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.