Is Quantum Above-Barrier Reflection Important for Molecular Barrier Crossing Rates?

Area of Science:

• Chemical kinetics
• Quantum mechanics
• Theoretical chemistry

Background:

• Quantum tunneling and above-barrier reflection significantly influence reaction rate constants.
• The parabolic barrier approximation is standard but may be insufficient, especially at moderate-to-high temperatures.
• Anharmonicity effects are increasingly recognized as important for accurate rate constant calculations.

Purpose of the Study:

• To develop and apply an analytic theory for thermal rates including anharmonicity up to order ℏ⁴.
• To investigate the impact of anharmonicity on quantum tunneling and above-barrier reflection.
• To provide a method for assessing the validity of the parabolic barrier approximation.

Main Methods:

• Analytic theory expansion of thermal rates up to order ℏ⁴.
• Computation of high-order potential derivatives at the barrier top.
• Application to six different chemical reactions.

Main Results:

• Anharmonicity can be straightforwardly incorporated into rate theories using high-order derivatives.
• A methodology is presented to assess the parabolic barrier approximation's validity.
• Large reaction asymmetries can lead to significant quantum above-barrier reflection and transmission coefficients less than unity.

Conclusions:

• The parabolic barrier approximation is insufficient when anharmonicity is significant.
• Accurate quantum chemical kinetics requires considering anharmonic effects, especially for asymmetric potentials.
• The developed methodology aids in understanding and predicting reaction dynamics under quantum mechanical influence.