The stabilities of ground state multiplicities in triradicals and their dependence on configuration and conformation

A series of triradicals derived from methine radical centers (CR) and phenylene coupling units in different configurations and conformations are compared using semiempirical AM1-CI calculations. It is found that the stability of high-spin ground states depends on both topology and conformation. It follows that branched 1,3,5-benzenetriyl triradicals do not always possess more stable quartet ground state than linear 1,3,1′,3′-phenylene bridged triradicals. A generalized structural rule is suggested that for each configuration (topology), small twisting between radical centers and coupling units (central part between two or more radical centers) favors a stable high-spin ground state.