Infrared Spectroscopy of Protonated Ethane in Helium Droplets

Protonated saturated hydrocarbons like C₂H₇⁺ often have unique structures and multiple isomers, but they are notoriously difficult to study due to their short-lived nature and tendency to fragment or react to produce smaller ions. Here we report the infrared spectrum of protonated ethane (C₂H₇⁺) in helium droplets and of the weakly bound C₂H₅⁺-H₂ complex. We found that C₂H₇⁺ could be formed by H₃⁺ protonation of C₂H₆ molecules as well as ionization of methane dimers. The spectrum displayed in this work is in agreement with previous results obtained for the bridged isomer of protonated ethane in the gas phase. However, we could not identify the classical isomer with a pentacoordinated carbon atom. Anharmonic calculations, performed using second-order vibrational perturbation theory (VPT2) based on electronic structure calculations performed at the B2PLYP-D3/aug-cc-pVTZ level of theory/basis, corroborate our assignments and reveal that broad C₂H₇⁺ bands previously assigned to single fundamental CH₃ stretches may be due to overlapping fundamental and combination bands.