Uncovering the Role of Fluorine Positioning on the Cationic Properties of 2,4-Difluoropyridine

Abstract

Fluorine substitution exerts a profound influence on the electronic structures, ionisation behaviours, and vibrational dynamics of pyridine systems, enabling the precise tuning of their molecular properties. 2,4-Difluoropyridine (2,4-DFP) exhibits a unique combination of inductive and resonance effects owing to the presence of fluorine atoms at the ortho and para positions relative to the nitrogen atom. However, studies on its ionisation-induced structural changes and vibrational dynamics remain limited. In this study, high-resolution vacuum-ultraviolet mass-analysed threshold ionisation mass spectroscopy and Franck–Condon analysis were employed to investigate the ionisation-induced structural and electronic properties of 2,4-DFP. The spatial arrangement of fluorine atoms selectively stabilised the nitrogen lone pair and the π orbitals in the pyridine ring, significantly affecting valence orbital energies. Precise measurements of the adiabatic ionisation energies of 2,4-DFP highlighted the synergistic effects of para-resonance and ortho-inductive withdrawal, thereby elucidating their roles in molecular stabilisation. Vibrational analyses of the proximate cationic states revealed mode-specific structural distortions, underscoring the dynamic changes induced by ionisation. A comparative evaluation of 2,3-, 2,5-, and 2,6-DFP derivatives demonstrated how fluorine positioning governs ionisation energies, molecular geometries, and vibronic interactions. This study advances the understanding of the chemistry of fluorinated pyridines and offers insights into their potential applications in the tailored molecular design of functional materials and chemical systems.