Is Quantum Above-Barrier\nReflection Important for\nMolecular Barrier Crossing Rates?

Understanding\nquantum tunneling and above-barrier reflection effects\non unimolecular and bimolecular reaction rate constants remains challenging\nto this very day. In many applications, especially when considering\nmoderate-to-high temperatures, the “standard” procedure\nis to use the parabolic barrier approximation. Recent work has shown\nthough that this may be insufficient, and one cannot ignore anharmonicity.\nIn this work, we study the analytic theory, including anharmonicity\nobtained when expanding the thermal rate up to order <i>ℏ</i><sup>4</sup>. Such theories need high-order derivatives of the potential\nat the barrier top. We show that such derivatives are computed straightforwardly\nfor six different reactions. We suggest a straightforward methodology\nfor assessing whether the parabolic barrier approximation is valid\nand show that when the reaction asymmetry is large, this may lead\nto significant quantum above-barrier reflection and transmission coefficients,\nwhich are less than unity.