Revealing the Ultrafast Energy Transfer Pathways in Energetic Materials: Time-Dependent and Quantum State-Resolved.

Intramolecular vibrational energy transfer is gaining tremendous attention as a regulator of condensed-phase behavior and reactions. In polyatomic molecules, this transfer is an ultrafast process involving multiple modes with numerous quantum states. The inherent complexity and rapid evolution of these processes pose significant challenges to experimental observation, and the high computational costs make full quantum mechanical calculations impractical with current technology. In the intramolecular energy transfer process, whether the doorway modes are intermediaries for transferring energy from lattice phonons to high-frequency intramolecular vibrational modes has been a controversial issue. However, the broad range of doorway modes complicates the experimental identification of a specific doorway in the transfer process corresponding to a specific end point. Here, for the first time, we utilize a combination of vibrational projection, statistical analysis, and the local quantum vibrational embedding (LQVE) method to elucidate the ultrafast energy transfer pathways that upconvert energy from lattice phonons to intramolecular modes in the typical energetic material β-HMX. This approach enables us to resolve the coupled vibrational mode groups, identify the most probable energy transfer pathways corresponding to the different final modes, and clearly confirm that the doorway region is a mandatory pathway for energy transfer. The LQVE method's time-dependent and quantum state-resolved advantages are leveraged to reveal the microscopic mechanism of the energy transfer process. The time scale of these processes is determined at about 1 ps, and the first theoretical two-dimensional infrared spectroscopy evidence is provided, which is confirmed by the experimental results. These findings deliver important insights into the fundamental mechanisms of ultrafast energy transfer in energetic materials, providing theoretical support for controlling explosive behavior and designing new explosives. The methodologies developed in this work can be extended to other condensed phase materials and used to evaluate the coupling between multiple vibrational modes.