Most explosives used worldwide are in solid states. However, intermolecular interactions in the dimers usually reflect the basic features of molecular packing in the solid state. Consequently, intermolecular interactions in solid states as well as in dimers have great influences on the mechanics, sensitivities and molecular packing et al. The binding energies of intermolecular interactions are much less than those of chemical bonding, and yet they play significant roles in a wide range of physical, chemical and biological fields. Theoretical treatment of intermolecular interactions can be expected to elucidate the nature of the forces stabilizing these systems that will be of great value for the design of energetic composites. In this paper, systematical studies on intermolecular interactions were presented for some typical energetic systems in both the gaseous dimers and the solid states. Results are as follows:
1 Four stable dimers (Ⅰ,Ⅱ,ⅢandⅣ) of 1,1-diamino-2,2-dinitroethylene were located using density functional method. The corrected binding energy of the most stable dimerⅣis predicted to be–38.15 kJ/mol at the B3LYP/6-311++G** level. It was found that the structure of the most stable dimer is just the basic packing pattern in the wave-shaped layer of 1,1-diamino-2,2-dinitroethylene solid phase. Vibrational modes associated with the N-C-N rocking exhibit blue shifts with large intensities as the results of large dipole moment changes, whereas those assigned to the stretching of N-H, which is bound by another submolecule, exhibit large red shifts (over -21cm-1) with respect to those of the monomer. The changes of Gibbs free energies (ΔG) in the processes from the monomer to the dimers at 298.15K are 16.46, 16.01, 11.85 and–1.78 kJ/mol for dimersⅠ,Ⅱ,ⅢandⅣ, respectively. DimerⅣcan be spontaneously produced from the isolated monomer at room temperature. The calculated lattice energy is–105.81 kJ/mol, and this value decreases to–114.06 kJ/mol when a 50% correction of the basis set superposition error is adopted, which is in good agreement with the theoretical values already reported. The frontier bands are quite flat. Judged from the value of band gap of 4.0 eV, it may be predicted that 1,1-diamino-2,2-dinitroethylene is an insulator. The frontier crystalline orbitals are formed by contributions from the C, N and O atoms of the C-NO2 group, indicating that there exists a strong conjugation in the molecule and that the C-NO2 group is the most reactive part of it. The population of the C-NO2 bond is much less than those of the other bonds and the detonation may be initiated by the breakdown of this bond.
2 DFT calculations at the B3LYP/6-21G* level were performed on crystalline octanitrocubane. The carbon atoms make up both the lower and the higher energy bands. The projection of density of state as well as the Mulliken populations obviously demonstrates that the C-C bonds are the weakest, indicating that the cubic cage skeleton is the most reactive parts of the molecule. An anisotropic impact on the bulk makes the electron transfer from carbon and nitrogen atoms to oxygen atoms. The high electronic density at the center of the cubic cage attributes some stabilization to the bulk. The crystal lattice energy is predicted to be -40.55 kJ/mol.
3 Density functional method with different basis sets was applied to the study of the highly efficient and low sensitive explosive 3-nitro-1,2,4-triazole-5-one (NTO) in both gaseous dimer and its bulk state. The binding energies have been corrected for the basis set superposition errors. Six stable dimers (Ⅱ~Ⅶ) were located. The