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Structural studies of the water hexamer
Gina Hincapié1, Nancy Acelas, Marcela Castaño
1Instituto de Química, Universidad de Antioquia, AA 1226 Medellín, Colombia.
The Journal of Physical Chemistry. A
|July 2, 2010
Summary
The study reveals complex structures for water hexamers, with many isomers contributing significantly. Compact forms are favored by electronic energy, while cyclic structures dominate when considering temperature and entropy.
Area of Science:
- Computational Chemistry
- Physical Chemistry
- Molecular Modeling
Background:
- Water clusters, particularly the hexamer (H2O)6, exhibit complex potential energy surfaces.
- Understanding the structural diversity and stability of water hexamers is crucial for solvation and atmospheric chemistry.
Purpose of the Study:
- To identify and characterize structural isomers of the water hexamer.
- To analyze the potential energy surface and relative stabilities of these isomers.
- To investigate the influence of electronic energy versus thermodynamic factors on structural preferences.
Main Methods:
- Ab initio calculations using Møller–Plesset perturbation theory (MP2) with the 6-311++g** basis set.
- High-level coupled-cluster calculations [Coupled Cluster with Singles and Doubles (CCSD(T))] with augmented correlation-consistent polarized valence double-zeta (aug-cc-pvdz) basis set for verification.
- Analysis of potential energy surfaces, including zero-point energy corrections and finite temperature/entropy effects.
Main Results:
- Identified 24 structural isomers of the water hexamer on the MP2/6-311++g** potential energy surface.
- Found at least 15 isomers within 3 kcal/mol of the most stable conformation, indicating a complex surface.
- Observed a quadratic correlation between proton-to-hydrogen bond center distance and oxygen-oxygen distances.
- MP2 and CCSD(T) methods showed agreement in binding energies but differed in stabilization orderings.
- Electronic energies favored compact structures, while thermodynamic considerations favored noncompact cyclic structures.
Conclusions:
- The water hexamer possesses a highly complex potential energy surface with numerous low-energy isomers.
- Structural preferences depend significantly on whether electronic energy or thermodynamic factors (temperature, entropy) are considered.
- This complexity has implications for modeling water's behavior in various environments.
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