Substituent effects in mass spectrometry—III: Substituent effects in the dissociation of the molecular ions of para and meta substituted benzoic acids

The effect of substituents on the electron-impact-induced fragmentation of the molecular ions of para and meta substituted benzoic acids has been examined. The substituent is observed to exert an effect on the ionisation potential of the molecular ion, on the appearance potentials of the primary daughter ions and on the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ analogues prior to the loss of hydroxyl therefrom. The energy of activation for the loss of hydroxyl from the molecular ion is in general dependent upon the nature but not the position of the substituent, while the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ derivative is dependent upon both the nature and the position of the substituent. No correlation of the relative ion abundances with σ⁺ constants was observed. The results are consistent with the molecular ions of each compound having a dissimilar energy distribution, which could arise either by different energy transfers from the electron beam to the molecule or by the participation of different isolated electronic excited states (or similar states but to varying extents) in the dissociation of the molecular ions.