Relation between the substituent effect and aromaticity in tetrazoles, protonated tetrazoles and tetrazolate derivatives

The energies, geometries and aromaticity of a series of 5R tetrazoles [R = NH2, OH, OCH3, SCH3, H, CH3, F, Cl, BH2, CF3, CN, NO, NO2], their anions and protonated forms in the gas phase have been calculated with the DFT/B3LYP method at the 6-31++G∗∗ level. We have analyzed the change of local aromaticity by NICS and HOMA and found a considerable ring aromaticity by tetrazole and tetrazolate anions and their protonated forms. In analysis by NICS, anion forms have the least aromaticity and in neutral forms 1-H tetrazoles have less aromaticity character than 2-H ones. Interestingly among of protonated forms 2,3-H forms have the most aromaticity character and others follow: 1,2-H > 1,3-H > 1,4-H. However, analysis by HOMA index showed that aromaticity is related to nature of groups at the 5 position. The protonated form of 1,4-H tetrazoles have the least aromaticity character.