Triradicals.

By using novel spin-flip approach, we characterized electronic structure of several prototypical triradicals. Interaction of unpaired electrons in triradicals results in unusual bonding patterns. We determined structural, spectroscopic, and thermochemical consequences of the partial bonds formed between the radical centers. The strength of these bonds varies from a rather weak (but stabilizing) interaction of about 0.5 kcal/mol up to 32 kcal/mol, which is one-third of a typical chemical bond energy. The structural signature of these bonds is shorter distances between the radical centers relative to the closed-shell precursors. Depending on the nature of triradical's molecular orbitals, the ground state in these species varies from quartet, to closed- or open-shell doublet.