Stereodefined Precursor-Based Preparation and Isomer-Resolved Spectroscopic Validation of trans-3-Penten-1-yne.

The preparation and characterization of volatile cis/trans stereoisomers require controlled access to defined isomeric forms and independent validation of their molecular compositions under spectroscopic conditions. Here, we report the integrated synthetic-spectroscopic characterization of trans-3-penten-1-yne, a volatile C5H6 enyne stereoisomer. A stereochemistry-preserving preparation route using trans-crotonaldehyde, combined with Corey-Fuchs-type dibromoolefin formation, a trimethylsilyl-protected enyne intermediate, mild deprotection, and freeze distillation, enables the low-temperature isolation of a trans-enriched sample while minimizing evaporative loss. The isomeric composition is first determined via nuclear magnetic resonance (NMR) spectroscopy and then independently evaluated under jet-cooled gas-phase conditions using vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The good consistency between the NMR- and VUV-MATI-derived trans/cis ratios indicates that the trans-enrichment of the isolated sample is retained during vaporization and supersonic expansion. The VUV-MATI spectrum yields the precise adiabatic ionization energy of the trans-isomer and enables the differentiation of its threshold features from those of the residual cis-component. Infrared-dip VUV-MATI spectroscopy, supported by anharmonic simulation, confirms the neutral-state vibrational fingerprint of trans-3-penten-1-yne, and Franck-Condon analysis reveals that the cationic vibronic structure is consistent with the ionization of the well-defined trans neutral precursor. This combined approach establishes VUV-MATI spectroscopy as a gas-phase analytical complement to NMR spectroscopy in analyzing volatile stereoisomeric hydrocarbons and provides a reliable framework for interpreting the congested ionization spectra of labile enyne systems.