The primary gas phase hydration shell of hydroxide.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The coordination number of hydroxide (OH-) in its primary hydration shell has been a subject of debate, with experimental and theoretical studies yielding conflicting results (three vs. four water molecules).
• Understanding the hydration structure of ions is crucial for various chemical and biological processes.

Purpose of the Study:

• To provide direct experimental and computational evidence for the number of water molecules in the primary hydration shell of hydroxide.
• To resolve the discrepancy between experimental and theoretical findings regarding hydroxide's hydration number.

Main Methods:

• High-resolution cryogenic experimental photoelectron spectroscopy was employed to study hydroxide-water clusters (OH-(H2O)n, n=2-5).
• High-level quantum chemical computations were performed to complement experimental data.
• Analysis of electron binding energies served as key signatures for determining molecular conformations.

Main Results:

• Well-defined photoelectron spectra provided accurate electron binding energies for hydroxide-water clusters.
• While OH-(H2O)3 and OH-(H2O)4 clusters showed similar, hard-to-distinguish binding energies, OH-(H2O)5 exhibited a predominant conformation.
• The predominant conformation of OH-(H2O)5 revealed a four-coordinated hydroxide binding motif.

Conclusions:

• The study provides unambiguous evidence that the gas-phase coordination number of hydroxide is four.
• This resolves the long-standing debate regarding the number of water molecules in hydroxide's primary hydration shell.
• The combined spectroscopic and computational approach offers a powerful method for elucidating ion hydration structures.