Evidence of anharmonicity in the vibrational spectrum of protonated ethylene

Abstract

The vibrational spectrum of protonated ethylene, H+(C2H4), in the 2000 to 3500 cm−1 region is re-investigated. The spectrum is obtained by predissociation of the H+(C2H4)Ar2 complex following vibrational excitation, which leads to elimination of an argon atom. The equilibrium structure of this cation has the excess proton in a bridged position, where it undergoes large amplitude motions in the CH+C plane. These large amplitude vibrational motions result in features in the spectrum that cannot be assigned to Δv=1 transitions predicted by harmonic treatments. The origins of the anharmonicities that lead to these higher order transitions are explored. Based on vibrational perturbation theory and reduced dimensional calculations, transitions to states with two quanta in the HCH bending vibrations and the state with one quantum of excitation in the two vibrations of the bridging proton in the CH+C plane are found to carry significant intensity in this region of the spectrum.