Fluorinated pyridines, particularly difluoropyridines, exhibit position-dependent electronic properties arising from the interplay of inductive, resonance, and hyperconjugative effects introduced by fluorine substitution. In this study, the ionization-induced electronic and structural characteristics of 3,5-difluoropyridine (3,5-DFP)─a previously uncharacterized meta–meta disubstituted isomer─were investigated using high-resolution vacuum ultraviolet mass-analyzed threshold ionization spectroscopy in conjunction with quantum-chemical calculations. The adiabatic ionization energy was determined to be 9.6829 ± 0.0004 eV, indicating an intermediate degree of electronic stabilization between the 2,3- and 2,5-isomers. Natural bond orbital analysis showed that the π-type HOMO of 3,5-DFP experiences moderate delocalization over the aromatic framework, whereas the σ-type HOMO–1 exhibits minimal interaction with the fluorine substituents, resulting in reduced stabilization. Franck–Condon analysis and vibrational spectroscopy revealed distinct ionization-driven geometric distortions dominated by ring-bending and stretching modes. The small D0–D1 energy separation (0.185 eV) supports assignment of the 44 cm–1 feature as the 0–0 transition of the D1 state, yielding a second adiabatic ionization energy of 9.6884 ± 0.0004 eV. These results demonstrate how fluorine substitution patterns regulate orbital ordering, structural relaxation, and vibronic coupling in pyridine cations. By elucidating the effects of meta–meta fluorination, this work fills a critical gap in difluoropyridine chemistry and provides fundamental insight relevant to the precision design of functional aromatic molecules for applications in materials science, catalysis, and pharmaceutical chemistry.
Kim et al. (Tue,) studied this question.