Substituent effects on the second ionization energies of hydroxy- and methoxy benzenes

The second ionization energies of phenol and the isomeric dihydroxybenzenes as well as their methyl ethers are determined by means of charge-stripping mass spectrometry. Compared to the parent molecule benzene, introduction of a hydroxy substituent lowers the second ionization energy by ca. 0.6 eV, whereas a methoxy substituent exerts a much more pronounced effect of about 1.6 eV. With regard to disubstitution, the effects behave almost completely additive for the para-derivatives, whereas the ortho-compounds are stabilized only about half that much by the second substituent, and for the meta-derivatives the influence of a second donor substituent is even smaller. The trends observed highlight the role of quinoid resonance structures for the stabilization of the dicationic compounds.