Theoretical study on effect of substituent on aromaticity of tetrazole ring

Tetrazole compounds are a category of energetic materials with great potential. Their stability is inherently related to their aromatics. On the basis of research in DFT at b3lyp/6-311** level on three compounds interested, viz. 5-amino-1 H-tetrazole, 5-carboxyl-1 H-tetrazole, and 2-methyl-5-carboxyl-tetrazole, their aromatic characters are extensively analyzed by virtue of different estimation method. Analyses show that these methods, Nucleus-independent Chemical Shift, iso-chemical shielding surface, adaptive natural density partitioning, pi-electrons density, and localized orbital locator of pi-electron density are efficient for substituted-tetrazole ring aromaticity estimation. Conclusion denotes that both substituent and substituted position can significantly affect the aromaticity of tetrazole ring of tetrazole compound. Among these given substituents, electrowithdrawing group -COOH significantly increases the aromaticity of the tetrazole rings due to the withdrawing to pi-electrons thereof but electro-donating group -NH2 weakens the one of the tetrazole rings due to the donating to pi-electrons thereof. But for 2-methyl-5-carboxyl-tetrazole, the aromaticity increasing of -COOH imposed on pi-electrons of tetrazole ring thereof is partially balanced by the existence of the electron-donating group -CH3 at asymmetric meta-position of tetrazole ring. Consequently, the aromaticity order is 5-Carboxyl-1 H-tetrazole > 2-methyl-5-Carboxyl-tetrazole > 5-amino-1 H-tetrazole.