Modeling Photolytic Decomposition of Energetically Functionalized Dodecanes

Abstract

The photolytic stability of explosives and energetic functional groups is of importance for those who regularly handle or are exposed to explosives in typical environmental conditions. This study models the photolytic degradation of dodecane substituted with various energetic functional groups: azide, nitro, nitrate ester, and nitramine. For the studied molecules, it was found that excitons localize on the energetic functional group, no matter where they were initially formed, and thus, the predominant degradation pathway involves the degradation of the energetic functional group. The relative trends for both 4 and 8 eV excitation energies followed with what is expected from the relative stability of the energetic functional groups to thermal and sub-shock degradation. The one notable exception was the azide functional group; more work should be done to further understand the photolytic effects on the azide functional group.

Radiolytic degradation of dodecane substituted with common energetic functional groups

“…To the extent that 1 H-NMR spectroscopy results could be collected, the integrated amount of decomposition products relative to the parent peaks show a similar trend to the total as yield analysis, as shown in Table 1; the uncertainties listed are systematic uncertainties that were propagated through for the G -value calculation. Interestingly, the order of the results matches well with both photolytic molecular dynamics simulations of these molecules at 8 eV 14 as well as the general trend for the sub-shock impact sensitivity of various explosives. 31–33 Although a more limited study, these results also provide an explanation why, for a previous study exploring radiolytic damage to a variety of explosives, 25 the only explosive that was found to degrade with low-level radiation in large enough quantities to be detectable using 1 H-NMR spectroscopy was pentaerythritol tetranitrate (PETN), a nitrate ester-based explosive.…”

“…Exciton migration and localization on the energetic FG was also seen with non-adiabatic excited state molecular dynamics (NEXMD) calculation performed on the same molecules. 14 By concentrating the energy on the energetic FGs, less degradation of the dodecane backbone could occur and thus most of the H 2 gas measured must be due to direct interactions with the C–H bonds of the dodecane molecule. Another possible explanation for the decrease in the hydrogen gas G -value for the substituted dodecane molecules with respect to pure dodecane involve the general delocalization of the molecular exciton in alkanes rather than localized transport.…”

“…It is interesting to compare the above results with the 8 eV results found in the photolytic NEXMD calculations completed on the same molecules. 14 Of course, radiolysis and photolysis are not the same process, but a significant portion of the primary decomposition in alkanes can be attributed to low-energy electronically excited states generated through absorption of scattered radiation. 22 Additionally, the photolytic NEXMD calculations allow for a look at the fast time scales for degradation involving the S 1 excited state, an insight that is unachievable for the radiolysis experiments in this study.…”

“…In the 8 eV computations involving the substituted dodecane molecules, the exciton was initially localized on the dodecane backbone where it then quickly relaxed to the S 1 excited state and moved to the energetic FG to undergo degradation. 14 For the D-N 3 molecule, the degradation would mostly involve the scission of the N-N bond, but contributions involving the scission of the C-N 3 as well as degradation to the dodecane backbone were also seen. For the D-NO 2 molecule, only the C-NO 2 bond was broken.…”

“…These same molecules were also the focus of an earlier photolytic computational study. 14 Although photolysis and radiolysis are different processes, it is still interesting to make broad comparisons across the different energy regimes.…”