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

Chiara Aieta¹, Michele Ceotto¹, Eli Pollak²

¹Dipartimento di Chimica, Università degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy.

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|September 12, 2024

Summary

This summary is machine-generated.

Quantum tunneling and reflection effects are crucial for reaction rates. This study includes anharmonicity in calculations, revealing its impact on reaction dynamics and the limitations of standard approximations.

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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.