organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, Baird's rule estimates whether the lowest

triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = � ...

of planar, cyclic structures will have

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

properties or not. The

quantum mechanic Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

al basis for its formulation was first worked out by physical chemist N. Colin Baird at the

University of Western Ontario The University of Western Ontario (UWO; branded as Western University) is a Public university, public research university in London, Ontario, Canada. The main campus is located on of land, surrounded by residential neighbourhoods and the Thame ...

in 1972. The lowest triplet state of an annulene is, according to Baird's rule, aromatic when it has 4''n'' π-electrons and antiaromatic when the π-electron count is 4''n'' + 2, where ''n'' is any positive

integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...

. This trend is opposite to that predicted by Hückel's rule for the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...

, which is usually the lowest

singlet state In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number s=0. A ...

(S₀). Baird's rule has thus become known as the photochemical analogue of Hückel's rule. Through various theoretical investigations, this rule has also been found to extend to the lowest lying singlet excited state (S₁) of small annulenes.

References

{{DEFAULTSORT:Bairds Rule Eponymous chemical rules Physical organic chemistry Rules of thumb

See also

References