1. UV-VIS Spectroscopy By
Dr.Jagadeesh

2. Introduction
UV-VIS spectroscopy deals with electronic excitations and de-excitations of atoms and molecules
The UV region corresponds to 200nm -400nm
Energy corresponding to this region is kcal / mole
The Visible region corresponds to 400nm-800nm
Energy corresponding to this region is kcal/mole

3. Principle of UV-VIS spectroscopy
Electrons jump from lower to higher energy orbitals absorbing light and jump back emitting the absorbed light.
In atoms the electron excitations and de-excitations take place in atomic orbitals.
In molecules the electron excitations and de-excitations take place in molecular orbitals

4. Electrons in molecules
Some are in closed ( completed) shells- The excitation energy is very high-not in UV-VIS region
Some are in molecular orbitals. The orbitals are of three types- Sigma(σ), Pi (π) and N (non-bonding)

5. Energy Profile of Molecular orbitals
Electrons in Sigma bonding orbitals require high energy –below 200nm
Electrons in Pi and Non bonding orbitals can get excited by absorbing energy in UV-VIS region.

6. Molecular orbitals
Sigma molecular orbitals associated with single covalent bonds ( Sigma bonds)
Pi- molecular orbitals associated with Pi-bond ( multiple bonds)
Non bonding orbitals associated with paired non bonding outermost shell. Provided by atoms like S, N, O and Halogens.

7. Molecular Orbitals Bonding Orbitals Antibonding orbitals
Non bonding orbitals

8. Allowed Excitations
Sigma to Sigma *
Pi to Pi *
N to Pi*
N to Sigma*