A theoretical study of the 1,2,3-triazolo-iminodiazomethane isomerism in the gas phase

The ring opening reactions of the 1,2,3-triazolate anion, of the three taut omers of NH-1,2,3-triazole, and of different 1,2,3-triazolium cations, to g ive the corresponding iminodiazomethanes, have been studied by means of ab initio calculations at the HF/6-31G** level. The 1,5-electrocyclization rea ctions of 1,2,3-triazolate and 1H-1,2,3-triazole proceed through planar tra nsition states with high and medium energy barriers, respectively. In both cases the cyclic forms are thermodynamically more stable than the open-ring ones. In contrast, the decomposition of the 1,2-H-1,2,3-triazolium cation into protonated iminodiazomethane is an exothermic process. In the last cas e, the preferable way of the reaction proceeds via non-aromatic intermediat es (1,4H(2)- and 1,1H(2)-1,2,3-triazolium ions) which have a very low activ ation energy towards the ring opening. It has been found that there is a hi gh pi pi-conjugation between diazomethyl and imine fragments in the chain f orms.