UV-VIS Spectroscopy By Dr.Jagadeesh

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 150-72 kcal / mole The Visible region corresponds to 400nm-800nm Energy corresponding to this region is 72-36 kcal/mole

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

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)

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.

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.

Molecular Orbitals Bonding Orbitals Antibonding orbitals Non bonding orbitals

Allowed Excitations Sigma to Sigma * Pi to Pi * N to Pi* N to Sigma*