title: On the electronic structures of the 1,3-diboracyclobutane-1,3-diyls and their valence isomers with a B2E2 skeleton (E=N, P, As).
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 (PH(2) (+), AsH(2) (+)), while E fragments with triplet character (NH(2) (+)) 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 (D(2h)) 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.
authors.affiliation: Fakultät für Chemie der Universität Bielefeld, Postfach 10 01 31, 33503 Bielefeld, Germany. wolfgang.schoeller@uni-bielefeld.de
authors.forename: Wolfgang W
authors.initials: WW
authors.lastname: Schoeller
authors.forename: Alexander
authors.initials: A
authors.lastname: Rozhenko
authors.forename: Didier
authors.initials: D
authors.lastname: Bourissou
authors.forename: Guy
authors.initials: G
authors.lastname: Bertrand