Aspects of mass spectra of organic compounds. Part VIII. Calculation of ionization potentials of disubstituted benzenes and their importance in Hammett correlations in mass spectrometry

Abstract

Molecular ionization potentials are well-defined quantities and provide the most reliable energy term in attempts to correlate mass spectrometric fragmentation reactions with Hammett-type σ-constants. Ionization potentials are important in these correlations and sometimes appear to become the most important term. Some of the factors affecting the relationship between ionization potentials and σ-constants are examined. Equations are derived which allow the calculation of the ionization potentials of disubstituted benzene compounds and provide also a means of examining the relationship between them and σ-constants. For molecular ionization potentials it appears that yet another set of Hammett constants is necessary, but that these are not necessarily effective overall in mass spectrometric fragmentation reactions.