the vacuum UV region.

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n s Transitions

• n s Transitions Saturated compounds containing
  atoms with lone pairs (non-bonding electrons) are
  capable of n s transitions.
• These transitions usually need less energy than s
  s transitions.
• They can be initiated by light whose wavelength
  is in the range 150 - 250 nm.
• The number of organic functional groups with n
  s peaks in the UV region is small.
• E.g., CH3Cl.

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n p Transitions

• n p transitions These are the transitions in
  which an electron in a non bonding atomic
  orbital is promoted to an antibonding p orbital.
• Compounds having double bonds between
  heteroatoms, e.g., CO, CS, and NO.
• For e.g. ,the gtCO group of saturated aldehydes
  or ketones exhibit an absorption of low intensity
  at about 285 nm.
• These transitions require only small amounts of
  energy and takes place with in the range of
  ordinary UV spectrophotometer.
• These are generally forbidden transitions.

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p p Transitions

• p p transitions These are the transitions in
  which an electron in a p electron is promoted to
  an antibonding p orbital.
• These transitions require relatively higher
  amount of energy than n p transitions.
• For e.g. ,the gtCO group of saturated aldehydes
  or ketones exhibit an absorption of high
  intensity at about 180 nm.

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Selection Rules of electronic transition

• Electronic transitions may be classified as
  intense or weak according to the magnitude of
  emax that corresponds to allowed or forbidden
  transition
• 1)Allowed transitions These are transitions have
  emax 104 or more and probability of their
  occurrence is very high. These are generally due
  to p p transition
• For e.g., p p transition in 1,3-butadiene
  which shows absorption at 217 nm and emax 20900
  represents an allowed transition.

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Forbidden Transition

• These are usually related to n p transition.
• These are the transitions for which emax is
  generally less than 104 .
• For e.g., n p transition of a saturated
  aldehydes or ketones exhibit a weak absorption of
  low intensity near about 285 nm and having emax
  less than 100 is a forbidden transition..

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• Chromophores
• But the term chromophore was originally used to
  denote a functional group or some other
  structural feature, the presence of which imparts
  a colour to a compound.
• A functional group which exhibits absorption of
  electromagnetic radiations in the visible or
  ultraviolet region is called a chromophore
  (colour loving).

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Chromophore Excitation lmax, nm Solvent
CC p?p 171 hexane
CO n?pp?p 290180 hexanehexane
NO n?pp?p 275200 ethanolethanol
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AUXOCHROME

• An auxochrome is a group which itself does
  not act as a chromophore but when attached to a
  chromophore it shifts the absorption maximum
  towards longer wavelength along with an increase
  in the intensity of absorption.
  
  

NH2
Benzene
Aniline
?max 254nm max203 nm
? max 280 nm max1430 nm
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Change in position and intensity of absorption
1) Bathochromic shift or red shift It involves
the shift of absorption maximum towards longer
wavelength. It can be brought about by three
different methods as given below a) By an
attachment of an auxochrome to the
chromophore, b) By conjugation of two or more
chromophoric groups, c) By using solvent of lower
polarity.
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2)Hypsochromic shift or blue shift It involves
the shift of absorption maximum towards shorter
wavelength.It may be brought about as follows a)
By removal of conjugation in a system, c) By
using solvent of higher polarity.
3)Hyperchromic Effect This effect involves an
increase in the intensity of
absorption.It is brought about by introduction of
an auxochrome. 4)Hypochromic Effect This
effect involves a decrease in the intensity of
absorption. It is brought about by groups which
distort the geometry of the molecule.
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HYPERCHROMIC
HYPSOCHROMIC
BATHOCHROMIC
HYPOCHROMIC
ABSORBANCE
? max
?
Terminology of shifts in the position of an
absorption band
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Choice of solvents

• They need to be transparent and do not affect the
  fine structure arising from the vibrational
  effects

• Polar solvents generally tend to cause this
  problem

• Same solvent must be used when comparing
  absorption spectra for identification purpose.

