PULSE-RADIOLYSIS OF AQUEOUS HYDRAZINE SOLUTIONS - TRIAZENE SPECIES

Pulse radiolysis of aqueous hydrazine solutions has been studied in the pH range 2-13. At times greater than about 1 ms after the pulse, a single transient species is observed (λ, 230 nm). This species decayed by first-order kinetics under all conditions studied. A mechanism which accounts satisfactorily for the observed kinetic behavior is proposed. From the available evidence, this species, identified as triazene (N3H3), is amphiprotic and the following acid-base equilibria are rapidly established: N3H4+ ⇄ N3H3 + H+(pK1 = 4.95, ΔH1° = -4.9 kcal mol-1, ΔS1° = -39.2 eu) and N3H3 ⇄ N3H2- + H+ or N3H3 + OH- ⇄ N3H4(T (pK2 = 11.37, ΔH2° = -4.2 kcal mol-1, ΔS2° = -67 eu). pK values derived from the dependence of the first-order rate constant on pH, from the dependence of initial absorbance on pH, and from the dependence of the first-order rate constant on temperature at selected values of pH are in satisfactory agreement. The rate constants for decomposition of the acidic and basic forms of triazene are kN3H4+ = 1.97 × 1011 exp(-12600/RT) s-1 and kN3H4- = 2.14 × 1014· exp(-19200/RT) s-1. For N3H3, which is stable relative to the faster reaction rates of its conjugate acid and base forms, k is estimated to be ≤0.001 s-1 at 24 °C. The dependence of the observed first-order rate constant on pH at constant temperature is expressed by the following equation: kobsd = [k3/{l + (1 + [H+])}] + [k4/{l + ([H+]/K2)}] (k3 = kN3H4+ = 133 s-1, k4 = kN2H2- = 2 s-1, pK1 = 4.95 and pK2 = 11.37). Phosphate is a catalyst for the decomposition of triazene. From studies on the salt effect, it is shown that the conjugate acid of neutral triazene has unit positive charge and its conjugate base has unit negative charge. © 1979 American Chemical Society.