Baird's rule

In organic chemistry, Baird's rule estimates whether the lowest triplet state of planar, cyclic structures will have aromatic properties or not. The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird at the University of Western Ontario in 1972.^[1][2]

The lowest triplet state of an annulene is, according to Baird's rule, aromatic when it has 4n π-electrons and antiaromatic when the π-electron count is 4n + 2, where n is any positive integer. This trend is opposite to that predicted by Hückel's rule for the ground state, which is usually the lowest singlet state (S₀). Baird's rule has thus become known as the photochemical 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]

See also

• Möbius–Hückel concept
• Möbius aromaticity

References

1. Baird, N. Colin (1972), "Quantum organic photochemistry. II. Resonance and aromaticity in the lowest ³ππ* state of cyclic hydrocarbons", Journal of the American Chemical Society, 94 (7/12): 4941–4948, doi:10.1021/ja00769a025
2. Ottosson, Henrik (2012), "Organic photochemistry: Exciting excited-state aromaticity", Nature Chemistry, 4 (12): 969–971, Bibcode:2012NatCh...4..969O, doi:10.1038/nchem.1518, PMID 23174974
3. Rosenberg, Martin; Dahlstrand, Christian; Kilså, Kristine; Ottosson, Henrik (2014-05-28). "Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations". Chemical Reviews. 114 (10): 5379–5425. doi:10.1021/cr300471v. ISSN 0009-2665. PMID 24712859.