Exploring the Correlation between Azido-Tetrazolo Tautomerizations and Isomer Structures: Electron Density of Bonding...

Azido-tetrazolo tautomerizations between azido N-heteroaromatic compounds and tetrazole-fused energetic materials can produce a new generation of high-energy density compounds. Density functional theory (DFT) computations are performed to explore the relationship between reaction barriers and electron densities of bonding N atoms, i.e., the terminal N₁ and heterocyclic N₂ atoms, for six reported tautomerizations. The results reveal four linear correlations between reverse reaction barriers (G_r) and the electron densities of N₁ and N₂ atoms in the product. N₁ electron density (ρ_N) and N-N bond polarity, as measured by the difference between the electron densities on the two N atoms (Δρ_N = ρ_N - ρ_N) in products, are inversely proportional to the reverse reaction barriers. They are also proportional to the energy barrier differences between the forward and reverse reactions (ΔG = G_f - G_r). Polar solvents, including DMSO, water, and acetone, can effectively increase the reverse reaction barriers (G_r) by improving the stability of products. This regularity is further confirmed by its application to four additional tautomerizations and can be used to screen out unfavorable azido-tetrazolo tautomerization reactions and increase the success rate of such synthesis.