spectroscopyyyyyyyyyyyyyyyyyy Final.pptx

Uv visible spectroscopy
Also named as electronic spectroscopy.
CHEM-309

Submitted to:
sir Dr. Ghulam Mustafa

Submitted by :
Tazeem Basharat(22011507-020)
Esha Afzaal (22011507-036)
Meryam Munir (22011507-046)
Manahil Fatima (22011507-048)
Chem–309 organic chemistry (II)

01 03
02 04
Introduction to spectroscopy UV Visible spectroscopy
electromagnetic spectrum types of electronic transitions
TABLE OF CONTENTS

05 07
06 08
Electronic spectroscopy Effect of solvent on lamda max
transition probabilities
Formation of Absorption band
TABLE OF CONTENTS

1. Introduction
of spectroscopy and basic concept of UV visible
spectroscopy.

1. Introduction to spectroscopy
 Branch of science that involves study of interaction
of electromagnetic radiation with matter
 Study of the absorption and emission of light and
other radiation by matter.
 Spectroscopy deals with emission as well as
absorption spectra.
 An emission spectrum is produced by the
emission of radiant energy by an excited atom.
Reference: Spectroscopy by Y.R Sharma page no. 2-3

1.3 Electromagnetic radiation
 Also known as radiant energy.
 These are produced by the oscillation of electric
charge and magnetic field residing on the atom.
 The electric and magnetic components are
mutually perpendicular to each other and are
coplanar.
 These are characterised by their wavelengths or
frequencies or wavenumbers.
Reference: Spectroscopy by Y.R Sharma page number 1.

Electromagnetic radiation
 The energy carried by an electromagnetic radiation is
directly proportional to its frequency.
 The emission or absorption of radiation is quantised
and each quantum of radiation is called a photon.
 All types of radiations travel with the same velocity
and no medium is required for their propagation.
They can travel through vacuum.
 They travel with speed of light c(3x108
m/s ).
Reference: Spectroscopy by Y.R Sharma page no.1-5

 Wavelength
It is the distance between the two adjacent crests (C—C)
or troughs (T—T) in a particular wave. It is denoted by
the letter(lambda). It can be expressed in Angstrom units
or in millimicrons (m )
μ
 Wave number
It is the reciprocal of wavelength and it is expressed in
per centimeter. In other words, it is defined as the total
number of waves which can pass through a space of one
cm. It is expressed as v. Reference: Spectroscopy by Y.R Sharma page no. 4

 Frequency
It is defined as the number of waves which can pass through
a point in one second. It is expressed as (nu) in cycles per
second or in Hertz (Hz) where 1 Hz = 1 cycle sec –1
Frequency is inversely proportional to wavelength.
 Energy of a wave
Energy of a wave of the particular radiation can also be
calculated by applying the relation. E=hf
Reference: Spectroscopy by Y.R Sharma page number 1-5

Energy of wave
Energy of wave can be
calculated by the
formula given as:

Electromagnetic spectrum
 Arrangement of all types of electromagnetic
radiations in order of their increasing wavelengths or
decreasing frequencies is known as complete
electromagnetic spectrum.
 The visible spectrum (from violet to red through
rainbow colours) represents only a small portion of
the electromagnetic spectrum.
 Cosmic rays carry high energy while radiowaves are
least energetic. Reference: Spectroscopy by Y.R Sharma page no. 2-3

Electromagnetic spectrum
 Cosmic rays are used in treatment of cancer.
 Microwaves have larger wavelengths and are used in
telephone transmission.
 X-rays can pass through glass and muscle tissues.
 Radiowaves can pass through air and used in
transmission of signals of telephone and radars.
 Visible, Ultraviolet and Infra-red radiations are
used in spectroscopic techniques.

