Why mixtures of hydrazine and dinitrogen tetroxide are hypergolic?

► This Letter elucidates how the hypergolic reaction of N2H4 and N2O4 occurs. ► The reactions of N2H4 with N2O4 isomers have been investigated computationally. ► N2H4 can react spontaneously with trans-ONONO2 (Cs) and cis-ONONO2 (C1). ► The H2NN(H)NO formed in the reactions can rapidly produce N2H3 radical. ► N2H3 radical can initiate chain reactions with NO2 and N2O4.