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 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 (ρN1 ) and N-N bond polarity, as measured by the difference between the electron densities on the two N atoms (ΔρN = ρN1 - ρN2 ) 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 G = 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.