Quantum-chemical study of five-membered nitrogen-containing heterocycles: III. Prototropic tautomerism of 1-aryltetrazol-5-ones and 1-aryltetrazole-5-thiones

Prototropic tautomerism of 1-phenyl- and 1-(4-nitrophenyl)tetrazol-5-ones and l-phenyl- and 1-(4-nitrophenyl)tetrazole-5-thiones in the gas phase was studied by the AM1 semiempirical method. A satisfactory agrrement with the experimental data and the results of ab initio calculations with the 6-31G** basis set was observed for some prototropic forms of tetrazol-5-ones and tetrazole-5-thiones calculated with correction for mutual repulsion of lone electron pairs of adjacent nitrogen atoms. The most thermodynamically stable neutral forms are 4,5-dihydro-1H-tetrazol-5-ones and 4,5-dihydro-1H-tetrazole-5-thiones whose protonation yields mainly 5-hydroxy(or mercapto)-1,3-dihydrotetrazolium ions.