Two-state model of antiaromaticity: The triplet state. Is Hund's rule violated?

A theoretical and computational study of the lowest lying triplet state df cyclic hydrocarbons having an even number (2n) of pi electron bonds (antiar omatic compounds) is presented. In these systems, the ground singlet state of the most symmetric structure is distortive, being a transition state for the reaction exchanging two bond-alternating structures. As a resonance hy brid of two equivalent valence bond (VB) structures, this singlet is a stab ilized biradical of B-1g symmetry. The lowest lying triplet of the most sym metric form is strongly bound, similar in geometry to the 1(1)B(1g) singlet transition state, and is always higher in energy. The energy difference be tween the two states is remarkably constant regardless of the ring size. Th is apparent violation of Hund's rule is derived from the symmetry propertie s of the system. The triplet state is treated as a resonance hybrid of n eq uivalent covalent structures, each having n - 1 singlet electron pairs and one pair of two spin parallel electrons (triplet pair); part of the exchang e resonance stabilization is lost in the triplet, making the singlet more s table. Thus, this effect is due to the difference between the static resona nce stabilization of the triplet and the singlet states. In contrast, Hund' s rule always holds for biradical systems having only one dominant VB struc ture. Spectroscopic observation of these biradical triplets is possible by photodetaching an electron from the monoanion, as recently demonstrated exp erimentally. The model predictions are confirmed computationally for severa l examples including H-4, H-8, cyclobutadiene, cyclooctatetraene, pentalene , and heptalene.