Deciphering the Electronic Structure and Conformational Stability of 2-Pyridinecarboxaldehyde

The conformational structures and ionisation dynamics of 2-pyridinecarboxaldehyde (2-PCA) were explored using high-resolution vacuum ultraviolet mass-analysed threshold ionisation (VUV-MATI) spectroscopy, complemented by Franck–Condon (FC) simulations and quantum chemical calculations. The precise adiabatic ionisation energy of 2-PCA was determined to be 76,589 ± 4 cm⁻¹ (9.4958 ± 0.0005 eV), which is notably lower than the previous values obtained from electron impact ionisation studies. The vibrationally resolved VUV-MATI spectrum of the molecule confirmed that ionisation predominantly originates from its s-trans conformer, with no significant contribution from its s-cis conformer, indicating that the interconversion barrier effectively limits the population of this species under supersonic expansion conditions. Molecular and natural bond orbital analyses revealed that the highest occupied molecular orbital of the s-trans conformer is primarily composed of a nitrogen nonbonding orbital, which interacts with the oxygen lone pairs of the formyl group. This interaction stabilises the electronic structure of the conformer, resulting in an increased ionisation energy compared with pyridine. FC analysis further demonstrated that vibrational excitations in the cationic state are predominantly associated with the in-plane ring and formyl bending modes, producing distinct vibrational progressions in the VUV-MATI spectrum. These findings provide not only valuable insights into the electronic structure, conformational stability, and ionisation dynamics of 2-PCA, but also a deeper understanding of the effect of functional-group substitution in pyridine derivatives. Moreover, the results underscore the effectiveness of VUV-MATI spectroscopy in resolving conformer-specific ionisation processes, paving the way for further investigations into the electronic properties of heterocyclic molecules.