Why mixtures of hydrazine and dinitrogen tetroxide are hypergolic?

The reactions of N ₂H ₄ with N ₂O ₄ isomers have been investigated at the G2M(CC3)//B3LYP/6-311++G(3df,2p) level. The results show that the reactions of N ₂H ₄ with sym-N ₂O ₄ (D _2h), cis-ONONO ₂ (C _s), and NO ₂ have to overcome 14.2, 10.6, and 7.6 kcal/mol barriers, respectively. However, N ₂H ₄ can react spontaneously with trans-ONONO ₂ (C _s) and cis-ONONO ₂ (C ₁) to produce HONO ₂ + H ₂NN(H)NO with the near gas-kinetic rate constant, 4 × 10 ^-10 cm ³ molecule ^-1 s ^-1 above 250 K. The H ₂NN(H)NO thus formed can rapidly fragment to give reactive N ₂H ₃ radical with rate constant > 1 × 10 ⁷ s ^-1 at 1000 K, which is fast enough to initiate the hypergolic chain reactions with NO ₂ and N ₂O ₄.