Date of this Version
Published in Accounts of Chemical Research Vol. 11, 1978, pp 341-348
The concept of occupied and unoccupied orbitals has provided a useful means for visualizing many of the most important properties of molecular systems. Yet, there is a curious imbalance in our experimental knowledge of the energies of occupied and unoccupied orbitals. Whereas photoelectron spectroscopy has provided a wealth of data on positive ion states and has established that they can be associated, within the context of Koopmans’ theorem, with the occupied orbitals of the neutral molecule, the corresponding information for the negative ion states, associated with the normally unoccupied orbitals, is sparse. In part this reflects the experimental difficulties connected with measuring the electron affinities of molecules which possess stable anions. A more interesting aspect, in our view, is that for many molecules even the ground state of the anion lies above the ground state of the neutral molecule and thus is unstable with respect to autodetachment of the additional electron. The temporary anions which we will discuss in this Account possess lifetimes in the range from 10–12 to 10–15 s in the gas phase. Ephemeral as these anions might appear, their existence is amply demonstrated in electron-scattering experiments. At energies in which an impacting electron may be temporarily captured into a normally unoccupied orbital, the electron-scattering cross section undergoes a pronounced change in magnitude, commonly referred to as a “resonance” by those in the scattering field.
A number of electron-scattering techniques may be used to investigate temporary anion formation. In this article we will describe only one, the electron transmission method in the format devised by Sanche and Schulz. This technique has numerous assets: it is both conceptually and experimentally straightforward, and it is likely the most sensitive means for detection of temporary anions.
During the last few years, electron transmission spectroscopy has seen increasing application to organic molecules. In the following sections, we discuss our results and those of other workers which bear on the characteristics of the temporary anion states associated with low-lying π* orbitals in a number of classes of hydrocarbons. In addition we will describe briefly the relationship of this work to other chemical studies, such as optical absorption measurements on anions in solution, molecular orbital theories of reactivity, and studies of low-lying electronic states of neutral molecules.