Identification and composition of conformational isomers and their cations in crotonaldehyde by VUV-MATI spectroscopy

Crotonaldehyde is a simple α,β-unsaturated aldehyde that reacts stereochemically with nucleophilic reagents according to its conformational structure. Identifiable vibrational spectra of the cationic crotonaldehyde conformers were measured using one-photon vacuum ultraviolet mass-analysed threshold ionization (VUV-MATI) spectroscopy. From the 0–0 bands for the individual conformers confirmed by Franck–Condon (FC) simulations, the precise adiabatic ionization energies were determined to be 9.7501 ± 0.0004 eV (78 640 ± 3 cm−1), 9.7620 ± 0.0004 eV (78 736 ± 3 cm−1), 9.7122 ± 0.0004 eV (78 334 ± 3 cm−1), and 9.6480 ± 0.0004 eV (77 816 ± 3 cm−1) for the trans-s-trans (tt)-, trans-s-cis (tc)-, cis-s-trans (ct)-, and cis-s-cis (cc)-crotonaldehyde, respectively. The complete vibrational assignments were accomplished for the peaks observed in the VUV-MATI spectrum from the calculated vibrational frequencies and the FC factors according to the dipole selection rules for one-photon absorption. In addition, the composition at ambient temperature was determined to be 1.000 (93.0%): 0.037 (3.4%): 0.036 (3.4%): 0.002 (0.2%) for the tt-/tc-/ct-/cc-conformers from the relative intensities of the 0–0 bands in the MATI spectrum normalized with the calculated dipole transition probabilities as proven by the NMR data for the trans- and cis-stereoisomers. This study ascertained the existence of the s-cis conformers of crotonaldehyde for the first time.