1.1 Absorption spectroscopy
 If electromagnetic radiations (of certain wavelength range) are
passed through the substance under analysis for sometime, then
radiations of certain wavelengths are absorbed by the substance.
 The wavelengths which are absorbed characterise some
particular functional groups present in the compound or the
compound itself.
 This dark pattern of lines which correspond to the wavelengths
absorbed is called Absorption spectrum.
 After absorption, the transmitted light is analysed by the
spectrometer relative to the incident light of a given frequency.

1.2 Emission spectroscopy
 An emission spectrum is produced by the emission of radiant energy
by an excited atom. The excitation of atoms can be brought about
thermally (by heating the substance strongly) or electrically (by
passing electric discharge through the vapours of the substance at a
very low pressure).
 Energy is absorbed and electrons in the ground state are promoted to
meta-stable states.
 Electrons from the meta stable state jump to the lower energy state,
then some energy of definite frequency is released as radiation.
 Radiation emitted is analysed with the help of a spectroscope, an
emission spectrum is observed.

2.
UV VISIBLE SPECTROSCOPY
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2. UV visible spectroscopy
 Absorption of electromagnetic radiation by an
organic sample in both UV(10-400nm) and visible
region(400-800nm).
 Alternate title for this technique is Electronic
Spectroscopy.
 Because involves the promotion of electrons (sigma,
pi and n* electrons) from the ground state to the
higher energy state.
Reference: Organic spectroscopy by Dr.Younas page no. 21

2. UV visible spectroscopy
 Detection of a functional group and impurities.
 to determine extent of conjugation in a system
 To differentiate aromatic system from non aromatic
systems
 To differentiate between cis and trans geometrical
isomers
 For quantitative analysis e.g single component and
multiple component analysis.

Regions of uv visible spectroscopy

2. What is exctinction coefficient?
 The extinction coefficient ( ) is a measure of how strongly
ε
a substance absorbs light at a specific wavelength.
 also known as molar absorptivity or molar extinction
coefficient
 The extinction coefficient is defined as the absorbance (A)
of a solution per unit concentration (c) and path length
(l)
= A / (c × l)
ε
 Units of extinction coefficient:
L mol^-1 cm^-1 (liters per mole per centimeter)

2. Properties of Emax
 Concentration-dependent:
max is directly proportional to the concentration of the
ε
substance.
 Solvent-dependent:
max can vary depending on the solvent used.
ε
 Temperature-dependent:
max can be affected by temperature changes.
ε
 Molecular structure-dependent:
max is influenced by the molecular structure of the
ε
substance.
Reference: Spectroscopy by Y.R Sharma page no. 21

2. Graph of absorption (Emax) and max
Λ

2. What is lambda max?
 max
Λ refers to the wavelength at which a substance
absorbs the most light, resulting in the maximum
absorption of radiation
 the wavelength at which the absorption spectrum of a
substance shows a peak,
 Different substances have unique max values, which can
Λ
be used to identify them.
 The intensity of absorption at max can be used to
Λ
determine the concentration of a substance.

2. Properties of max
Λ
 Unique to each substance:
Each substance has a unique and constant max value, which
λ
can be used to identify it.
 Wavelength of maximum absorption:
max is the wavelength at which a substance absorbs the most
λ
light.
 Independent of concentration:
max is independent of the concentration of the substance.
λ
 Dependent on solvent:
max can vary depending on the solvent used.
λ
 Dependent on temperature and pH:
max can vary slightly with temperature and pH changes.
λ

2. max independent of concentration
Λ

2.Why graph is like hump of a camel?
 Energy levels:
The energy levels of the molecule are not infinitely sharp, but rather have
a finite width. This width leads to a broadening of the absorption
spectrum, resulting in a hump-shaped curve.
 Molecular vibrations:
Molecular vibrations and rotations also contribute to the broadening of
the absorption spectrum. These vibrations and rotations lead to a range of
energy levels, resulting in a hump-shaped curve.
 Instrumental broadening:
The instrumentation used to measure the absorption spectrum can also
contribute to the broadening of the spectrum. This instrumental
broadening can result in a hump-shaped curve.

