Impact of chlorine substitution on valence orbitals and ionization dynamics in 3-chloropyridine: Insights from high-resolution vacuum ultraviolet mass-analyzed threshold ionization study.

This article focuses on the investigation of the effects of chlorine substitution on the valence orbitals and electronic states of 3-chloropyridine (3-CP) using high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy combined with computational methods. The study accurately determined the adiabatic ionization energies (AIEs) of 3-CP isotopomers and revealed two closely spaced cationic electronic states (D0 and D1) arising from the highest occupied molecular orbital (HOMO), primarily a π orbital of the pyridine ring, and the HOMO-1, consisting of nonbonding orbitals. Franck–Condon simulations supported the assignment of vibrational spectra to these states, highlighting that meta-chlorine substitution in 3-CP stabilizes nonbonding orbitals less effectively than ortho substitution in 2-chloropyridine, influencing the electronic structure and ionization dynamics. Minor discrepancies in vibrational frequencies and intensities suggest vibronic coupling effects, indicating the need for further theoretical and experimental studies to fully understand these interactions. Overall, the research advances knowledge of how halogen substitution position affects molecular orbitals and cationic structures in pyridine derivatives.