Application of natural bond orbital analysis to delocalization and aromaticity in C-substituted tetrazoles

Energies of two tautomeric forms of 10 tetrazole derivatives substituted at C5 were established by DFT/B3LYP calculations carried out at the 6-311++G** level. In each case the calculated energy of the 2H-tautomer was lower than that of the 1H. Furthermore, three geometric aromaticity indices of both forms were calculated, as were the values of nuclear independent nuclear shift and aromatic stabilization energy. The electronic properties were evaluated with the help of the natural bonding orbital theory. Following this a new. pi -delocalization parameter, the root-mean square of pi -electron density localized on the atoms of the five-membered tetrazole ring, SDn, was introduced. It was concluded that the electronic delocalization can be described equally well by three different parameters: SDn, the extent of the transfer of electron density from the p(z) orbital of one nitrogen to the rest of the pi electron system, and population of two antibonding pi* orbitals. Arguably, the information provided by the electronic parameters is similar to that contained in the geometric (structural) aromaticity indices except for tetrazole substituted by -BH2.