Influence of Intramolecular and Intermolecular Interactions on Internal Rotation around C=C Bond in FOX-7 Molecules and Its Derivatives

The internal rotation barriers in 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7) and 2‑(nitromethylene)-1,3-diazacycloheptane (NMDACH) are calculated in this work at the PBE/cc-pVDZ level of theory. The calculation for isolated molecules shows that the rotation in them around the C=C bond is quite possible. The rotation barriers are 19 and 16 kcal/mol for DADNE and NMDACH, respectively. The complexes of DADNE and NMDACH with two nitromethane molecules are considered to study the influence of intermolecular interaction on this barrier. The latter are used as models of DADNE and NMDACH. The simulation shows that intermolecular interaction decreases the internal rotation barrier. This barrier is 12 and 10 kcal/mol in the DADNE and NMDACH complexes with two nitromethane molecules, respectively. In addition, the state, in which nitro group is rotated by 120° relative to the NH–C–NH plane, appears to be the most energetically favorable in the NMDACH complex with two nitromethane molecules.