Infrared Spectroscopy of CH5 + Cations in Helium Nanodroplets

The methanium CH5 + is a prototypical fluxional ion whose infrared spectra remain unassigned. Here we report on the infrared spectra of CH5 + cations and its deuterated isotopomer, CH4D+, in helium droplets at a low temperature of 0.38 K. The ions were produced upon protonation of CH4 molecules, a technique that was developed in this work. The spectra of CH5 + around 3000 cm-1 show two strong and broad infrared bands and a weak shoulder, reflecting its highly fluxional nature. The spectrum of CH4D+ shows a much sharper infrared band, indicating a partial quenching of the exchange of H/D atoms. This work also reports on the infrared spectrum of the methane dimer radical cations (CH4)2 +.