Quantifying entropic barriers in single-molecule surface diffusion.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• The role of entropy in surface diffusion for molecules with multiple degrees of freedom remains poorly understood.
• Quantitative analysis of entropic contributions to diffusion barriers is essential for complex molecular systems.

Purpose of the Study:

• To quantify entropic diffusion barriers and attempt frequencies for oligophenyl molecules on amorphous silica.
• To systematically decompose the Arrhenius equation and analyze the influence of molecular length on entropic effects.

Main Methods:

• Extensive molecular dynamics simulations were performed on oligophenyl molecules (benzene to six phenyl rings) on an amorphous silica surface.
• Velocity auto-correlation functions were used to evaluate attempt frequencies.
• The Arrhenius equation was decomposed to analyze entropic contributions to the diffusion free energy barrier.

Main Results:

• Attempt frequencies align with the transition state theory prediction of k_BT/h.
• Entropy contributes significantly to the free energy barrier, up to 55%, increasing with molecular length (4.1 kJ/mol/phenyl ring).
• The entropic barrier represents 40%-60% of the molecule's surface adsorption free energy, indicating liberation of conformational states at transition states.

Conclusions:

• Internal entropic effects are substantial and essential for accurate quantitative studies of surface diffusion in complex molecules.
• The findings highlight the importance of considering molecular flexibility and internal degrees of freedom in surface diffusion processes.
• This research provides a quantitative framework for understanding the entropic contribution to surface diffusion barriers.