Two novel types of intermolecular hetero cycloadditions in the participation of the nitro group are put forward in the dimerization of TNT, in comparison with Diels-Alder cross-linking of benzene ring skeletons. Possible transition states and products; for example, their geometrical details, vibrational frequencies, and energies are verified at the B3LYP/6-31+G(d,p) level. Contrary to the hetero Diels-Alder reaction, the folding of the benzene ring side endo is slightly selective specific in 1,3-dipolar cycloaddition. The substituent pattern on reactivity indicates that the methyl group at the bridged sites significantly retards the reaction. Two new sigma bonds are formed kinetically and thermodynamically through the single state; however, the first sigma bond is more easily generated by the triplet state. The shock-wave-produced chemically bound dimer of TNT is most likely to be the oxygen-carbon linkage product. The initial chemical events in TNT under high pressure can be extended to interpret the shock insensitivity of other unsaturated nitro-explosives. The calculated results agree well with some experimental observations.