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

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 sigma(co)-pi or oxygen lone pair-pi mixing in the alcohols nor in the epoxides. Although the energy perturbations, caused by the hydroxy and epoxy substituents, to adjacent filled pi orbitals are small, the electron transmission spectra show that the empty pi* 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 pi* orbitals and low-energy empty sigma* orbitals localised at the three-membered cycle. The experimental data have been compared with the orbital energies predicted by ab initio 3-21G calculations.