Fluorine position-dependent ionization dynamics and structural distortion in 2,3-difluoropyridine: a VUV-MATI and theoretical study

Pyridine derivatives are integral to fields such as organic chemistry, materials science, and pharmaceuticals owing to their distinct electronic and structural properties. While fluorine substitution on the pyridine ring is known to modulate these properties, the specific effects of fluorine positioning-particularly at the <i>ortho</i> and <i>meta</i> positions-on the molecular orbitals, ionization energies, and cationic structures remain insufficiently understood. In this study, we investigate the ionization-induced structural dynamics of 2,3-difluoropyridine (2,3-DFP), which incorporates both <i>ortho</i>- and <i>meta</i>-fluorine substituents, using high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy in conjunction with Franck-Condon (FC) simulations and natural bond orbital analysis. Precise measurement of the adiabatic ionization energy (AIE) yields a value of 9.6958 ± 0.0004 eV for 2,3-DFP, which is lower than that of 2,6-DFP due to weaker hyperconjugative stabilization of the highest occupied molecular orbital (HOMO) by <i>meta</i>-fluorine. The VUV-MATI spectrum reveals significant geometric distortion upon ionization, notably in out-of-plane vibrational modes, which become FC-active due to symmetry lowering. FC fitting with a refined, slightly nonplanar geometry closely reproduces the experimental spectrum. Additional unassigned peaks are attributed to the D₁ state, originating from ionization of the HOMO-1 nonbonding orbital, and a second AIE of 9.7643 ± 0.0004 eV is proposed. These findings clarify how fluorine substitution patterns modulate valence orbital energies, cationic structures, and vibrational dynamics. This work provides a detailed framework for understanding the stereoelectronic effects of fluorination in heteroaromatic systems and offers practical insight for designing functional materials and pharmaceuticals with tunable electronic properties.