engleski

Zwitterionic structures of strained cis-pyramidalised disilenes – Fact or artifact

A series of cis-pyramidalized disilenes was modeled by ab initio and DFT methods with the special emphasis on character of the wavefunction. Spin restricted DFT (RDFT) and MP2 approaches predicted qualitatively different structures of disilenes as the minima (deformed zwitterionic and symmetrical diradicaloid, respectively). Additional CASSCF and broken symmetry spin unrestricted DFT (BS-UDFT) calculations confirmed symmetrical structures as more reliable minimum. The only exception is highly strained 1, 4-disilabicyclo[2.2.0]hex-1, 4-ene where deformed zwitterionic structure is corroborated by most of the applied methods including high-level MR-AQCC calculation. Analysis of the wavefunction obtained for both deformed and symmetrical minima indicates a strong tendency of the RDFT method to prefer structures possessing a closed shell wavefunction even though a strong electron density shift occurs. The results obtained at CASSCF level of theory showed that for a correct description of geometries of such systems, a two-configurational wavefunction is needed at least. The BS-UDFT and spin restricted MP2 methods describe such “ diradicaloid” geometries of cis-pyramidalized disilenes qualitatively correct although from different reasons.

quantum chemical calculations ; disilenes ; pyramidalization ; diradical character ; ring strain

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