Predictions of Molecular Geometries and Electronic Spectra of Complex Unsaturated Molecules from MC-LCAO-MO Method. II. 1,3-Butadiene and Cyclobutadiene

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Abstract
A multi-configuration (MC)-LCAO-MO method is applied to 1,3-butadiene and cyclobutadiene, predicting their molecular geometries and electronic spectra. The molecular ground state geometry, electronic spectrum and stabilization energy of the trans- to the cis-isomer of 1,3-butadiene are predicted to be in good accord with experiments and with those derived from the ordinary semi-empirical SCF-CI method. The results obtained for cyclobutadiene indicate that (a) the Jahn-Teller distortion occurs to favor a rectangular (D2h) form, the stabilization energy being 0.3–0.9 eV depending on approximations, and (b) even in a square (D4h) form a singlet (ground) state is more stable than a triplet state by ca. 0.3 eV. It is also shown that the lowest excited singlet and triplet states (3A2g and 1B1g) in the square form are not subjected to the Jahn-Teller effect. A discussion is given on a possible error in a CI treatment arising when degenerate molecular orbitals are expressed by real coefficients of atomic orbitals.