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

Protonated ethylene cations, C2H5+, and their C2H5+Arn 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 C2H5+ structure. The signature bridged-proton stretch, observed at 2058 cm (1) for C2H5Ar2, 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. (C) 2009 Elsevier B.V. All rights reserved.