CL-20 photodecomposition: ab initio foundations for identification of products

1,5-Dihydrodiimidazo[4,5-b:4'5'e]pyrazine, 1H-imidazo[4,5-b]pyrazine, and 1H-imidazole were considered as possible products of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) photodecomposition. Since we took as a reference the product obtained after CL-20 irradiation in methanol solution, the nature of intermolecular bonds between heterocycles under study and methanol molecules was analyzed in detail. Existing hydrogen bonds were found to be quite strong, so dependence of calculations results on an influence of solvent was taken into account using both the polarizable continuum model (PCM) and the supermolecular approach. Electronic spectra of 1,5-dihydrodiimidazo[4,5-b:4'5'e]pyrazine, 1H-imidazo[4,5-b]pyrazine and 1H-imidazole were simulated using time dependent density functional theory (TD-DFT) and single-excitation configuration interaction (CIS) method. We observed that TD-DFT excitation energies are lower if compared to corresponding values obtained by the CIS method. Results of calculations with PCM and the supermolecular approach are very close. It was found that differences between calculated gas phase excitation energies and those values obtained by applying solvent models increases when the number of conjugated bonds in a molecule increases. Oscillator strengths of UV bands of the considered molecules are higher in the gas phase than in modeled methanol solutions. We found that the predicted spectrum of 1H-imidazole is in close agreement with the experimental UV spectrum of the CL-20 photolysis product.