40min:

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 C_s 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 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.