The concepts of aromaticity, antiaromaticity, and Dewar resonance energy are extended to the lowest mr* triplet state of conjugated hydrocarbons by comparing the bonding energy of triplets to the most stable biradical reference structure. Arguments based upon simple perturbation theory indicate that the rules for ground state aromaticity are reversed in the ~T T * state (4n rings display "aromatic" character whereas 4n + 2 systems display "antiaromaticity"). Semiempirical SCF-LCAO-MO calculations by the NNDO method confirm these predictions, and are used to predict transition, bonding, and stabilization energies for a wide range of triplets including those for cyclobutadiene and derivatives, cyclooctatetraene and derivatives, nonclassical polyenes, three-to sevenmembered rings containing exocyclic carbon atoms, benzenoid hydrocarbons, butalene, azulene, and cyclodecapentaene. The preference of certain hydrocarbon triplets for a completely planar rather than 90" twisted structure (such as methylenecyclopropene, fulvene, and heptafulvene) is analyzed by perturbation theory. The consequences of aromatic and antiaromatic character to the exothermicity of ortho addition are explored for several hydrocarbon triplets.he concepts of resonance energy and of aromaticity

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In the present paper, some of the general properties of the Dewar Resonance Energy definition are developed. In particular, the DRE value for a compound is shown to be independent of the numerical values used to bond energies, and the use of DRE in judging the aromaticity of organic molecules is illustrated.

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