Question

Is the ground state of cyclobutadiene a singlet or triplet?

Answer 1

Cyclobutadiene has singlet ground state with D₂h symmetry. This can be explained on the basis of john-teller distortion. According to that principle, the non linear molecule having degenerate ground state can undergo a distortion by reducing its symmetry. In case of cyclobutadiene the expected symmetry is D4h but due to john teller distortion it behaves as a rectangular instead of perfectly square planar and hence the symmetry is D₂h. As the distortion makes the structure change from square planar to rectangular shape, there is the conformation of two single bond is longer than two double bond which is also experimentally proved. So that when it undergoes from square planar to rectangular shape, the degenerate orbital in square planar state also became non-degenerate in rectangular shape. out of the two degenerate orbital one go higher energy and other goes lower energy to balance overall energy and hence the all electrons in the ground state of rectangular D₂h become paired and hence it behaves as ground state triplet. this can be better explained by following image.