Comparison of Theoretical Methods for Predicting Tunneling Rates: The Alanine + Hydrogen Atom Reactions at Low...

Area of Science:

• Astrochemistry
• Chemical Kinetics
• Quantum Mechanics

Background:

• Hydrogen-atom-abstraction reactions are crucial in astrochemistry.
• Quantum tunneling significantly influences reaction mechanisms at low interstellar medium temperatures.
• The accuracy of theoretical methods for predicting reaction rates under these conditions is uncertain.

Purpose of the Study:

• To compare the reliability of theoretical methods for calculating reaction rate constants in astrochemistry.
• To evaluate classical transition state theory (TST), semiclassical transition state theory (SCTST), and ring-polymer instanton theory for H- and D-atom abstraction reactions of alanine.
• To identify the limitations of SCTST at low temperatures.

Main Methods:

• Utilized ring-polymer instanton rate theory, a method known for accurate tunneling-driven reaction rates.
• Employed classical transition state theory (TST) and semiclassical transition state theory (SCTST) for comparison.
• Analyzed H- and D-atom abstraction reactions of the astrochemically relevant molecule alanine.

Main Results:

• Classical TST significantly underestimates reaction rates without tunneling corrections.
• SCTST provides good agreement with instanton theory across a wide temperature range, including deep tunneling regimes.
• SCTST deviates from instanton rates at very low temperatures, indicating limitations in describing relevant potential energy surface regions.

Conclusions:

• Ring-polymer instanton theory offers accurate predictions for tunneling-driven reactions in astrochemistry.
• SCTST has limitations at very low temperatures due to inadequate treatment of the potential energy surface.
• Findings can guide strategies for improving the applicability of SCTST in astrochemistry.