Magnetic circular dichroism of the triphenylcarbenium ion and some symmetrically para-substituted derivatives

Abstract

Polarization and MCD measurements indicate that in solution the triphenylcarbenium cation (φ₃C⁺) has D ₃ symmetry. The magnetic moment μ in the lowest excited singlet state of φ₃C⁺, which is of E symmetry, is large and positive. Its sign is opposite to that in other aromatic systems of high symmetry and at variance with predictions from PPP-SCF-CI theory. Upon the introduction of three identical para-substituents the magnetic moment decreases. It is shown that the large value of μ in the lowest E state of φ₃C⁺ arises from the high local symmetry of the individual phenyl rings and that its sign results from the interaction between the rings. The quenching of the magnetic moment upon substitution is described in terms of a simple inductive perturbation.