UV spectroscopy explores electronic transitions within molecules, where absorbed ultraviolet or visible light promotes electrons to higher energy levels. These transitions involve the movement of electrons between different molecular orbitals, and the specific types of transitions depend on the molecule's structure and the presence of chromophores (light-absorbing groups). Common transitions include sigma-sigma* (σ-σ*), pi-pi* (π-π*), and n-pi* (n-σ*), each associated with characteristic absorption wavelengths.

1. UV VISIBLE SPECTROSCOPY
Prepared by
Ms. B. Poojitha
B.Pharm 4th
Year
Ratnam Institute of Pharmacy
Nellore
Andhrapradesh
Under the guidance of
Dr.M.Suchitra,
Professor & HOD,
Department of Pharmaceutical Chemistry &Analysis
Ratnam Institute of Pharmacy
Nellore
Andhrapradesh

2. UV Visible spectroscopy
INTRODUCTION
UV-Visible Spectroscopy (Ultraviolet-Visible Spectroscopy) is an analytical technique used to
measure the absorbance or transmission of ultraviolet or visible light by a substance.
When UV (200–400 nm) or visible (400–800 nm) light is passed through a sample, some
wavelengths are absorbed.
The amount of light absorbed depends on the molecular structure and concentration of the
compound

4. ELECTRONIC TRANSITIONS
electronic transitions refer to the excitation of electrons from lower
energy molecular orbitals to higher energy orbitals when a molecule
absorbs light. These transitions occur primarily in molecules with
conjugated systems or lone pair electrons.
TYPES OF ELECTRONS
π-Present in saturated compounds.
σ- present in un saturated compounds.
 n- these are non bonding electrons.

5. Types of electronic transitions
Electronic transitions refer to the movement of electrons from one energy level
to another within an atom or molecule.
There are several types of electronic transitions
1 .σ → σ* transition
2. n → σ* transition
3. n → π* transition
4. π → π Transition

7. 1.σ → σ* transition
- It Involve in the movement of an electron from a σ bonding orbital
to a σ* antibonding orbital.
- Typically occur at high energies (short wavelengths), often in the
vacuum ultraviolet region.
2.n → σ* transition
- Involve the promotion of an electron from a non-bonding (n) orbital
to a σ* antibonding orbital.
- Often observed in molecules with lone pairs of electrons, such as
those containing oxygen, nitrogen, or sulfur.
- n → π* transition
- - Involve the promotion of an electron from a π bonding orbital to a
π* antibonding orbital.- Often observed in molecules with conjugated
systems, such as alkenes, alkynes, or aromatic compounds.

8. - Involve the movement of an electron from a π bonding orbital
to a π* antibonding orbital.
- Often observed in molecules with conjugated systems, such as
alkenes, alkynes, or aromatic compounds.
4.π → π Transitions
3.n → π* transition
- Involve the movement of an electron from a π bonding orbital
to a π* antibonding orbital.
- - Often observed in molecules with conjugated systems, such as
alkenes, alkynes, or aromatic compounds.

9. Chromophores
Definition
Chromophores are groups of atoms within a molecule that absorb light in the
visible or ultraviolet (UV) region of the electromagnetic spectrum.
These groups are responsible for the color of a compound, as they absorb
certain wavelengths of light and reflect or transmit others.
Chromophores typically contain multiple bonds, such as double or triple bonds,
or aromatic rings, which are capable of absorbing light energy.
The energy absorbed by chromophores can lead to electronic transitions,
where electrons are promoted from a lower-energy state to a higher-energy
state.

10. Types of chromophores
1.Independent Chromophores
 It Can absorb light on their own
Eg: –NO₂, –C=O
2.Conjugated Chromophores
 It Need to be part of a conjugated system to absorb visible light
Eg: –C=C–C=C–

11. Auxochromes
An auxochrome is a functional group that does not itself absorb visible light but,
when attached to a chromophore, it absorbs light .
Auxochromes typically contain lone pairs of electrons, such as -OH, -NH2, or -
OR, which can interact with the π electrons of the chromophore.
This interaction can lead to a change in the energy levels of the chromophore,
resulting in a shift in its absorption spectrum.
Auxochromes can also influence the color of a compound by enhancing or
modifying the absorption of light by the chromophore.

12. Types of auxochromes
1.Electron-donating groups:
These are groups that can donate electrons to the chromophore,
Eg: such as -OH, -NH2, or -OR.
2. Electron-withdrawing groups:
Although not typically considered auxochromes in the traditional sense, some
electron-withdrawing groups can influence the absorption spectrum of a
chromophore.
 Eg: Hydroxyl (-OH)- Amino (-NH2)- Alkoxy (-OR)- Alkylamino (-NHR or -NR2)

13. Spectral shifts
Spectral shifts refer to changes in the position of spectral
lines or bands in a spectrum, often due to changes in the
molecular or atomic environment.
These shifts can provide valuable information about the
structure, interactions, and dynamics of molecules.

15. Types of spectral shifts
• Bathochromic Shift (Red Shift)
• Hypsochromic Shift (Blue Shift):
• Hyperchromic Shift
• Hypochromic Shift

16. 1.Bathochromic Shift (Red Shift):
A shift to longer wavelengths (lower energy) in a spectrum,
often due to increased conjugation, solvent effects, or
changes in molecular structure.
2.Hypsochromic Shift (Blue Shift):
A shift to shorter wavelengths (higher energy) in a spectrum,
often due to decreased conjugation, changes in solvent
polarity, or alterations in molecular structure.

17. 1.Hyperchromic Shift:
An increase in the intensity of a spectral band, often due
to increased transition probability or changes in molecular
structure.
2.Hypochromic Shift:
A decrease in the intensity of a spectral band, often due
to decreased transition probability or changes in molecular
structure..

18. Solvent effect on absorption spectra
Solvent Effects on Absorption Spectra The absorption spectrum of
a molecule can be influenced by the solvent in which it is dissolved.
Solvent effects can cause changes in the position, intensity, and
shape of absorption bands.
Types of Solvent Effects:
Polarity Effects
Hydrogen Bonding Effects
Dispersion Effects

19.  Polarity Effects:
- Polar solvents can interact with polar or charged molecules,
causing shifts in absorption bands.
- For example, a polar solvent can stabilize a charge-transfer
state, leading to a bathochromic shift (red shift).
 Hydrogen Bonding Effects:
- Solvents that can form hydrogen bonds with the solute can
cause changes in absorption spectra. Hydrogen bonding can
either stabilize or destabilize certain electronic states, leading
to shifts in absorption bands.

20.  Dispersion Effects:
Non-polar solvents can interact with non-polar
molecules through dispersion forces, causing changes
in absorption spectra.

21. Beer–Lambert Law
Definition:
The Beer–Lambert Law states that the absorbance (A) of a solution is
directly proportional to both the concentration (c) of the absorbing
species and the path length (l) of the sample.
Mathematical Expression:
A =Ɛc l

22. A=Absorbance (no unit)
Ɛ=Molar absorptivity (L·mol⁻¹·cm⁻¹)
C=Concentration of the solution (mol/L)
l=Path length of the cuvette (cm)