Isomerization of N₂O₄ in Solid N₂H₄ and Its Implication for the Explosion of N₂O₄-N₂H₄ Solid Mixtures

The mechanism responsible for the explosion of solid mixtures of nitrogen tetroxide (N₂O₄) and hydrazine (HZ) or methyl-substituted HZs, detected experimentally by slow warming from 77 to 203-223 K, has been elucidated by quantum chemical calculations using the Vienna ab initio simulation package code. The result of the calculation for the reaction of a N₂O₄ molecule embedded in the middle of the N₂H₄ molecular crystal, N₂O₄@HZ₂₃, indicates that a loose nonconventional transition state (TS) occurring by stretching the O₂N-NO₂ bond up to 2.18 Å with the concerted rotation of one of the NO₂ groups producing the reactive ONONO₂ isomer (ONONO₂@HZ₂₃) has a low 13.1 kcal/mol barrier at TS1; the process is exothermic by 45 kcal/mol, reflecting the much stronger ONONO₂ binding with N₂H₄. A further simultaneous reaction of ONONO₂ with 2N₂H₄ in the same unit cell occurs with a small 1.4 kcal/mol barrier producing NO₃ ^- + NH₂N(H)NO + N₂H₅ ⁺ with an overall exothermicity of 70.2 kcal/mol. The mechanism for this last-step reaction is distinctly different from that in the gas phase taking place via a five-centered concert mechanism giving N₂H₃NO and HNO₃, which can further produce N₂H₅ ⁺NO₃ ^- by the rapid acid-base neutralization process. On the basis of the predicted structure, energy, and vibrational frequencies at TS1, we estimated the rate constant at 218 K for the N₂O₄-ONONO₂ isomerization reaction in solid N₂H₄ to be 1.35 s^-1, giving the half-life of N₂O₄@HZ₂₃ to be as short as 0.5 s. This result can explain why the slow warming of the solid mixtures of N₂O₄ and N₂H₄ from 77 K exploded reproducibly at 218 K.