The Base Strength of Diradicals

A diradical is likely to be more polarizable and a stronger base than the corresponding singlet state. If so, reaction with an acid should shift the equilibrium enough for the diradical to be detectable in otherwise diamagnetic substances. This seems to be the case for the substances shown below, in which paramagnetic resonance absorption indicates the presence of the diradical conjugate acid to the extent 

Bianthrone and thianthrene also dissolve in aluminium chloride and ether to give red solutions exhibiting paramagnetic resonance. 

The marked shift in the singlet-triplet equilibrium produced by sulfuric acid and aluminium chloride is important because it undermines one of the commonly applied criteria for polar mechanisms we can no longer be quite certain that an acid-catalyzed reaction is a polar one on the basis of the catalysis alone. Fortunately, the radical mechanism can often be ruled out on other grounds. 

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