Chromophores
 Any isolated covalently bonded group that shows a characteristic
absorption in the ultraviolet or the visible region
 There are two types of chromophores :
 Chromophore-a
Chromophores in which the group contains  electrons and they
undergo to * transitions. Such as ethylenes, acetylenes etc.
 Chromophores-b
Chromophores which contain both electrons and n(non-bonding)
electrons. Such chromophores undergo two types of transitions i.e.,
to* and n to *Examples of this type are carbonyls, nitriles, azo
compounds, nitro compounds etc.

Auxochromes
 any group-which does not itself act as a chromophore but
whose presence brings about a shift of the absorption band
towards the red end of the spectrum (longer wavelength).
 The absorption at longer wavelength is due to the
combination of a chromophore and an auxochrome to give
rise to another chromophore.
 An auxochromic group is called colour enhancing group.
Some common auxochromic groups are —OH, —OR, —
NH2, —NHR, —NR2, —SH etc.
 a new chromophore results which has a different value of the
absorption maximum as well as the extinction coefficient.

Absorption and Intensity Shifts
 Bathochromic effect:
When the absorption maximum is shifted towards longer
wavelength due to the presence of an auxochrome or by
the change of solvent then the effect is called bathochromic
shift. Such shift towards longer wavelength is called red
shift.
 Groups like OH, CH3, OCH3 shows this effect.
 Example:
Benzene shows an absorption maximum at 255nm [Emax 203] whereas
aniline absorbs at 280nm [Emax 1430]. Hence, amino (—NH2) group is
an auxochrome.

 Hypsochromic shift:
When the absorption wavelength is shifted toward the shorter
wavelength(blue shift) then the effect is called hypsochromic
shift. This is commonly known as blue shift. The absorption
maximum of aniline is 280nm whereas in acidic medium it is
shifted to 203 nm.
.

Hyperchromic shift:
When the intensity of the absorption maximum is increased the effect is called
Hyperchromic shift. i.e. Emax increases. The incorporation of an auxochrome in
an organic molecule usually increases the intensity of absorption and hence
exhibits hyperchromic shift.
Pyridine(257nm with Emax 2750.) 2,methyl pyridine (262nm with Emax 3560)

 Hypochromic shift:
It is the shift where intensity of absorption maximum
decrease. When the geometry of a molecule is distorted due
to the introduction of group this Type of shift may take place.
For example, biphenyl absorbs at 250nm with Emax 19000,
whereas 2-methyl biphenyl absorbs at 237 nm with value of
Emax 10,250.

UV active and inactive compounds
 It is not always necessary that the excitation of an electron takes place
from a bonding orbital or lone pair to an antibonding or nonbonding
orbital when a compound is exposed to UV or visible light.
Allowed transitions:
The transitions with values of (extinction coefficient) Emax, more than
104 are usually called allowed transitions. They generally arise due to *
transitions.
Example
1,3, Butadiene has the absorption at 217nm and Emax 21,000 is an
example of allowed transition.
 The compounds that can have such transitions are called UV active
compounds.

UV active and inactive compounds
Forbidden transition
is a result of the excitation of one electron from the lone pair
present on the heteroatom to an antibonding *orbital. n *
transition near 300 m in case of carbonyl compounds with
μ
Emax value between 10–100, is the result of forbidden
transition. The values of Emax for forbidden transition are
generally below 104 .
Example of benzophenone
The two types of transitions observed in this case are :
 252 nm Emax 20,000 (allowed)
 325 nm Emax 180 (forbidden).

Symmetry restrictions in Electronic transitions
 The transition (allowed or forbidden) is related with the geometries of
the lower and the higher energy molecular orbitals and also on the
symmetry of the molecule as a whole.
 Symmetrical molecules have more restrictions on their electronic
transitions than less symmetrical molecules.
 For example
 benzene is a highly symmetrical molecule. Thus, many restrictions
apply to the electronic transitions of the benzene molecule and thus,
its electronic absorption spectrum is simple.
 For unsymmetrical molecule, no symmetry restrictions apply to
the electronic transitions so that transitions may be observed among all
of its molecular orbitals except among filled orbitals, a complex
electronic absorption spectrum will result.

