An Ab Initio Study of the Structures and Selected Properties of 1,2-Dihydro-1,2-azaborine and Related Molecules.

An ab initio study has been carried out to investigate the effect of replacing [HC-CH]n linkages in benzene by the isoelectronic [HN-BH]n linkages for n = 1, 2, and 3. Such replacements give rise to azaborine, a set of diazaborines, borazine, and pseudoborazine. These replacements lead to significant rearrangements of electron densities in these molecules due primarily to the introduction of the polar B-N bond. As a result, azaborine and diazaborines exhibit much more localized structures than that of benzene. They are also less aromatic than benzene but have a higher degree of aromaticity than borazine. The bonding patterns can be related to the relative stabilities of the diazaborines. Among these molecules, the most stable isomer contains an N-B-N-B linkage, while the two least stable isomers have either a B-B or a N-N bond. Changes in bonding patterns are also reflected in changes in the N1-B2 coupling constant. When N1 and B2 are bonded to the less electronegative atoms C and B, (1)J(N1-B2) increases relative to borazine, but when either N1 or B2 is bonded to N, (1)J(N1-B2) decreases. Computed NMR chemical shifts and coupling constants are in good agreement with available experimental data.