A master equation simulation for the ^•OH + CH₃OH reaction

Area of Science:

• Chemical Kinetics
• Atmospheric Chemistry
• Astrochemistry

Background:

• The reaction between hydroxyl radical (•OH) and methanol (CH3OH) is crucial in atmospheric and interstellar chemistry.
• Accurate rate coefficients are needed for modeling chemical processes in diverse environments.

Purpose of the Study:

• To compute reaction rate coefficients for •OH + CH3OH over wide temperature (10-2500 K) and pressure (10⁻¹-10⁴ Torr) ranges.
• To elucidate the temperature and pressure dependence of the reaction mechanism.
• To investigate the role of tunneling at low temperatures.

Main Methods:

• A combined theoretical approach: fixed-J two-dimensional master-equation, semi-classical transition state theory, and variational Rice-Ramsperger-Kassel-Marcus (RRKM) theory.
• Utilized a potential energy surface derived from the high accuracy extrapolated ab initio thermochemistry (HEAT) protocol.
• Calculated reaction rate coefficients and analyzed reaction pathways.

Main Results:

• The reaction is nearly pressure-independent above 250 K but strongly pressure-dependent at lower temperatures.
• At very low temperatures (≤ 50 K), tunneling dominates the CH3OH + •OH → CH3O• + H2O pathway, with rate constants increasing as temperature decreases.
• At higher temperatures (≥ 200 K), the thermodynamically favored CH3OH + •OH → •CH2OH + H2O pathway becomes dominant.
• Adjusting ab initio barrier heights allowed satisfactory reproduction of experimental rate constants from 200 to 1250 K.

Conclusions:

• The study provides accurate theoretical rate coefficients for the •OH + CH3OH reaction across a wide range of conditions.
• Distinct reaction mechanisms prevail at low (interstellar) and high (atmospheric/combustion) temperatures, with tunneling being significant at low temperatures.
• The theoretical model successfully reproduces experimental data, validating its accuracy for chemical modeling.