Symmetry restrictions in Electronic transitions
To decide whether the transition is allowed or forbidden for
such molecules, it is important to consider:
 the geometry of the molecular orbital in the ground state
 the geometry of the molecular orbital in the excited state
 the orientation of the electric dipole of the incident light
that might induce the transition.
NOTE:
The transition will be an allowed transition if the above three
factors have an appropriate symmetry relationship.

2.Sigma and pi bonds formation
Sigma ( ) Bond
σ
Formed when atomic orbitals overlap head-on, with the electron
density between the nuclei of the bonding atoms. Sigma bonds
are the strongest covalent bonds and are always the first bond to
form . The electrons participating in a bond are commonly
σ
referred to as electrons.
σ
Pi (π) Bond
Formed when atomic orbitals overlap side-to-side, with the
electron density above and below the nuclei of the bonding
atoms. Pi bonds are weaker than sigma bonds and often occur in
double and triple bonds.

2.Types of Electronic
Transitions

2.Electronic transitions
When a molecule is excited by the absorption of energy its
electrons are promoted from a bonding to an antibonding
orbital.
 The antibonding orbital which is associated with the
excitation of electron is called *† antibonding orbital. to *
transition takes place when electron is promoted to
antibonding orbital.
It is represented as to * transition.

2.Electronic transitions
 When a non-bonding electron ** (n) gets promoted to an
antibonding sigma orbital ( *), then it represents n to *
transition.
 * transition represents the promotion of electrons to an
antibonding orbital, * orbital.
 When an n-electron (non-bonding) is promoted to
antibonding orbital, it represents n to * transition.
 The energy required for various transitions obey the following
order  
 * > n to * > to * > n to *

Electronic excitation energies

Electronic excitation energy differences

2.1 to*transition
 It is a high energy process since bonds are, in general, very
strong.
 Organic compounds in which all the valence shell electrons
are involved in the formation of sigma bonds do not show
absorption in the normal ultra-violet region 180–400 m .
μ
 . For saturated hydrocarbons, like methane, propane etc.
absorption occurs near 150 m .
μ

2.1 to*transition
 The usual spectroscopic technique cannot be
used below 200 m , since oxygen begins to
μ
absorb strongly.
 Thus, the region below 200 m is commonly
μ
called Vacuum Ultraviolet region.
 For example:
 methane,ethane propane.

2.2 nto* transitions
 This type of transition takes place in saturated compounds
containing one hetero atom with unshared pair of electrons .
 Some compounds undergoing this type of transitions are
saturated halides, alcohols, ethers, aldehydes, ketones, amines
etc.
 Such transitions require comparatively less energy than that
required for * transitions.
 Water absorbs at 167 m, methyl alcohol at 174 m and
μ
methyl chloride absorbs at 169 m .
μ

2.2 nto* transitions
 In saturated alkyl halides, the energy required for transition
decreases with the increase in the size of the halogen atom.
compare nto * transition in methyl chloride and methyl iodide.
 Due to the greater electronegativity of chlorine atom, the n
electrons on chlorine atom are difficult to excite*. The
absorption maximum for methyl chloride is 172–175mμ
whereas that for methyl iodide is 258 .

2.3 * transitions
 This type of transition occurs in the unsaturated centres of
the molecule.
 In compounds containing double or triple bonds and also in
aromatics.
 The excitation of electron requires smaller energy and
transition of this type occurs at longer wavelength.
 A electron of a double bond is excited to * orbital.
 For example:
 Alkenes, alkynes, carbonyl compounds, cyanides, azo
compounds .

2.4 nto * transitions
 In this type of transition, an electron of unshared
electron pair on hetero atom gets excited to *
antibonding orbital.
 This type of transition requires least amount of energy
out of all the transitions discussed above and hence
occurs at longer wavelengths.

