Theoretical Studies of Borazynes and Azaborines

Four isomers of didehydroborazine, B3N3H4, borazyne, and three isomers of azaborine, C4H4BN, are studied by DFT, CCSD, and CCSD(T) computational methods. The singlets of 1,2-borazyne (I) and 1,4-borazyne (IV) have angles about the boron of ca. 150°. In 1,2-azaborine (V), the angles are ca. 140°, while the N angles are ca. 112° except in IV (127°) and 1,4- azaborine (VII, 120°). These geometries are almost reversed in the triplets. The 1,3-borazyne (III) shows more bicyclic character than the corresponding m-benzyne, with an N−N distance of 1.7 Å. In all cases I was found to be lower in energy than the 2,4-borazyne (II), III, and IV. The order of stability of the azaborines is V > VII > 1,3- azaborine (VI). The nucleus-independent chemical shift (NICS) indicates that all isomers of borazyne are less aromatic than benzene and all isomers of azaborine are about as aromatic as benzene. Time-dependent DFT (TD-DFT) is used to study the excited states of the singlets. The significant structural differences between the singlet and triplet states suggest long phosphoresence lifetimes.