The CH(X²Π) + H₂O reaction: two transition state kinetics

Area of Science:

• Chemical Kinetics
• Theoretical Chemistry
• Astrochemistry

Background:

• Methylidyne (CH) is a key radical in interstellar clouds.
• Understanding CH reactions with abundant molecules like water is vital for astrochemistry.
• Previous theoretical studies on CH + H2O reaction kinetics were limited.

Purpose of the Study:

• To theoretically re-investigate the reaction kinetics of ground state methylidyne (CH) with water vapor (H2O).
• To characterize the reaction mechanism, including potential energy surfaces and transition states.
• To calculate the temperature-dependent rate coefficient for the CH + H2O reaction.

Main Methods:

• High-level coupled cluster computations for electronic structure.
• Semi-classical transition state theory (SCTST) for rate calculations.
• Two-dimensional master equation simulations to account for complex reaction dynamics.

Main Results:

• The reaction proceeds via insertion into an H-O bond, forming a pre-reaction van der Waals complex (PRC) and subsequent isomerization.
• The calculated rate coefficient k(T) exhibits a pronounced 'down-up' behavior with temperature.
• Tunneling and variational effects were found to be minor; the rate is nearly pressure-independent.

Conclusions:

• The study provides accurate theoretical rate coefficients for the CH + H2O reaction over a wide temperature range (50–3500 K).
• The complex temperature dependence of the rate coefficient is crucial for modeling interstellar environments.
• The findings contribute to a better understanding of chemical processes in interstellar clouds.