Ionization-Induced Electronic and Vibronic Structure of 3,5-Difluoropyridine Probed by VUV-MATI Spectroscopy

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.