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.
|Original language||English (US)|
|Number of pages||4|
|Journal||Journal of Organic Chemistry|
|State||Published - 1988|
All Science Journal Classification (ASJC) codes
- Organic Chemistry