Theoretical studies of -NH₂ and -NO₂ substituted dipyridines

In this work, the experimental synthesized bipyridines 3,3'-Dinitro-2,2'-bipyridine (DNBPy), 3,3'-Dinitro-2,2'-bipyridine-1,1'-dioxide (DNBPyO), and (3-Nitro-2-pyridyl)(5-nitro-2-pyridyl) amine (NPyA), and a set of designed dipyridines that have similar frameworks but different linkages and substituents with NPyA were studied theoretically at the B3LYP/6-31G* level of density functional theory. The gas-phase heats of formation were predicted based on the isodesmic reactions and the condensed-phase heats of formation and heats of sublimation were estimated in the framework of the Politzer approach. The crystal densities have been computed from molecular packing. Results show that this method gives a good estimation of density in comparison with the available experimental data for DNBPy, DNBPyO, and NPyA. The predicted detonation velocities and pressures indicate that the performance of dipyridines linked with -O-, -NH-, or -CH₂- bridges have not been improved compared with that of the directly linked dipyridines, but all derivatives have better detonation properties than DNBPy, DNBPyO, and NPyA because of the presence of more nitro groups. An analysis of the bond dissociation energies (BDEs) or the impact sensitivity (h₅₀) suggests that introduction of different bridges but not substituents has little influence on thermal stability. The calculated h₅₀ may be more reliable than BDE for predicting stability. Four bridged bipyridines have quite good detonation performance and low sensitivity.