The relation between structure and reactivity in five-membered heteroaromatic compounds—I: Effect of assumed molecular geometry on CNDO/2 electron distribution for some tetrazole derivatives

Abstract

Two parallel sets of CNDO/2 calculations have been carried out for 1H-5-aminotetrazole ( 1), 2-methyl-5-aminotetrazole ( 2), 1,3-dimethyl-5-aminotetrazolium cation ( 3) and the tetrazolate anion ( 4). In one set, the molecular geometry was represented by a regular pentagon 1·33 Å on a side, and in the other the experimentally determined molecular geometries were used. The calculated electronic structures from the two sets of calculations are compared; both sets predict similar trends in variation of atomic charges and bond indices with structure. Where comparison is possible, these trends seem to be generally in agreement with expectations based on classical concepts such as resonance. It is noted that the relative magnitudes of σ-electron bond indices seem to parallel those of the π-electron bond indices in many cases, although the σ-bond indices show much smaller variation in absolute magnitude as compared with the π-bond indices. A brief discussion of the relative merits of various geometrical models is presented; it is suggested that, in order to combine the advantages of both approaches, standard and experimentally-determined geometries should be used side-by-side whenever possible. A procedure for speeding convergence by use of a damping factor is described.