It is possible to interconvert cis and trans alkenes, but the `pi` bond must be broken first. This requires a considerable amout of energy -around `260 kJ "mol" ^(-1)` . One way to break the `pi` bond would b to promote an electron from the `pi` orbitla to the `pi^(*)` -orbital. If this were to happen , there would be one electron in the bonding `pi` orbital and one in the anitbonding `pi^(*)` -orbital and hence no overall bonding. Electromagnetic radiation of the correct energy could promote the electron from HOMD to LUMO . The correct energy actually corresponds to light in the ultraviolte (UV) region of the specturm. Thus shining its antibonding `pi^(*)` molecular ortbial. thereby breaking the `pi` bond (but not the `sigma` bond ) and allowing rotation to occur.
