The water dimer reaction OH + (H₂O)₂ → (H₂O)-OH + H₂O

Area of Science:

• Chemical Kinetics
• Atmospheric Chemistry
• Computational Chemistry

Background:

• The hydroxyl radical (OH) is a key species in atmospheric chemistry.
• Water clusters, like the water dimer ((H2O)2), play a significant role in atmospheric processes.
• Understanding the reaction dynamics of OH with water clusters is crucial for atmospheric modeling.

Purpose of the Study:

• To investigate the stationary points and reaction pathways for the OH + (H2O)2 reaction.
• To determine the energetic profile of the reaction, including barriers and complex stability.
• To provide theoretical data for experimental validation in spectroscopy.

Main Methods:

• High-level ab initio calculations using the coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) method.
• Utilized correlation-consistent basis sets up to quadruple zeta (cc-pVQZ) for high accuracy.
• Calculated harmonic vibrational frequencies and zero-point vibrational energies for stationary points.

Main Results:

• The entrance complex, (H2O)2OH, is found to be stable, lying 10.8 kcal mol-1 below reactants.
• Seven unique transition states were identified.
• One barrierless pathway exists where OH captures a water molecule. The lowest hydrogen abstraction barrier (TS1) is 5.9 kcal mol-1, with others at 17.8 (TS2) and 18.4 (TS3) kcal mol-1.

Conclusions:

• The reaction between OH and water dimer is energetically favorable and proceeds through multiple pathways.
• The identified stable complex and low-barrier pathways suggest potential observability and reactivity in atmospheric conditions.
• Theoretical findings provide a foundation for experimental spectroscopic studies and further atmospheric mechanism development.