Valence bond theory spin-coupled pairs

The choice of a single function from either set (36) or (37) does not permit such a useful physical interpretation, and may indeed lead to difficulties as the internuclear distance is varied. Thus if one chooses just the perfectly paired function from the set (36), as -R-> 00 one finds each N atom is described by a curious non-stationary state - the so-called valence state of the atom, about which there has been so much discussion in the literature.18 The choice of the set of functions (36) in which orbitals participating in a bond are directly coupled to each other is just the VB theory as proposed by Slater and Pauling,19 whereas the set (37) formed from atoms in specific L-S coupled states corresponds to the spin-valence theory employed by Heitler.20... 

Other theoretical treatments which have been applied to considerations of antiaromaticity include spin-coupled theory, topological methods,and quantum statistical definitions. The relevance of spin-coupled theory has been described The comparison between the (spin-coupled) descriptions of cyclobutadiene, benzene, and cyclooctatetraene clearly indicates that the reason for the lower stability and higher reactivity of antiaromatic systems is due to a simultaneous unfavorable coupling of the spins of all valence orbitals to triplet pairs, which discourages bonding interactions and suggests diradical character. ... 

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