Three-Body Collisions Driving the Ion-Molecule Reaction C₂^- + H₂ at Low Temperatures

Area of Science:

• Physical Chemistry
• Chemical Physics
• Astrochemistry

Background:

• Understanding ion-molecule reactions is crucial for various fields, including astrochemistry and plasma physics.
• The reaction between carbon dimer anions (C₂⁻) and hydrogen (H₂) is relevant for interstellar medium chemistry.
• Previous studies have explored similar reactions, but detailed low-temperature rate coefficients and mechanisms remain areas of active research.

Purpose of the Study:

• To experimentally determine the three-body reaction rate of C₂⁻ with H₂ at cryogenic temperatures.
• To theoretically investigate the reaction mechanism using ab initio calculations and variational transition state theory (VTST).
• To elucidate the role of quantum effects, such as tunneling, in the reaction dynamics.

Main Methods:

• Experiments were conducted using a cryogenic 16-pole radio frequency ion trap.
• Measurements were performed in the temperature range of 10–28 K.
• Ab initio calculations and variational transition state theory (VTST) were employed for theoretical analysis.

Main Results:

• The reaction was found to proceed exclusively via three-body collisions at low temperatures.
• The experimentally determined termolecular rate coefficient was fitted to a temperature-dependent form.
• VTST calculations, incorporating quantum effects, successfully reproduced the experimental rate coefficients and explained the dominance of three-body pathways.

Conclusions:

• The study provides crucial experimental and theoretical data for the C₂⁻ + H₂ reaction at low temperatures.
• Variational transition state theory, including quantum tunneling, is essential for accurately describing the reaction mechanism.
• The findings contribute to a deeper understanding of ion-molecule reactions in cold environments, relevant to astrochemistry.