Appearance Energies of Fluorinated and Fluorine‐Free Fragment‐Ion Series Formed by Electron Ionization of 2‐Fluoropyridine

Rationale Electron‐impact ionization and fragmentation of pyridine have been investigated extensively, whereas systematic appearance‐energy data for fluorinated pyridines are scarce. In particular, the appearance energies of the principal high‐mass fragment ions of 2‐fluoropyridine have not been reported. Methods Energy‐resolved electron‐impact mass spectrometry was used to record ion‐yield curves of 2‐fluoropyridine between 5 and 45 eV. Appearance energies were obtained by fitting the threshold regions with the ThreSpect program using the Wannier threshold law convoluted with the experimental electron‐energy distribution. Mass spectra were also recorded at 100 eV. Results Appearance energies were determined for the fluorinated (C 5 H x FN + ) and fluorine‐free (C 5 H x N + ) fragment‐ion series ( x = 0–4), allowing their systematic comparison. The fluorinated series showed a nonmonotonic variation in appearance energies with decreasing hydrogen content, whereas the fluorine‐free series showed an almost monotonic increase after the first two members of the series. Two appearance thresholds were observed for the C 5 N + ion. Comparison with pyridine showed that fluorine substitution changes the appearance energies of individual fragment ions without producing a uniform energetic shift. Conclusions The appearance energies reported here provide the first systematic data for the principal fluorinated and fluorine‐free high‐mass fragment ions of 2‐fluoropyridine. Fluorine substitution changes the energetics of high‐mass fragmentation without changing the principal fragmentation pattern.