Spin of Electrons

When an electron is excited to a higher energy orbital, it can either have the same spin as the remaining electron in the original HOMO or it can have the opposite. The same spin is known as parallel and the opposite known as antiparallel. Each electron has spin = 1/2 so depending on which way the excited electron orients itself there will be either a zero net spin or a non-zero net spin.

Parallel: electrons have same spin: 1/2 + 1/2 = 1 = S (total spin)

Antiparallel: electrons have opposite spin: 1/2 - 1/2 = 0 = S

There is a formula, 2S+1, which is applied to the total spin to give the multiplicity.

Parallel: 2S+1 = (2x1)+1 = 3 = Triplet state

Antiparallel: 2S+1 = (2x0)+1 = 1 = Singlet state.

There is a rule that states that the spin of an electron can't change during an electronic transition i.e. DS = 0. Therefore, singlet to singlet and triplet to triplet transitions are allowed but singlet to triplet is spin forbidden.