Electronic structure of a stable phenalenyl radical in crystalline state as studied by SQUID measurements, cw-ESR, and 13C CP MAS NMR spectroscopy

A neat single crystal of phenalenyl radical was obtained for the first time by introducing the substituent group of tert-butyl. X-ray structure analysis of the crystal of 2,5,8-tri-tert-butylphenalenyl(2) showed that the two planar radicals face each other with inversion symmetry and are dimerized in the monoclinic crystal. This is supported by the magnetic susceptibility(chi) measurement of the crystalline solid state of 2. The observed temperature dependence of chi T was reproduced by assuming the thermal equilibrium between a singlet ground state and excited tripler stare of the dimer (2J/k(B)=-2000K) with traces of paramagnetic impurity molecules (0.3mol%). The electronic structure of the radical dimer in the crystalline state has been identified also by single-crystal ESR and C-13 CP/MAS NMR measurements. The origin of the large antiferromagnetic interaction is discussed in terms of both the experimentally determined pi-spin density distribution of monomer radicals and pi-MO calculations.