The three azaborine isomers with the formula C&BN, 1,2-, 1,4-, and 1,3-azaborine (I, 11, and III), have been examined using MNDO bodified neglect of diatomic nverlap) calculations. The most stable azaborine was I (heat of formation -8.147 kcal/mol), followed by I1 (+11.60 kcal/mol) and 111 (+16.64 kcal/mol). Qualitatively, although the r -and r*-orbitals calculated for the azaborines exhibited an ordering similar to that in benzene and borazine, the HOMO/LUMO energy differences (9.27, 9.68, and 8.44 eV, respectively) were smaller than was the difference calculated for borazine (12.81 eV), but of the same magnitude as the difference for benzene (9.76 eV). With the exception of borazine, each molecule had a r-orbital for the HOMO and a r*-orbital for the LUMO; borazine's LUMO was a u*-orbital. The calculated shapes and atomic contributions for the rand r*-orbitals of the azaborines were best described as "hybrids" of the r -and r*-orbitals of benzene and borazine. As was observed for the r -and +-orbitals of borazine, the azaborines exhibited increased orbital density at the nitrogen atom in the r-bonding orbitals and at boron in the r-antibonding orbitals, as would be predicted from electronegativity considerations. Although I and I1 exhibited significant double-and single-bond localization, all of the ring bonds in 111 were delocalized. The delocalization in 111 was not uniform but, rather, resembled two inequivalent fused ally1 systems. The cyclotrimer (IV) of 1,2-azaborine (heat of formation -44.07 kcal/mol), based purely on thermodynamic considerations, was predicted to form spontaneously from three monomer molecules with the concurrent loss of three molecules of dihydrogen. The cyclotrimers that could theoretically be produced from 1,2-azaborine without the loss of dihydrogen (IVc and IVt) were each calculated to be less stable (heats of formation +24.45, and +33.29 kcal/mol, respectively) than was the experimentally observed IV. The carbon molecules triphenylene (TP) and cis-and tran~-4a,4b$a,8b,12a,12b-hexahydrotriphenylene (TPc and TPt) (heats of formation +76.79, +101.6, and +103.1 kcal/mol, respectively) were each calculated to be less stable than were the azaborine cyclotrimer analogs, as was observed in comparisons of benzene with the azaborines and borazine.
