The Far Ultraviolet Absorption Spectra of Organic Molecules: Lone Pairs and Double Bonds

Abstract

The far-ultraviolet spectra of organic molecules can be conveniently discussed in terms of the following categories: saturated molecules containing bonding σ-electrons only; saturated heteroatomic molecules with both bonding σ-electrons and lone pairs of electrons; unsaturated molecules containing bonding σ and π-electrons; unsaturated molecules with bonding σ and π electrons and lone pairs of electrons. In a previous lecture at this Institute Dr. M.B. Robin stressed the principal characteristics of the far-ultraviolet spectroscopy of saturated molecules. In one simple sentence one might say that electrons in bonding σ- orbitals start absorbing at 170 or 160 nm and when the molecule contains heteroatoms then, except for fluorine, the bands due to the transitions of their lone pair electrons dominate the spectrum up to, at least 120 nm and begin at longer wavelengths. Almost all these bands can be interpreted as Rydberg bands converging to the first (lowest) ionization potential of these molecules. The Rydberg nature of most of the bands encountered in the far-UV spectra of organic molecules establishes a close connection between optical vacuum ultraviolet (VUV) and photoelectron (PE) spectroscopy.