Q-band ESR studies on the origin of magnetic anisotropy in tetrahalogenated semiquinones

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Abstract
The g‐tensor components associated with tetrachloro‐, tetrabromo‐, and tetraiodo‐benzosemiquinones in frozen solutions of dimethylsulfoxide and ethanol‐D at 135 K have been measured by ESR spectroscopy at Q band (35 GHz). Spin‐orbit interactions on the halogens make a significant contribution to the g tensor; components in the plane of the aromatic ring, (gx and gy) are enhanced, whereas the component perpendicular to the ring (gz) is decreased. Related solvent effects have been observed in the spectrum of tetraiodobenzosemiquinone; in going from dimethylsulfoxide to ethanol gx and gy were found to increase, whereas gz decreased. It is argued that the positive contribution to gx and gy arises from the participation of halogen pz orbitals in the molecular π orbitals and that the negative contribution to gz in part reflects spin density on the halogens due to σ–π interaction induced configurational admixture in the wave function. Possible participation of halogen d orbitals is discussed. The dominant part of the solvent dependence of the g tensor components is suggested to result from changes in the energy separation between the orbital of the unpaired electron and lone‐pair orbitals on the halogens.