Scattering off molecules far from equilibrium

Area of Science:

• Physical Chemistry
• Molecular Dynamics
• X-ray Scattering

Background:

• Pump-probe X-ray scattering reveals molecular structures far from equilibrium.
• Thermal excitation and energy randomization create complex molecular systems.
• Traditional methods fail for large amplitude vibrational motions in polyatomic molecules.

Purpose of the Study:

• Introduce a novel method using molecular dynamics trajectories for analyzing X-ray scattering data.
• Apply the method to hot, vibrating molecules at thermal equilibrium.
• Address limitations of traditional analytical solutions for complex molecular structures.

Main Methods:

• Utilized pump-probe gas phase X-ray scattering experiments.
• Employed molecular dynamics simulations.
• Analyzed data from excited 1,3-cyclohexadiene (CHD) and 1,3,5-hexatriene (HT) molecules.

Main Results:

• The novel method accurately describes molecular structures deviating significantly from equilibrium.
• Experimental and theoretical results for CHD and HT showed excellent agreement.
• Identified transition state structures near the inversion barrier of CHD contributing to scattering signals.
• Clarified that previous inconsistent structural parameters for HT were artifacts of inapplicable analytical equations.

Conclusions:

• The molecular dynamics approach provides accurate structural information for molecules far from equilibrium.
• This method overcomes limitations of traditional harmonic approximations for anharmonic systems.
• The findings resolve inconsistencies in previous structural determinations for 1,3,5-hexatriene.