Electronic structure of a stable phenalenyl radical as studied by ESR/ENDOR, paramagnetic NMR spectroscopy and SQUID measurements

We have studied magnetic properties and electronic structures of a novel pe rsistent neutral radical, 2,5,8-tri-tert-butylphenalenyl(2), which is the f irst example of alternant hydrocarbon pi-radical isolated in the crystallin e state. The observed magnetic susceptibilities of polycrystalline sample 2 were reproduced by assuming the thermal equilibrium between singlet ground state and excited triplet state of the dimer (2J/k(B)= -2x10(3)K) with tra ces of paramagnetic impurity molecules (0.3mol%). The pi-spin density distr ibution of radical 2 was determined by liquid-phase cw-ESR, H-1-ENDOR/TRIPL E, and paramagnetic H-1-NMR measurements. The origin of the large inter-mol ecular antiferromagnetic interaction in the crystal 2 was explained by an o ccurrence of effective SOMO-SOMO overlap between the two radicals on the si x equivalent alpha-positions with large positive spin densities of the carb on sites.