A dimer of HCN, N-cyanomethanimine 6, was prepared by flash pyrolysis (gas phase, 10-2 mbar) from 3-azido-1, 2, 4-triazole 1 a and detected by ultraviolet photoelectron spectroscopy (HeI). Its ionization potentials were measured and the ionic states sequence proposed, with the aid of ab initio-CI (configuration interaction) calculations. The study of the MNDO potential hypersurface enabled us to propose a mechanism of the thermal ...