1, 3,4,6,3a, 5,7, 1,3,4,6, and 2,3a,5,7, 3,4,6, 2,3a,5,7,. Both molecules optimize to planar structures, have exothermic heats of formation respectively), and exhibit some bond delocalization. The relationship of these t w o PAPBAZ isomers to their organic parent molecule, azulene, is discussed.

Modified neglect of diatomic overlap (MNDO) calculations have been used to investigate the ground state properties of the 130 nonfully boron-nitrogenalternating isomers of pentaazapentaboraazulene (PAPBAZ), N5B5H8. The two most stable of these nonfully boron-nitrogen-alternating isomers, 1,2,3a, 5,7-pentaaza-3,4,tS,8,8a-pentaboraazulene, ( I ,2,3a,5, (3), and I ,3,3a,5,7-PAPBAZ (4) have strongly exothermic heats of formation (-142.5 and -142.4 kcallmol, respectively), while the two least stable isomers, 1,6,7,8,2,3,3a,, are calculated to have endothermic heats of formation (+95.43 and +96.31 kcal/mol, respectively). Each isomer optimized as a slightly to extremely nonplanar molecule, but each remained a bicyclic system containing a seven-membered ring fused to a five-membered ring. Thus, no ring-opened structures were observed. An empirical relationship between the heat of formation and the number, position, and type of homonuclear (N-N or B-B) bonds present in the molecules could be established, but as the heat of formation for the molecules became more positive, the relationship became less strong. A general trend of diminishing energy differ-Dedicated to Prof. James Cullen Martin on the occasion of his