Investigating Valence Orbitals and Cationic Structure of 2,6-Difluoropyridine via High-Resolution VUV-MATI Spectroscopy and Franck–Condon Simulations

Pyridine derivatives are fundamental in fields such as organic chemistry, materials science, and pharmaceuticals, largely due to their versatile electronic properties. Fluorination of pyridine significantly alters these properties, yet the specific effects of the position and number of fluorine atoms on valence orbitals and cationic structures remain not fully understood. This study examines the impact of fluorine substitution on the valence orbitals and cationic structures of various pyridine derivatives, with a particular emphasis on 2,6-difluoropyridine (2,6-DFP). Using high-resolution vacuum ultraviolet mass-analysed threshold ionisation (VUV-MATI) spectroscopy, the adiabatic ionisation energy of 2,6-DFP was determined to be 78,365 ± 3 cm⁻¹ (9.7160 ± 0.0004 eV). Franck–Condon simulations were conducted to interpret the VUV-MATI spectra, providing detailed insights into the molecular structure and vibrational modes of the cationic form. The analysis indicated a symmetry shift from C2V to C1 upon ionisation, highlighted by the presence of out-of-plane ring-bending modes. Natural bond orbital analysis identified the highest occupied molecular orbital (HOMO) and HOMO-1 as π-orbitals, with HOMO-2 being a nonbonding orbital. The introduction of two ortho-fluorine substitutions in 2,6-DFP significantly influenced this electronic configuration, stabilising the nonbonding orbital through interactions with the two fluorine σ-type lone pairs. This stabilisation notably altered the valence orbital ordering compared to that of 2-fluoropyridine, resulting in a substantial difference in the binding energies between the HOMO and HOMO-1. This research provides a deeper understanding of how halogen substitution affects the electronic properties of pyridine derivatives, promoting research in the field of physical chemistry.