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

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 N and heterocyclic N atoms, for six reported tautomerizations. The results reveal four linear correlations between reverse reaction barriers () and the electron densities of N and N atoms in the product. N electron density (ρ) and N-N bond polarity, as measured by the difference between the electron densities on the two N atoms (Δρ = ρ - ρ) 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 (Δ = - ). Polar solvents, including DMSO, water, and acetone, can effectively increase the reverse reaction barriers () 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.