Why mixtures of hydrazine and dinitrogen tetroxide are hypergolic?

The reactions of N2H4 with N2O4 isomers have been investigated at the G2M(CC3)//B3LYP/6-311++G(3df,2p) level. The results show that the reactions of N2H4 with sym-N2O4 (D2h), cis-ONONO2 (Cs), and NO2 have to overcome 14.2, 10.6, and 7.6 kcal/mol barriers, respectively. However, N2H4 can react spontaneously with trans-ONONO2 (Cs) and cis-ONONO2 (C1) to produce HONO2 + H2NN(H)NO with the near gas-kinetic rate constant, 4 × 10−10 cm3 molecule−1 s−1 above 250 K. The H2NN(H)NO thus formed can rapidly fragment to give reactive N2H3 radical with rate constant > 1 × 107 s−1 at 1000 K, which is fast enough to initiate the hypergolic chain reactions with NO2 and N2O4.