Tetrasilacyclobutadiylidene: The lowest energy cyclic isomer of singlet Si 4H 4?

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Ab initio molecular electronic structure theory has been used to continue the investigation of four-membered-ring structures on the Si 4H 4 singlet potential energy surface. At the double-zeta plus polarization (DZP) self-consistent-field (SCF) level of theory five new cyclic structures are predicted to be minima, with three of these lying lower in energy than the hitherto predicted lowest energy isomer, tetrasilacyclobutadiylidene. Thus tetrasilacyclobutadiylidene is not the lowest energy cyclic isomer of Si 4H 4. With inclusion of electron correlation through the configuration interaction (CI) method, the lowest-energy isomer is found to be a C s puckered-ring silicon analogue of 1,2-cyclobutadiene. This structure is predicted to lie 51 kcal mol −1 below the previously reported tetrasilatetrahedrane.