Frontier electronic structure in gas-phase epoxy and hydroxy cyclohexenes by means of photoelectron and electron transmission spectroscopies

Abstract

Ionisation energy (E_i) and electron affinity (E_a) values in benzene oxide, benzene hydrate, the corresponding monoenes and other related molecules have been measured by means of UV photoelectron spectroscopy and electron transmission spectroscopy. The valence filled levels do not show evidence for strong σ_co–π or oxygen lone pair–π mixing in the alcohols nor in the epoxides. Although the energy perturbations, caused by the hydroxy and epoxy substituents, to adjacent filled π orbitals are small, the electron transmission spectra show that the empty π* orbitals are affected to very different extents, the epoxy substituent producing a sizeable electron affinity increase. This finding indicates the occurrence of strong interaction between the empty π* orbitals and low-energy empty σ* orbitals localised at the three-membered cycle. The experimental data have been compared with the orbital energies predicted by ab initio 3-21G calculations.