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Using a minimum basis set of Slater-type orbitals, the equilibrium structure of cyclobutane is determined. The calculated structure has CC and CH bond distances and HCH bond angles, which are within the limits of error for the experimentally measured values. The importance of methylene rocking in the puckered structure, shown in an earlier semiempirical calculation, is verified by the ab initio results. However, only a very small double minimum puckering potential is found. Force constants calculated for some of the important normal modes of vibration are too large by a factor of two. The orbital energies obtained agree very well with the mea-sured photoelectron spectrum of cyclobutane. A potential surface for the coplanar (2s+ 2s) rectangular decom-position of cyclobutane to form two ethylenes shows a potential barrier of 156 kcal/mol. The properties of this surface are discussed with reference to the Woodward …