MNDO CALCULATIONS ON BORAZINE DERIVATIVES .2. SUBSTITUTION OF 2 [HNBH] FRAGMENTS FOR 2 [HCCH] FRAGMENTS IN BENZENE TO FORM THE DIAZADIBORINES AND THE NOVEL OPEN STRUCTURE OF THE 1,2,4,5-ISOMER

The 11 diazadiborine isomers of formula C2H6B2N2, 1,3,2,4-, 1,4,2,6-, 1,3,2,5-, 1,3,4,6-, 1,4,2,5-, 1,4,2,3-, 1,2,3,6-, 1.,3,4,5-, 1,2,3,5-, 1,2,3,4-, and 1,2,4,5-diazadiborine (4-14), have been examined by using MNDO (modified neglect of diatomic overlap) calculations. The thermodynamically most stable diazadiborine was 4, the 1,3,2,4-isomer (heat of formation -60.16 kcal/mol). With the exceptions of 12-14, the diazadiborines optimized as planar molecules. For the 1,2,4,5-isomer, 14, the cyclic structure was neither a local nor an absolute minimum on the MNDO potential energy surface. (No orbital overlap or bonding was calculated to exist between the two nitrogen atoms in the molecule.) Rather, 14 optimized as the novel chain structure HN = CH(BH)2CH = NH, 2,3-didehydro-2,3-diborabutane-1,4-diimine (the lowest energy structure of this molecule is 14o). The diazadiborines 4, 9, 10, and 13, which can be represented by classical, uncharged valence bond structures, optimized as molecules with localized double and single bonds. Of the remaining diazadiborines, none of which can be represented by classical, uncharged valence-bond structures and which are thus termed mesionic, 5, 6, and 8 optimized as molecules with approximately uniform bond delocalization among ring atoms, while 7, 11, and 12, although containing some delocalized bonds, exhibited single-bond localization between certain ring atoms.