Baird's rule

In organic chemistry, Baird's rule estimates whether a lowest triplet state of planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird in 1972.

A lowest triplet state of cyclic ring molecule follows Baird's rule when the number of its π-electrons equals 4n where n is any positive integer. However, in most cases the lowest triplet state of cyclic ring has higher energy then the lowest singlet state of cyclic ring due to Jahn–Teller effect.