Ultrafast diffraction and structural dynamics: the nature of complex molecules far from equilibrium

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Molecular structure studies traditionally focus on equilibrium geometries (bond distances, angles).
• Understanding far-from-equilibrium molecular structures, dynamics, and average geometries is challenging.
• Nonequilibrium structures are crucial for energy redistribution and chemical reactivity.

Purpose of the Study:

• To investigate transient molecular structures far from equilibrium using ultrafast electron diffraction.
• To compare the behavior of two cyclic hydrocarbons at high internal energies (approx. 4 eV).
• To probe the structural dynamics and energy redistribution in complex molecules under nonequilibrium conditions.

Main Methods:

• Utilized ultrafast electron diffraction (UED) to observe transient molecular structures.
• Studied two cyclic hydrocarbons: 1,3,5-cycloheptatriene and 1,3-cyclohexadiene.
• Analyzed molecular structures at high internal energies and over extended timescales (up to 400 ps).

Main Results:

• 1,3,5-cycloheptatriene formed hot ground-state structures with increased vibrational amplitudes but similar bond distances.
• Energy redistribution in 1,3,5-cycloheptatriene occurred within 5 ps, exhibiting negative temperature.
• 1,3-cyclohexadiene underwent ring-opening to hot structures with a significantly elongated CC bond (1.7 Å) and trapped energy in large-amplitude motions up to 400 ps.

Conclusions:

• Ultrafast electron diffraction provides direct insights into nonequilibrium molecular structures.
• Cyclic hydrocarbons exhibit divergent structural dynamics and energy relaxation pathways under high excitation.
• These findings open new avenues for studying structural dynamics in complex systems far from equilibrium.