Dynamics and mechanism of structural diffusion in linear hydrogen bond

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• Proton transfer in hydrogen-bonded chains is fundamental to many chemical and biological processes.
• Understanding the dynamics and mechanisms of proton transfer in model systems is crucial for elucidating complex phenomena.
• Protonated methanol complexes serve as valuable models for studying proton dynamics in hydrogen-bond networks.

Purpose of the Study:

• To investigate the dynamics and mechanism of proton transfer in protonated methanol hydrogen-bond chains (CH(3)OH(2)(+)(CH(3)OH)(n), n=1-4).
• To compare proton transfer characteristics in methanol systems with those in analogous water systems.
• To elucidate the role of thermal fluctuations and molecular dynamics in promoting proton transfer.

Main Methods:

• B3LYP/TZVP density functional theory calculations were employed.
• Born-Oppenheimer Molecular Dynamics (BOMD) simulations were performed at 350 K.
• Analysis included characteristic hydrogen-bond structures, energetics, and infrared (IR) spectra.

Main Results:

• Hydrogen-bond chains with n=1 and n=3 were identified as the most active intermediate states for proton transfer.
• The CH(3)OH(2)(+)(CH(3)OH)(n) complexes exhibited the lowest threshold frequency for proton transfer.
• BOMD simulations revealed that thermal energy fluctuations enhance proton transfer in shared-proton structures (n=3) by facilitating interconversion between oscillatory shuttling and structural diffusion.

Conclusions:

• The n=3 shared-proton structure demonstrates a higher population of structural diffusion motion compared to the n=1 structure, indicating enhanced proton mobility.
• Quasi-dynamic equilibria between shared-proton (n=3) and close-contact (n≥4) structures influence the concertedness of proton transfer reactions.
• The lifetime of the shared-proton intermediate state dictates the rate of structural diffusion, preventing a fully concerted proton transfer mechanism.