Molecular orbital studies of tautomerism in tetrazole

Results from ab-initio calculations of the fully optimized structures of the two tautomers of tetrazole indicate that in the gas phase the 2H tautomer is the energetically preferred form. Inclusion of polarization functions and electron correlation allowed an estimation of the energy difference between the tautomers as ca. 2 kcal/mol. Vertical ionization potentials calculated from the difference between the energies of the radical cations and the neutral molecules agree with observed photoelectron spectra. Structure optimization of the radical cations indicates a greater susceptibility to fragmentation of the π over the σ radicals. The preference of the 2H tautomer in gas phase taken together with suggested fragmentation from structure optimization explains the observed fragmentation in mass spectra of tetrazole and 5-methyltetrazole.