Understanding Isotope Substitution Effects in Water Using the Potential Energy Landscape Formalism for Quantum...

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Statistical Mechanics

Background:

• Isotope substitution significantly impacts water's thermodynamic, dynamic, and structural properties, especially at low temperatures.
• Understanding these effects is crucial for accurately modeling water behavior across various conditions.

Purpose of the Study:

• To rigorously describe H, D, and T isotope substitution effects in water using the potential energy landscape (PEL) formalism.
• To elucidate how isotopic changes influence the quantum and classical behavior of water molecules.

Main Methods:

• Path-integral (PI) computer simulations were performed for H2O, HDO, D2O, and T2O.
• Simulations covered temperatures from 200 to 400 K at a specific molar volume (18.0 cm³/mol).
• The potential energy landscape (PEL) formalism was applied to analyze the simulation data.

Main Results:

• The same potential energy minima (inherent structures, IS) were identified in the PEL of quantum and classical water and its isotopes.
• Isotope substitution primarily alters the curvature and shape of PEL basins, leading to wider basins with decreased atom delocalization (H2O → HDO → D2O → T2O).
• Thermodynamic analysis showed that different isotopes sample PELs with varying curvatures at a given temperature, explaining property variations.

Conclusions:

• The PEL formalism provides an intuitive and rigorous framework for understanding isotopic effects in quantum liquids.
• The topography of the PEL dictates the system's dynamics (molecular diffusion), as evidenced by the continued validity of the Adam-Gibbs relation for all isotopes.
• Isotope substitution in water affects the dynamics and thermodynamics by modifying the PEL's basin shapes around inherent structures.