Electronic Structure of a Stable Phenalenyl Radical as Studied by ESR/ENDOR, Paramagnetic NMR Spectroscopy and SQUID Measurements

Abstract

We have studied magnetic properties and electronic structures of a novel persistent neutral radical, 2,5,8-tri-tert-butylphenalenyl(2), which is the first example of alternant hydrocarbon π-radical isolated in the crystalline 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 = −2×10³ K) with traces of paramagnetic impurity molecules (0.3mol%). The π-spin density distribution of radical 2 was determined by liquid-phase cw-ESR, ¹H-ENDOR/TRIPLE, and paramagnetic ¹H-NMR measurements. The origin of the large inter-molecular antiferromagnetic interaction in the crystal 2 was explained by an occurrence of effective SOMO-SOMO overlap between the two radicals on the six equivalent α-positions with large positive spin densities of the carbon sites.

Key Words:

• persistent phenalenyl radical
• magnetic susceptibility
• ENDOR/TRIPLE
• paramagnetic NMR
• spin density distribution
• pancake bonding