Inorganic chemistry,Baird's rule estimates whether the lowesttriplet state ofplanar,cyclic structures will havearomatic properties or not. Thequantum mechanical basis for its formulation was first worked out byphysical chemistN. Colin Baird at theUniversity of Western Ontario in 1972.[1][2]
The lowest triplet state of anannulene is, according to Baird's rule, aromatic when it has 4nπ-electrons andantiaromatic when the π-electron count is 4n + 2, wheren is any positiveinteger. This trend is opposite to that predicted byHückel's rule for theground state, which is usually the lowestsinglet state (S0). Baird's rule has thus become known as thephotochemical analogue of Hückel's rule.[3]
Through various theoretical investigations, this rule has also been found to extend to the lowest lying singlet excited state (S1) of small annulenes.[3]
