Aspects of structure and bonding in carbocations and plasma polymers, with particular emphasis on the use of ESCA

Abstract

Since photoionisation occurs on a timescale which is rapid compared with that of nuclear notion. X-ray Photoelectron Spectroscopy (XPS or ESCA) is an ideal technique to study (controversial) carbocations in which potentially rapid rearrangement may be occurring. Previous attempts at (parts of) this work have not produced totally accepted results. In this work a method is presented for the preparation of carbocations in situ in the source of an ESCA spectrometer, by the use of molecular beams. Preparation of the t-butyl cation by ionisation of the chloride using antimony pentafluoride was concluded to be a gas-phase reaction. This was contrasted with the probable gas-surface nature of the protonation of alkenes using fluorosulphuric acid, Which has led to the successful ESCA dbservation of both the 2-norbomyl and sec-butyl cations. The experimental C(_1s) core-hole spectra of the solid-state species are close to those predicted theoretically for the nonclassical 2-norbomyl cation and the partially methyl-bridged sec-butyl cation. The ability to study amorphous thin films is another important facet of the ESCA technique which has been exploited in the study of plasma polymers produced in an inductively-coupled RF glow-discharge flow reactor. The study was specifically aimed at the incorporation of metals into a perfluorobenzene plasma polymer by attempting to evaporate the metal from a resistively heated molybdenum coil. This was unsuccessful and the major metal incorporation was of molybdenum oxides, though these were present in amounts comparable with those in samples previously produced by an etching mechanism. Since theoretical calculations are one method of "probing" reactive media, semi-enpirical molecular-orbital calculations (MNDO) have been performed on an extensive series of species vAiich are potential intermediates in perfluorodiaza- benzene plasmas (in particular the valence isomers) to gain some insight into these systems in an attempt to rationalise experimentally determined (ESCA) data.