On the Electronic Structures of the 1,3-Diboracyclobutane-1,3-diyls and Their Valence Isomers with a B2E2 Skeleton (E=N, P, As)

Authors

  • Wolfgang W. Schoeller Prof. Dr.,

    1. Fakultät für Chemie der Universität Bielefeld Postfach 10 01 31, 33503 Bielefeld, Germany, Fax: (+49) 521-106-6146
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  • Alexander Rozhenko Dr.,

    1. Fakultät für Chemie der Universität Bielefeld Postfach 10 01 31, 33503 Bielefeld, Germany, Fax: (+49) 521-106-6146
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  • Didier Bourissou Dr.,

    1. Laboratoire d'Hétérochimie Fondamentale et Appliqué CNRS (UMR 5069) Université Paul Sabatier 118, route de Narbonne, Toulouse Cedex 04, France
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  • Guy Bertrand Prof.

    1. Laboratoire d'Hétérochimie Fondamentale et Appliqué CNRS (UMR 5069) Université Paul Sabatier 118, route de Narbonne, Toulouse Cedex 04, France
    2. UCR-CNRS Joint Research Chemistry Laboratory (UMR 2282) Department of Chemistry, University of California Riverside, CA 92521-0403, USA
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Abstract

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH2+, AsH2+), while E fragments with triplet character (NH2+) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MR-MP2 level, and 3) two different types of density functional levels for the planar (D2h) geometries. The singlet–triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.

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