Mechanistic Inside into the Gas-Phase Reaction.

Area of Science:

• Atmospheric Chemistry
• Quantum Chemistry
• Chemical Kinetics

Background:

• The reaction between hydroxyl radical (OH) and nitrogen dioxide (NO2) is crucial for nighttime atmospheric chemistry.
• This reaction is known to proceed through two potential pathways: formation of nitric acid (HNO3) or nitryl radical (NO3).
• Previous experimental studies have yielded conflicting results regarding the dominant or exclusive pathway.

Purpose of the Study:

• To elucidate the reaction mechanism between OH and NO2 using advanced computational methods.
• To determine the exclusive reaction channel at play in nighttime atmospheric conditions.

Main Methods:

• High-level quantum chemical calculations, including coupled-cluster methods with triple and partial quadratic excitation corrections, were employed for accurate energy calculations.
• A master equation approach was utilized to compute rate constants over a temperature range of 213–400 K.
• Theoretical investigation of reaction energetics and dynamics.

Main Results:

• The study's findings strongly suggest that the reaction between OH and NO2 proceeds exclusively through the nitryl radical (NO3) formation pathway.
• Calculated rate constants support the dominance of the NO3 channel under relevant atmospheric conditions.
• The nitric acid (HNO3) formation pathway was found to be negligible.

Conclusions:

• The reaction between OH and NO2 exclusively forms NO3, resolving a long-standing debate in atmospheric chemistry.
• This finding has significant implications for atmospheric models and understanding the fate of nitrogen oxides.
• The computational approach provides a robust mechanism for studying atmospheric reactions.