Resolving s-trans-dominant ionization dynamics in tiglic aldehyde using VUV-MATI spectroscopy and PES analysis

Abstract

Tiglic aldehyde (TA), an α,β-unsaturated aldehyde bearing an α-methyl substituent, serves as a prototypical system for exploring conformer-specific ionization and electronic dynamics in conjugated carbonyl compounds. Using high-resolution vacuum-ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy combined with infrared double-resonance techniques, Franck–Condon simulations, and two-dimensional potential energy surface (2D-PES) mapping, we characterized the conformational and ionization behaviour of TA. Under jet-cooled conditions, TA exists almost exclusively as the s-trans conformer, with an accurately determined adiabatic ionization energy of 77 183 ± 4 cm⁻¹. The 2D-PES analysis revealed that the s-trans conformer dominates in the neutral ground state but becomes less stable than the s-cis form upon ionization, consistent with electron removal from the carbonyl π orbital. The observed MATI vibrational progression arises mainly from excitation of the formyl torsional mode, which is strongly activated upon ionization. At higher vibrational energies, partial dephasing due to intramolecular vibrational redistribution among torsional sublevels likely contributes to the attenuation of spectral intensity. Theoretical results show that the shallow torsional potential and the sensitivity of the cationic geometry to electron correlation and basis-set effects underlie this conformational reordering. These findings emphasize the need for correlated ab initio and advanced DFT approaches—such as optimally tuned range-separated hybrids, Koopmans-compliant functionals, and self-interaction-corrected methods—to accurately describe the orbital response accompanying ionization. This work provides the first conformer-resolved view of TA and establishes a general framework for investigating conformer-specific ionization dynamics in α,β-unsaturated aldehydes.