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On the Electronic Structures of the 1,3‐Diboracyclobutane‐1,3‐diyls and Their Valence Isomers with a B 2 E 2 Skeleton (E=N, P, As)
Author(s) -
Schoeller Wolfgang W.,
Rozhenko Alexander,
Bourissou Didier,
Bertrand Guy
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200204508
Subject(s) - singlet state , chemistry , valence (chemistry) , cyclobutene , singlet fission , electronic structure , triplet state , computational chemistry , crystallography , atomic physics , molecule , ring (chemistry) , physics , excited state , organic chemistry
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 2 h ) 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.