Collision‐induced dissociation and charge‐reversal processes of isomeric C ₆ H ₄ X ⁻ (X = F, Cl and Br) anions

The NH ion reacts with fluoro‐ and chlorobenzene in the cell of a Fourier‐transform ion cyclotron resonance instrument only by proton abstraction, whereas the reaction with bromobenzene also yields Br − ions. The deprotonation of these halogen‐substituted benzenes occurs the 2‐, 3‐ or 4‐positions as revealed by deuterium labelling in combination with high kinetic energy (8keV) collision‐induced dissociation (CID) experiments performed with a double‐focusing quadrupole hybrid mass spectrometer. The 2‐fluoro‐ and 3‐ fluorophenyl anions eliminate HF following collision with an oxygen molecule. By contrast, the collisions between 4‐fluorophenyl anions and O 2 do not yield detectable amounts of negatively charged fragment ions owing to the exclusive occurrence of electrom detachment. Electron detachment is also the only process observed in the 8 keV CID experiments with chlorophenyl anions and 3‐ and 4‐bromophenyl anions, whereas the ion formed by proton abstraction from the 2‐position in bromobenzene dissociates to Br − and 1,2‐dehydrobenzene. The 8 keV collision‐induced charge‐reversal spectra of the anions formed by proton abstraction from the 2‐, 3‐ or 4‐positions in a given halogen‐substituted benzene are virtually identical, indicating that this method does not provide insight into the position of the charge with respect to the halogen atom in the substituted phenyl anions.