Orbital Spin States

•For some of the excited states, there are states with a pair of electrons having their spins parallel (in 2 orbitals), leading to total spin of 1 and multiplicities of 3.

lSinglet state (S):Most molecules have ground state with all electron spin paired & most excited state also have electron spin all paired, even though they may be one electron each lying in two different orbital.

lSuch states have zero total spin and spin multiplicities of 1, are called singlet (S) states.

•For Triplet state (T): Under the influence of external field, there are three values (i.e. 3 energy states) of +1, 0, -1 times the angular momentum.

•Such states are called Triplet states (T).

Selection Rules of electronic transition

lElectronic transitions may be classed as intense or weak according to the magnitude of εmax that corresponds to allowed or forbidden transition as governed by the following selection rules of electronic transition:

lSpin selection rule: there should be no change in spin orientation or no spin inversion during these transitions.

lThus, S→S, T→T, are allowed,

lbut S→T, T→S, are forbidden.

l(△S=0 transition allowed)