Unsymmetrical 1B2 (ππ1) state of ozone

Using ab initio SCF CI calculations it is shown that the ππ1 singlet state of ozone has an unsymmetrical equilibrium geometry with one long and one short OO bond. A similar behavior has been experimentally demonstrated on the parent SO₂ molecule. The double-minimum potential energy surface of this state results from an avoided crossing involving (1) the singlet ππ1 state itself which is stabilized by an unsymmetrical stretching of the two bonds and (2) a lower lying (nπ1)² doubly excited state which is destabilized by the same distortion.