ab initio Study on the Stability and Geometry of Cyclobutadiene

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Abstract
Ab initio SCF MO and CI calculations dealing with questions of molecular stability, geometry, and spectra of cyclobutadiene C4H4 are reported. Comparison is made between the ab initio and semiempirical Hückel methods for the treatment of molecules of this type and emphasis is placed upon the role configuration interaction plays in improving the representation of the closed-shell SCF molecular potential surface. The results of the CI calculations in turn indicate a singlet ground state for cyclobutadiene in a rectangular geometry and a lowest excited triplet of the molecule which possesses a potential minimum for a slightly nonsquare geometry and is everywhere more unstable than the aforementioned singlet; the molecule is not found to be stable with respect to its separation products (two acetylenes), although it appears to exist in metastable equilibrium in the cyclic configuration. An attempt is also made to relate the cyclobutadiene calculations to the study of the geometry of similar molecules; from this it is concluded that the cyclization process is strongly favored by the presence of CH bonding and that benzene is much more stable than cyclobutadiene mainly because of its greater ability to simultaneously form strong CC and CH bonds.
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