Relaxed structure of typical nitro explosives in the excited state: observation and implication

Understanding the structural, geometrical and chemical changes that occur after electronic excitation is essential to unraveling the inherent physical and chemical mechanisms of nitro explosives. In this work, the relaxed structure of some typical nitro explosives in the excited state, including RDX, HMX, CL-20, PETN and LLM-105, have been investigated by time-dependent density functional theory. During the excitation process, an electron is vertically excited into a low-lying excited state, imparting π-antibonding character onto the nitro group. The nitro group becomes activated by the excitation energy and then relaxes via vibrational cooling, leading to a relaxed excited-state structure. All five nitro explosives exhibit similar behavior in which impact sensitivity is related to the excitation energy of the relaxed structure. Insight into the relaxed structure of typical nitro explosives offers an efficient method of unraveling ultrafast and complex photo-initiated reactions and detonation physics.