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 (DeltaG298 = 19.7 kcal/mol for 2a and DeltaG298 = 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 DeltaG298 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 degrees 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.