Doi:10.1016/j.theochem.2003.10.019

A theoretical study of the tautomerism and ionization of 5-substituted NH-tetrazoles

The energies, geometries and charge distributions of the five more stable forms (anion, 1H- and 2H-neutral tautomers, 1,3-H,H⁺- and 1,4-H,H⁺-cation tautomers) of a series of 5-R tetrazoles [R=H, CH₃, C(CH₃)₃, Ph, Cl, CF₃, NO₂] in the gas phase have been calculated with the DFT/B3LYP method at the 6-31G* level. For tetrazolate anions, a shortening of the 1–2 bond and a simultaneous elongation of the 2–3 bond is observed when the value of the σ_p constant of the substituent increases. A considerable ring aromaticity is exhibited by tetrazolate anions, and also by the 2H-form of neutral tetrazoles and 1,3-H,H⁺-form of cations and it somewhat depends on the nature of the substituent. The calculated values of the dipole moments of 2H- and 1H-forms of neutral tetrazoles correlate well with the σ_p constants of substituents. The relative thermodynamic stability of 2H-forms as compared with 1H-ones does not practically depend on the nature of the substituent R. In contrast, in the case of tetrazolium cations, the most stable form is the 1,4-H,H⁺—for the electron-donating substituents and the 1,3-H,H⁺—for the electron-withdrawing ones. Good correlations have been observed between the energies of protonation of anions and neutral tetrazoles and the experimental pK_aand pK_BH+ values.