Infrared spectroscopy of protonated ethylene: The nature of proton binding in the non-classical structure

Protonated ethylene cations, C 2 H 5 + , and their C 2 H 5 + Ar n argon-tagged analogues, are produced with a pulsed discharge/supersonic beam source, mass-selected in a reflectron time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy. Spectra in both the higher frequency C–H stretching and lower frequency bridged-proton regions confirm that these complexes have the non-classical bridged-proton C 2 H 5 + structure. The signature bridged-proton stretch, observed at 2058 cm−1 for C 2 H 5 + Ar 2 , is investigated computationally and compared to those of other proton-bridged systems. This characteristic frequency does not scale with the proton affinities of these systems, but rather depends on the carbon hybridization at the binding sites.