Resolving ionization-induced distortions in 2,5-difluoropyridine: influences of meta-fluorination on electronic and vibrational structures

Abstract

We conduct a high-resolution vacuum ultraviolet mass-analyzed threshold ionization spectroscopic study of 2,5-difluoropyridine (2,5-DFP), supported by Franck–Condon (FC) simulation and natural bond orbital analysis, to investigate the stereoelectronic influences of ortho- and meta-fluorination on ionization dynamics. The adiabatic ionization energy (AIE) is precisely determined as 77 760 ± 3 cm⁻¹, which is lower than those of other DFP isomers owing to the reduced stabilization of the highest occupied molecular orbital (HOMO) by meta-substituted F. The mass-analyzed threshold ionization spectrum reveals well-resolved vibrational progressions and weak out-of-plane modes, indicating slight nonplanarity in the cationic D₀ state. These features are successfully reproduced by FC simulations incorporating minor dihedral distortions, confirming the subtle symmetry breaking upon ionization. A second AIE of 78 227 ± 3 cm⁻¹ is assigned to the D₁ state, corresponding to ionization from a lower-lying nonbonding orbital (HOMO−1). Compared to that of 2,3-DFP, 2,5-DFP exhibits less extensive geometric reorganization upon ionization, reflecting the π-character of its singly occupied molecular orbital and its more delocalized charge distribution. These results demonstrate how meta-fluorination on the opposite side modulates the cationic geometry and frontier orbital energetics, offering fresh insights into the substitution-dependent ionization behavior of fluorinated heteroaromatics.