Arylpentazoles Revisited:  Experimental and Theoretical Studies of 4-Hydroxyphenylpentazole and 4-Oxophenylpentazole Anion

Kinetic measurements for the degradation of 4-hydroxyphenylpentazole (2a) and its salt 2b-NBu₄ in CD₃OD and in CD₂Cl₂ provided a set of activation parameters. The resulting free energies of activation in methanol (ΔG^‡₂₉₈ = 19.7 kcal/mol for 2a and ΔG^‡₂₉₈ = 20.6 kcal/mol for 2b-NBu₄) were compared with previous results for the 4-chloro derivative, 2c, and collectively correlated with results of gas-phase calculations at the B3LYP/6-31+G(d,p) level of theory. This, and another linear correlation of the seven computed ΔG^‡₂₉₈ values with the previously reported kinetic data of Ugi and Huisgen, gave the basis for the estimation of the stability of pentazole anion (1) and its derivatives in solutions. Thus, N₅(−) is predicted to have t_1/2 = 2.2 d, while the half-lifetime for HN₅ is expected to be only about 10 min in methanol at 0 °C. Controlled ozonolysis of 2b-NBu₄ followed by ¹H and ¹⁵N NMR spectroscopy shows a preferential destruction of the N₅ ring, which excludes it from possible methods for preparation of the parent pentazole.