The Nature of the Singlet and Triplet States of Cyclobutadiene as Revealed by Quantum Interference

The generalized product function energy partitioning (GPF-EP) method is applied to the description of the cyclobutadiene molecule. The GPF wave function was built to reproduce generalized valence bond (GVB) and spin-coupled (SC) wave functions. The influence of quasiclassical and quantum interference contributions to each chemical bond of the system are analyzed along the automerization reaction coordinate for the lowest singlet and triplet states. The results show that the interference effect on the π space reduces the electronic energy of the singlet cyclobutadiene relative to the second-order Jahn–Teller distortion, which takes the molecule from a D4h to a D2h structure. Our results also suggest that the π space of the 1B1g state of the square cyclobutadiene is composed of a weak four center–four electron bond, whereas the 3A2g state has a four center–two electron π bond. Finally, we also show that, although strain effects are nonnegligible, the thermodynamics of the main decomposition pathway of cyclobutadiene in the gas phase is dominated by the π space interference.