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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*