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Solvents for the Ultraviolet and Visible regions
Solvent Lower wavelength limit (nm) Solvent Lower wavelength limit (nm)
Water 180 Diethyl ether 210
Ethanol 220
Hexane 195
Cyclohexane 195
Carbon tetrachloride 260
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Solvent effect
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Effects of solvents

• 1. Blue shift (n- p) (Hypsochromic shift)
• Increasing polarity of solvent ? better solvation
  of electron pairs (n level has lower E)
• ? peak shifts to the blue (more energetic)
• 30 nm (hydrogen bond energy)
• 2.Red shift (n- p and p p) (Bathochromic
  shift)
• Increasing polarity of solvent, then increase the
  attractive polarization forces between solvent
  and absorber, thus decreases the energy of the
  unexcited and excited states with the later
  greater
• ? peaks shift to the red
• 5 nm

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Effect of polar solvent and shift in n - p
transition
p
p
?E1
n
?E2
Non- polar solvent
n
Polar solvent
?E1
?E2gt
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Effect of polar solvent and shift in p - p
transition
p
?E1
p
?E2
p
Non- polar solvent
p
Polar solvent
?E1gt
?E2
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Woodward-Fieser Rules

• This quantification is referred to as the
  Woodward-Fieser Rules which we will apply to
  three specific chromophores
• Conjugated dienes
• Conjugated dienones

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Woodward-Fieser Rules Dienes
The rules begin with a base value for ? max of
the chromophore being observed acyclic
butadiene 217 nm The incremental contribution
of substituents is added to this base value from
the group tables
Group Increment
Extended conjugation 30
Each exo-cyclic CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
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Woodward-Fieser Rules - Dienes

• Isoprene - acyclic
  butadiene 217 nm
• 
  one alkyl subs. 5 nm
  
• 
  222 nm
• Experimental
  value 220 nm
• Allylidene cyclohexane
• -
  acyclic butadiene 217 nm
• one
  exocyclic CC 5 nm
• 2 alkyl
  subs. 10 nm
• 
  232 nm
• 
  Experimental value 237 nm

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There are two major types of cyclic dienes, with
two different base values Heteroannular
(transoid) Homoannular (cisoid) e 5,000
15,000 e 12,000-28,000 base ?max 214
base ?max 253 The increment table
is the same as for acyclic butadienes with a
couple additions
Woodward-Fieser Rules Cyclic Dienes
Group Increment
Additional homoannular 39
Where both types of diene are present, the one with the longer l becomes the base
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Woodward-Fieser Rules Cyclic Dienes

• In the pre-NMR era of organic spectral
  determination, the power of the method for
  discerning isomers is readily apparent
• Consider abietic vs. levopimaric acid

abietic acid
levopimaric acid
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• Woodward-Fieser Rules Cyclic Dienes

heteroannular diene 214 nm 4 alkyl subs. (4 x
5) 20 nm 1 exo CC 5 nm 239 nm
homoannular diene 253 nm 4 alkyl subs. (4 x
5) 20 nm 1 exo CC 5 nm
278 nm
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• Woodward-Fieser Rules Cyclic Dienes
• Be careful with your assignments three common
  errors

This compound has three exocyclic double bonds
the indicated bond is exocyclic to two rings
This is not a heteroannular diene you would use
the base value for an acyclic diene
Likewise, this is not a homoannular diene you
would use the base value for an acyclic diene
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Woodward-Fieser Rules - Enones
Group Increment
6-membered ring or acyclic enone Base 215 nm
5-membered ring parent enone Base 202 nm
Acyclic dienone Base 245 nm

Double bond extending conjugation 30
Alkyl group or ring residue a, b, g and higher 10, 12, 18
-OH a, b, g and higher 35, 30, 18
-OR a, b, g, d 35, 30, 17, 31
-O(CO)R a, b, d 6
-Cl a, b 15, 12
-Br a, b 25, 30
-NR2 b 95
Exocyclic double bond 5
Homocyclic diene component 39
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• Woodward-Fieser Rules Enones
• Aldehydes, esters and carboxylic acids have
  different base values than ketones

Unsaturated system Base Value
Aldehyde 208
With a or b alkyl groups 220
With a,b or b,b alkyl groups 230
With a,b,b alkyl groups 242

Acid or ester
With a or b alkyl groups 208
With a,b or b,b alkyl groups 217
Group value exocyclic a,b double bond 5
Group value endocyclic a,b bond in 5 or 7 membered ring 5
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• Woodward-Fieser Rules Enones
• cyclic enone 215
  nm
• 2 x b- alkyl
  sub (2 x 12) 24 nm
• 239 nm
• Experimental value
  238 nm
• cyclic enone 215 nm
• extended conj. 30 nm
• b-ring residue 12 nm d-ring
  residue 18 nm exocyclic double bond 5 nm
• 280 nm
• Experimental 280 nm

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