Predicting nonstatistical unimolecular reaction rates using Kramers’ theory

Abstract

Articles you may be interested inOn the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions A theory for nonisothermal unimolecular reaction rates Response to "Comment on 'On the relation between unimolecular reaction rates and overlapping resonances'" [A method for computing unimolecular reaction rate constants in the IVR-limited regime is presented. It is based on Kramers' energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. Applications to some bond fission reactions demonstrate that the method accurately predicts the rate constants for wide range of energies by using the result of a dynamical calculation of the reaction rate at a single energy to determine the friction coefficient. Examination of the energy exchange in the reaction coordinate provides a qualitative understanding of the validity of the approach for treating unimolecular reactions. Thus, the method provides a practical means of calculating reaction rates in the IVR-limited regime at considerable savings of computer time than that required by standard classical trajectory calculations.

Thermal and Microcanonical Rates of Unimolecular Reactions from an Energy Diffusion Theory Approach

“…The present approach enables us to extend our previous studies [10][11][12][13][14] of IVR-controlled reactions to the more general case whether the rate of IVR is faster or slower than the rate of reaction. Assuming constant friction, the method predicts the rate constants for a wide range of energies by requiring only one rate from a trajectory calculation at a single energy to determine the friction coefficient.…”

“…16 We have applied the energy diffusion theory to compute the microcanonical rates for large molecules in the high-energy (diffusion) regime by assuming that the IVR rate is much slower and thus determines the dynamic rate. [11][12][13][14] The present approach enables us to obtain the reaction rates more generally for the entire range of energies from the statistical to the diffusion limit.…”

“…[10][11][12][13][14][15] The basic idea is to consider the reaction coordinate as a subsystem that interacts with a bath consisting of the rest of the modes. The diffusion theory can then be applied in a straightforward way by taking the average energy per mode as the effective temperature.…”

Intramolecular dynamics diffusion theory approach to complex unimolecular reactions

“…͑4͒ is the energy diffusion coefficient D(E). As in our previous study, 7 we use Kramers' result 8,17 for a onedimensional particle interacting with a heat bath. Assuming constant friction, Kramers showed that the energy diffusion coefficient is…”

“…7 We applied the method to two bondfission reactions. The calculated rates from the IDDT are in good agreement with those from trajectory simulations, showing that the method is accurate for computing the unimolecular reaction rates.…”