Valence molecular orbitals and cationic structures of 2-fluoropyridine by high-resolution ion spectroscopy and...

The alteration of the valence molecular orbitals' ordering of halopyridine molecules, by the introduction of a halogen atom(s) as substituent on the pyridine ring, has spurred an extensive interest for their investigation. Herein, the effect of a fluorine substituent on the two outermost orbitals of pyridine was elucidated by investigating the photoionization dynamics of 2-fluoropyridine (2-FP), considering that the geometrical changes with respect to the neutral geometry induced by adiabatic ionic transition affect the vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectrum. The adiabatic ionization energy associated with the 0-0 band on the measured high-resolution VUV-MATI spectrum was determined to be 9.6702 ± 0.0004 eV (77 995 ± 3 cm^-1), which differs considerably from the 9.401 eV by two-color ionization spectroscopy. Franck-Condon simulation of the MATI spectrum corresponded quantitatively with the experimental results. Interestingly, among the forbidden transitions under C_S symmetry, an out-of-plane ring-bending mode resulting from the warped cationic structure of 2-FP with C₁ symmetry was discovered. Rigorously, among the unassigned peaks, the first prominent peak at 78 532 cm^-1 should rather be assigned as the origin of the excited electronic state (D₁) of the 2-FP cation, in accordance with time-dependent density functional theory calculations. Natural bond orbital analysis led to the conclusion that such observations could be induced by electron removal from the highest occupied molecular orbital (HOMO) consisting of the π orbital of the pyridine ring and lone-pair orbital of the fluorine atom or from the HOMO-1 of the molecular non-bonding orbitals, to generate the two proximate electronic states of the cation.