Formation of the cyclo-pentazolate N₅^- anion by high-energy dissociation of phenylpentazole anions

The recent successful preparation of the cyclo-pentazolate N ₅^- anion (cyclo-N₅^-) by the dissociation of the p-pentazolylphenolate anion using high-energy collisions is accounted for by considering the electronic structure of the system. It is shown that a symmetry-allowed conical intersection is involved, leading directly from an electronically excited state of the precursor to ground-state cyclo-N ₅^-. The presence of the conical intersection is manifested by the structure of the thermal transition state of the C-N bond dissociation reaction, which is shown to be bent. A similar mechanism is proposed for the formation of cyclo-N₅^- from the dimethylami- nophenylpentazole anion radical. High-level model calculations on the dissociation of these precursors and of the HN₅̇^- anion radical, which is the parent molecule of the larger aromatic pentazolates, support the proposed model.