2.4 nto * transitions
For example:
 Aldehyde ,ketones,esters,carboxlic acids.
 Saturated aldehydes show both the types of
transitions, low energy n to * and high energy *
occuring around 290 m and 180 m respectively.
μ μ

• Absorption: process of taken in Bands:
range/limit
• Absorption bands: Specific ranges of
UV-visible light absorbed by molecule,
measured to study molecular
structure.
• Main point: Each molecule has unique
absorption bands like fingerprints
What are absorption bands?
Organic chemistry by L.G.D Yadav

●Here are some of the main points we will discuss for
the formation of absorption bands:
1-Absorption Mechanism
2-Why sharp bands are not formed?
3-Spectral overlap
Formation of absorption bands

1-ABSORPTION MECHANISM
During absorption mechanism a molecule absorbs
electromagnetic radiation (UV-VISIBLE light) and promtes an
electrons from it’s ground state to an excited state.
• This energy corresponds to a specific wavelength resulting in
absorption bands.
• ∆E=hc/wavelength
• Intensity of absorption: Corresponds to the number of
moleculs which absorb the radiation of that wavelength
ABSORPTION MECHANISM
Organic chemistry by Dr.Yunous

●Each electronic energy levels associated with
rotational and vibrational energy levels
●Promote electrons from E1toE2
●Large number of possible transitions
responsible for band formation
●Each of them from lower energy levels to
higher energy levels
Electronic transition
Vibrational transition
Rotational transition
Why sharp bands are not formed?
Organic chemistry by YR Sharma

.
• Infrared radiation undergoes vibrational transition
• Micro radiation undergoes rotational transitions
• UV radiation, due to higher energy than both can give electronic,
vibrational and rotational transitions as well. Organic chemistry by LG wade

Spectral overlap
Occurs when absorption bands of different molecule overlaps,
making it challenging to resolve individual bands.
This can happen due to:
• Same energy level
• Instrumental limitations
Resolved by using these techniques:
• Derivative spectroscopy
• Multivariate analysis
• Spectral deconvolution
Organic chemistry by L.D.S Yadav

Theory of electronic
spectroscopy

Ground singlet state: In ground
state spins of electrons in each
molecular orbital are essentially paired.
In excited state:
Case 1: Excited singlet state: Spin of
electrons are in paired form
Case2: Excited triplet state: Spins of
electrons are in parallel form
Main terms:
Organic chemistry by YR sharma

●Excited singlet state is always higher in
energy as compared to excited triplet state
●Excited Triplet state is always more stable
than excited singlet state
●In TES electrons are farther apart in space
thus electron-electron repulsion is
minimized
●Highly probable transition involve
promotion of electrons from HOMO to
LUMO and thus several transition occurs.
Comparison of both excited states
Organic chemistry by YR sharma

EFFECT OF SOLVENT
ON LAMBDA MAX
Name: Meryam munir Roll number: 22011507-046

WHAT IS SOLVENT?
A solvent is a liquid that dissolves another solid,
liquid, or gaseous solute, resulting in a solution at
specified temperature.
Solvents can be broadly classified into two
categories:
 Polar.
 Non-Polar.
Organic-Spectroscopy-and-Chromatography-by-M-Younas-Third-Edition.

WHAT IS SOLVENT?
Mixture of
sample and
solvent.

Why we use SOLVENT?
 To make solution clear.
 Concentrated solution will cause reflection of uv
and visible rays(according to beer lambert law).
 To make solution more dilute.
Absorption Concentration of
solution.
∞

CHOICE OF SOLVENT:
Benzene, chloroform , carbon tetrachloride cannot be
used because they absorb in the range of 240-280 nm.
It should not itself absorb radiations between 200-
800nm. So it should be saturated.
It should be less polar so that it has minimum interaction
with the solute molecules.
Pavia-Introduction-to-Spectroscopy.

