To gain further insight concerning the dimerization process, we calculate the binding energies (BE) between nitrogen atoms and methylene bridges. In every dimer, two monomers are linked together via four N–CH2 bonds, see Fig. 3. The breaking of these bonds leads to the dimer separation into four fragments (two CH2s and two cages). So, we define BE of N–CH2 bonds as the quarter of energy difference between these fragments and entire dimer. Obtained BEs are also presented in Table 2. These values of BE are even higher than the value of BE calculated at the same level of theory for the CH3–NH2 molecule (4.20 eV). One can see that BEs for the Si-substituted dimers are significantly higher than the corresponding values for the CL-20 dimer. Therefore, Si-substituted cages possess a stronger connection with each other via the methylene bridges than the pristine CL-20s.
During the molecular dynamics simulation of heated dimers, we observe the same pyrolysis mechanisms as for the corresponding monomers (NO2 fission and O/F migration to Si). We have never observed the contribution of methylene bridges to the initial decomposition step. So, we conclude that dimers are approximately as kinetically stable as the corresponding monomers.
UV spectra of the CL-20 derivatives and their dimers
Optical spectra analysis is the common way for the experimental detection of dimerization of isolated CL-20 or its derivatives. According to our time-dependent density functional calculations, all considered cages, as well as their dimers, are not active in the visible range due to their hardness. However, they demonstrate an optical response in the UV range (150 ÷ 250 nm). UV spectra for the CL-20, CSi5H6N12O12, and C4Si2H6N12F12 systems as well as for the corresponding low-energy dimers are presented in Fig. 5. Dimer formation leads to the redshifts on the spectra for all considered cages. This effect is expected with regard to larger effective sizes of dimers in comparison with monomers. The values of redshifts are 29, 25, and 18 nm for the CL-20, CSi5H6N12O12, and C4Si2H6N12F12, respectively. These values are sufficient for the dimers formation detection using their spectral analysis.