Mechanism of electrocyclic ring-opening of diphenyloxirane: 40 years after Woodward and Hoffmann.

The photochemistry of diphenyloxirane has been investigated by using static density functional theory and first-principles molecular dynamics. We optimised potential-energy surfaces for both the disrotatory and the conrotatory pathway in the first excited state. Although the disrotatory pathway does not seem to be favoured energetically, we get only the disrotatory product during the molecular dynamics simulations. This can be attributed to the "on-the-fly" description of the electronic structure in a first-principles molecular dynamics simulation. The different photochemical behaviour of aryl oxiranes and unsubstituted oxirane is due to different shapes of the frontier orbitals.