High-Density "Windowpane" Coordination Patterns of Water Clusters and Their NBO/NRT Characterization

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Cluster mixture models indicate higher coordination and density are needed for water clusters at high pressures.
• Existing models often rely on tetrahedral hydrogen-bonding motifs.

Purpose of the Study:

• To demonstrate the formation of proton-ordered water clusters with increased coordination and density.
• To investigate novel cluster structures and their bonding characteristics.
• To challenge dichotomous views of intermolecular vs. intramolecular bonding.

Main Methods:

• Assembly of proton-ordered water clusters from cyclic tetramers or twisted bicyclic heptamers.
• Formation of extended "Aufbau" sequences of two-, three-, and four-coordinate "windowpane" motifs.
• Computation of free energy and natural resonance theory (NRT) bond orders.

Main Results:

• Stable "windowpane" motifs with sharply reduced bond angles (~90°) were formed.
• NRT bond orders quantitatively described cluster stability and linkage strengths.
• NRT demonstrated consistency across supra-integer and sub-integer bond orders.

Conclusions:

• Hydrogen bonding exemplifies resonance-covalent (fractional) bonding.
• The study discounts the strict separation of electrostatics and covalency in bonding descriptions.
• Novel water cluster structures provide insights into water behavior at high pressures.