Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO₄]^.+/C₂H₄ Couple by Gas‐Phase Vibrational Spectroscopy

The gas‐phase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150 K using cryogenic ion‐trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier‐free thermal C2H4→CH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified