If a molecule absorbs a photon of the appropriate energy, one of its electrons is raised to an orbital of higher energy. The resulting molecule can take part in a reaction while in this, or a related, excited state. The reaction so induced can be intramolecular or intermolecular. Photochemical nucleophilic substitution can follow a different course to the analogous reaction under thermal conditions. The distribution of charge in an excited state can be wholly different from that in the ground state, and special use has been made of this difference in controlling the positional selectivity in nucleophilic aromatic substitution. The photochemical addition between a carbonyl and an alkene is known as the Paterno–Buchi reaction. The addition normally occurs by reaction of the triplet of the carbonyl compound with the ground state of the alkene.