EFFECT OF SOLVENT
 A drug may show varied spectrum at particular
wavelength in one particular condition but shall
absorb partially at the same wavelength in another
conditions.
 This variation depend upon
1.Nature of solvent
2.Nature of the solute

EFFECT OF SOLVENT
The solvent exerts a profound influence on the quality
and shape of spectrum.
 A drug may absorb a maximum radiation energy at
particular wavelength in one solvent but shall
absorb partially at the same wavelength in another
solvent.
For example:
 acetone in n-hexane lambda max at 279nm.
 Acetone in water lambda max at 264.5nm.

POLAR SOLVENT
 Polarity means unequal sharing of electron.
 Polar solvents have large dipole moments and
charge separation, and they can dissolve ions, other
polar materials.

POLAR SOLVENT
 So due to there lone pair and electronegative charge
they start forming solute solvent complex.
 Thus Non- Bonding electron Can form Solute
Solvent Complex through H- Bonding.
 Due to this reason sample show increase or
decrease in lambda max.
REASON OF FORMING SOLUT SOLVENT
COMPLEX
Pavia-Introduction-to-Spectroscopy

POLAR SOLVENT
Also It in Seen that the Polar Solvent Interacting with
Sample Molecule Diminished the Hyperfine Structure
of UV Spectra, that Mean peak Become Broader.
Lambda max
Absorption
Lambda max
Absorption
Hyperfine
Structure
(a) (b)

POLAR SOLVENT

POLAR SOLVENT
V I B G Y O R
Wavelength
Energy

POLAR SOLVENT
Due to polar solvent the energy difference between
bonding and antibonding orbital increase or decrease.
This increase and decrease depend upon the type of
transition.
Mostly;
 Pi-pi*
 n-pi*

POLAR SOLVENT
 The sample like C=O show both pi-pi* and n-pi*
transition.
 Due to solvent polarity band undergoes red shift.
 Since a pi* orbital is more polar than a n orbital
so it will be get stable due to polar solvent. As a
result, the energy difference between the two
states decreases.
Pi-Pi* Transition
..
..
..

Pi-Pi* Transition
E2
E1
Polar solvent
*
𝝅
𝝅
*
More polar

Pi-Pi* Transition
E2
E1
Polar solvent
 E1>E2
 Energy ∞
 Show the longer wavelength
 Known as red shift.
 Also known as Bathochromic
shift.
1/wavelength
*
𝝅
𝝅
*

Pi-Pi* Transition
 Because of the polar solvent the Pi* orbital got
stable.
 Due to which the energy difference between
these orbital decrease and show increase in the
lambda max wavelength.
 Because wavelength and energy has inverse
relation sho red shift.

POLAR SOLVENT
n-Pi* Transition
 A n-pi* transition shows a shorter wavelength. This
is because the nonbonding orbital is more stabilized
than the Pi*orbital, due to hydrogen bonding or
electrostatic interaction with a polar solvent.
In this transition the polar solvent will show Blue
shift means shorter wavelength.

n-Pi* Transition
E2
E1
Polar solvent
 E2>E1
 Energy ∞
 Show the shorter wavelength
 Known as Blue shift.
 Also known as Hypsochromic
shift.
1/wavelength
*
*
𝑛
𝑛

NON-POLAR SOLVENT
 Non polar solvent arise due to equal sharing of
electron.
 Due to equal sharing they do not have a dipole
moment or partial charges.
 So they not show any interaction with sample..

NATURE OF SOLVENT
Most commonly used solvent is 95% ethanol.
 It is cheap
 Has good dissolving power
 Does not absorbs radiations above 210nm.
Most commonly used solvent
is 95% ethanol.
 It is cheap
 Has good dissolving power
 Does not absorbs radiations
above 210nm.
Pavia-Introduction-to-Spectroscopy

IDEAL SOLVENT
 Solvent should not absorb UV light in the region
where our sample is absorbing.
 Solvent should not interact chemically with the
sample molecules, even small hydrogen bonds affect
the peak position and its width.
 If possible avoid the use of polar solvents.
 It should be less polar so that it has minimum
interaction with the solute molecules.

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