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 sigma(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 sigma(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(a) and pK(BH+) values. (C) 2004 Elsevier B.V. All rights reserved.