Water inertial reorientation: hydrogen bond strength and the angular potential

Area of Science:

• Physical Chemistry
• Spectroscopy
• Molecular Dynamics

Background:

• Understanding water's short-time orientational dynamics is crucial for its unique properties.
• Previous studies have explored water relaxation, but ultrafast dynamics remain less understood.
• The role of hydrogen bonding in ultrafast water reorientation needs further investigation.

Purpose of the Study:

• To investigate the ultrafast orientational relaxation of water molecules.
• To explore the temperature dependence of inertial and slow orientational motion.
• To correlate orientational dynamics with hydrogen bond strength and network effects.

Main Methods:

• Utilized ultrafast infrared pump-probe spectroscopy on the OD stretching mode of HOD in H2O.
• Studied temperature effects ranging from 1°C to 65°C.
• Employed a new harmonic cone model to analyze experimental data.

Main Results:

• Observed a sharp inertial drop followed by a slower decay in anisotropy.
• Inertial component amplitude correlates with OD stretching frequency at high temperatures, but not low.
• Hydrogen bond strength correlates with inertial decay at high temperatures; collective effects dominate at low temperatures.

Conclusions:

• Water's short-time orientational relaxation involves both inertial and network-dependent dynamics.
• Hydrogen bond strength is a key factor at higher temperatures, while collective effects dominate at lower temperatures.
• The harmonic cone model provides estimates of intermolecular angular potential frequencies (~400 cm⁻¹).