Ab Initio Studies of Triazenes in Relation to Experimental Findings

The sites of protonation of triazene (HN═NNH₂), 1-methyltriazene, and 1,3-dimethyltriazene were investigated by ab initio molecular orbital calculations. Proton affinities, E(AH)⁺ - E(A), were calculated for each possible geometry-optimized triazene species. The proton affinities for (E)-triazene were calculated at the 3-21G basis set level; then single-point calculations of the 3-21G geometries were carried out with the 6-31G* basis to test the effect of a larger basis set. Møller-Plesset perturbation theory to the third order was used to obtain the correlation correction. The Hartree-Fock level calculations were found to be in agreement with those which included correlation in predicting that the preferred site of protonation was N₁. Proton affinities of 1-methyltriazene and 1,3-dimethyltriazene (both cisoid and transoid), calculated at the 3-21G level, also predicted that N₁ was the preferred site of protonation. Kinetic studies on the acid-catalyzed decomposition of triazenes have suggested that protonation at N₃ is the key step which leads to heterolysis. The calculated proton affinities reveal that protonation at N₃, although slightly less favorable than protonation at N₁, results in a molecule in which the N₂-N₃ bond is considerably elongated. These theoretical findings suggest that although protonation at N₁ may be thermodynamically preferred, protonation at N₃ leads to a structure which is predisposed to heterolysis of the N₂-N₃ bond. Proton affinity calculations on (Z)-triazene showed that the preferred site of protonation was N₁ but that attempts to attach a proton to N₃ resulted in the scission of the N₂-N₃ bond. Possible mechanistic implications of that finding are discussed.