STRUCTURES, BONDING AND ENERGIES OF N6 ISOMERS

The most stable N6 isomer, a twisted open-chain C2 structure, is 188.3 kcal/mol (MP4SDTQ/6-31G(d)//MP2(full)/6-31G(d)+ZPE(MP2/6-31G(d))) higher in energy than three N2 molecules. In contrast to benzene, hexazine, N6, prefers a non-planar twist-boat D2 structure, but this is 26.0 kcal/mol less stable than the C2 form. The D6h alternative is a second-order saddle point at MP2 (full)/6-3 IG(d) 2.1 kcal/mol higher in energy at MP4SDTQ/6-311 (+s)G(d)//MP2(full)/6-311 (+s)G(2d) +ZPE(MP2)/6-31 G(d)). The homodesmotic and hyperhomodesmotic reaction energies indicate that the D6h structure is destabilized (-17.6 and -10.4 kcal/mol, respectively), in contrast to the stabilization of benzene (23.9 and 20.3 kcal/mol, respectively, MP4SDTQ/6-31G(d, p)//MP2(full)/6-31G(d, p)). NBO analysis shows that none of the N atoms in the N6 open-chain structures forms more than four covalent bonds. The other N6 valence isomers, hexaaza-Dewar-benzene, hexaazabicyclopropenyl, and hexaazaprismane are higher in energy than hexazine (33.9, 29.6 and 115.8 kcal/mol, respectively) at MP4SDTQ(fc)/6-31G(d)/MP2(full)/6-31G(d)+ZPE(HF/6-31G(d)).