Proton quantal delocalization and H/D translocations in (MeOH)nH+ (n = 2, 3)

Area of Science:

• Computational Chemistry
• Quantum Dynamics
• Molecular Simulations

Background:

• Proton delocalization in hydrogen-bonded systems is crucial for chemical and physical properties.
• Understanding proton behavior in small methanol clusters ((MeOH)nH+) provides insights into larger systems.

Purpose of the Study:

• To characterize quantum spatial delocalizations of protons in OH bonds within (MeOH)2H+ and (MeOH)3H+.
• To investigate the influence of molecular structure on proton delocalization and isotopic effects.

Main Methods:

• Path integral molecular dynamics (PIMD) simulations were employed.
• A neural network fitting procedure based on second-order Møller-Plesset perturbation theory was used for force field flexibility.
• Analysis focused on structural characteristics and proton spatial distributions.

Main Results:

• In (MeOH)2H+, shared connective protons exhibit prolate-like delocalization (∼0.1 Å), while dangling protons are confined to a spherical layer (∼0.25 Å).
• Connective protons in (MeOH)3H+ show greater delocalization along the O-H bond and more localization perpendicular to it compared to dangling protons.
• Isotopic propensities (H vs. D) for localization in dangling and connective positions were examined.

Conclusions:

• Proton delocalization characteristics are dependent on the specific cluster structure and proton's role (connective vs. dangling).
• Thermodynamic trends can be explained by local geometry and intermolecular connectivity strengths.
• The study provides a detailed quantum mechanical picture of proton behavior in these methanol-water cation systems.