TAKESHI OKA, Department of Chemistry, Department of Astronomy and Astrophysics, and the Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637.

Protonated methane, CH5+, has unusual C-H bondings and is a new prototype of spectroscopic specimen. While all of the five protons are strongly bound to the central carbon atom with well defined C-H stretch potentials, angles between C-H bonds are highly fluxional. Ab initio theory predicts lowest energy for an "equilibrium structure" with Cs symmetry but the barriers separating the 24 equivalent structures are extremely low and almost non-existent when zero point vibrations are taken into account.

We have identified nearly 1000 spectral lines of CH5+ in a liquid N2-cooled hydrogen dominated plasma using a gas mixture of H2:CH4 sim 100:1. The spectral lines are weak and do not show obvious symmetry or regular pattern. The identification required extensive studies of the spectra of other carbocations CH3+ (and CH2+) and C2H3+ (and C2H2+) whose spectra appear in the same wavelength region with much higher intensities. Spectroscopy and plasma chemistry of those carbocations will be discussed and the raw observed spectrum of CH5+ will be presented without assignment nor even qualitative understanding.