SPIN-COUPLED DESCRIPTION OF CYCLOBUTADIENE AND 2,4-DIMETHYLENECYCLOBUTANE-1,3-DIYL - ANTIPAIRS

Spin-coupled theory is applied to the description of the pi electrons in cyclobutadiene, C4H4, and its dimethylene derivatives. This approach is greatly clarified by first considering the simple case of square-planar H-4. For C4H4, the theory predicts that the ground state is B-1(1g), with a 3A2g state lying at 39.6 kJ mol-1 (9.46 kcal mol-1) higher, which is in good agreement with experiment. The theory also shows that in the ground state the square-planar geometry is unstable and distorts to a rectangle, while in the triplet state the square-planar geometry is stable. This is also in accordance with observation. A central feature of the present description is that the shapes of the spin-coupled orbitals and the pairing of the spins in these states are highly unusual. Both orbitals and spin pairings are quite different from those of MO theory and bear little relation to those of classical valence bond theory. However by considering the related molecule 2,4-dimethylenecyclobutane-1,2-diyl (DMCBD), we show that our results are consistent and make excellent physical and chemical sense. In particular we see the emergence of what we believe to be a distinctive feature of antiaromatic systems: pairs of electrons in semilocalized orbitals whose spins are coupled to form a triplet. We call these antipairs. C4H4 possesses two such antipairs, whereas DMCBD has one. Preliminary results for BBB (bismethylenebiscyclobutylidene, C10H8, a non-Kekule isomer of naphthalene) and for the ground 1E2' state of C5H5+ show that these each have two antipairs.