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