Unraveling the Conformational and Electronic Landscape of 3-Pyridinecarboxaldehyde: A Spectroscopic Journey through Conformer-Specific Ionization

3-Pyridinecarboxaldehyde (3-PCA) is a versatile building block, but its conformational and ionization behavior remains underexplored. Using high-resolution IR-resonant vacuum ultraviolet mass-analyzed threshold ionization spectroscopy and Franck-Condon simulations, two conformers, s-<i>cis</i> and s-<i>trans</i>, were identified in the gas phase, distinguished by adiabatic ionization energies of 76 123 ± 4 and 76 173 ± 4 cm^-1, respectively, and revealing conformer-specific structural dynamics and frontier orbital changes during ionization. Computational analyses, including anharmonic and natural bond orbital studies, provided structural and electronic insights. Analysis of jet-cooled conformer populations indicated an S₀-state energy difference of 40 ± 14 cm^-1, while in the D₀ state, the two conformers are essentially near-isoenergetic within experimental uncertainty (≈ -10 ± 14 cm^-1). These results highlight the delicate balance of cationic conformer stability and its distinct ionization behavior. This work provides conformer-specific spectroscopic and theoretical benchmarks for understanding ionization-induced structural and stereoelectronic effects in formyl-substituted pyridines with implications for their reactivity and excited-state dynamics.