Effects of Temperature on Cs⁺(H₂O)₂₀ Clathrate Structure

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Cesium ion-water clusters (Cs+(H2O)20) exhibit exceptional stability due to a clathrate structure formed by three-coordinate water molecules.
• Understanding the temperature-dependent stability of these clathrate structures is crucial for various chemical and physical processes.

Purpose of the Study:

• To investigate the stability of Cs+(H2O)20 clathrate structures across a range of temperatures.
• To elucidate the structural transitions from clathrate to non-clathrate forms as a function of temperature.
• To explore the role of entropy in the stability of these water-ion clusters.

Main Methods:

• Infrared photodissociation (IRPD) spectroscopy was employed to analyze the vibrational modes of the water molecules.
• Spectra were recorded in the free-OH stretching region (approximately 3600-3800 cm-1).
• Experiments were conducted at ion cell temperatures ranging from 135 K to 355 K.

Main Results:

• At temperatures of 275 K and below, IRPD spectra showed a single acceptor-acceptor-donor band, characteristic of the stable clathrate structure.
• Above 275 K, a new, higher-energy acceptor-donor band appeared and increased in intensity, indicating the formation of non-clathrate structures.
• Comparison with Na+(H2O)20 revealed that non-clathrate structures contain both spectral bands, with stable intensities across the temperature range.

Conclusions:

• A rapid transition from clathrate to non-clathrate structures occurs with increasing temperature.
• The clathrate structure of Cs+(H2O)20 persists to some extent even at the highest temperatures studied.
• These findings provide novel insights into the influence of entropy on the stability of clathrate hydrates.