The explosive sensitivity upon the formation of molecule-cation interaction between the nitro group of 3,4-dinitropyrazole (DNP) and H(+), Li(+), Na(+), Be(2+) or Mg(2+) has been investigated using the B3LYP and MP2(full) methods with the 6-311++G** and 6-311++G(2df,2p) basis sets. The bond dissociation energy (BDE) of the C3-N7 trigger bond has also been discussed for the DNP monomer and the corresponding complex. The interaction between the oxygen atom of nitro group and H(+) in DNP…H(+) is partly covalent in nature. The molecule-cation interaction and bond dissociation energy of the C3-N7 trigger bond follow the order of DNP…Be(2+) > DNP…Mg(2+) > DNP…Li(+) > DNP…Na(+). Except for DNP…H(+), the increment of the trigger bond dissociation energy in comparison with the DNP monomer correlates well with the molecule-cation interaction energy, natural charge of the nitro group, electron density ρ(BCP(C3-N7)), delocalization energy E(2) and NBO charge transfer. The analyses of atoms in molecules (AIM), natural bond orbital (NBO) and electron density shifts have shown that the electron density of the nitro group shifts toward the C3-N7 trigger bond upon the formation of the molecule-cation interaction. Thus, the trigger bond is strengthened and the sensitivity of DNP is reduced.

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The asymmetric unit of the title compound, 4C3H2N2O4·C6H6, contains two independent dinitro­pyrazole mol­ecules and half a benzene solvent mol­ecule, which lies on a crystallographic inversion centre. Each pyrazole ring is essentially planar (mean deviations of 0.009 and 0.002 Å), with the two nitro groups rotated out of the plane [dihedral angles = 11.7 (2)/31.1 (1) and 21.8 (2)/25.0 (1)° for the two mol­ecules].

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The substituent effects on the intermolecular T-shaped F-H...π interactions are investigated between HF and LBBL (L = -H, : CO, :NN, -Cl, -CN and -NC) using the (U)MP2(full) and (U)CCSD(T) methods with the 6-311++G(2 d,p) basis set. The B ≡ B triple-bond contraction is found in the complexes with lone-pair-electron donors while the B = B double-bond is lengthened in the systems with the single-electron substituents upon complexation. The T-shaped F-H...π interaction energies follow the order of ClB = BCl...HF>HB = BH...HF>NNB ≡ BNN...HF>OCB ≡ BCO...HF>CNB = BNC...HF>NCB = BCN...HF. The electron-donating substituents : CO and :NN increases electron density of the B ≡ B triple bond by the delocalization interaction E ((2)) π ((CO/NN) → Lp(B)) while the electron-withdrawing substituents -CN and -NC decrease electron density of the B = B double bond by means of the π-π conjugative effect. The analyses of the APT atomic charge, "truncated" model, natural bond orbital (NBO), atoms in molecules (AIM) and electron density shifts reveal the nature of the substituent effect and explain the origin of the B ≡ B bond contraction.