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

Abstract

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 A 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-st...