Intramolecular dynamics diffusion theory approach to complex unimolecular reactions

Abstract

A theory for nonisothermal unimolecular reaction rates We consider the relation between observed pump-probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant within the bandwidth of the pump laser which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration.

Recent applications of boxed molecular dynamics: a simple multiscale technique for atomistic simulations

“…For us, BXD has its origin in the intramolecular dynamics diffusion theory (IDDT) [ 4 – 8 ], which has demonstrated that long-time reaction rates can be recovered from a set of short-time MD simulations [ 8 ]. Longer time-scale dynamics may be calculated from the set of rate constants obtained from short-time simulations within each box.…”

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.…”

Predicting nonstatistical unimolecular reaction rates using Kramers’ theory

“…Using the idea of intrinsic reaction coordinate, We have applied it to the case of ring fission in RDX ͑hexahydro-1,3,5-trinitro-1,3,5-triazine͒ molecule where the reaction coordinate is a combination of several modes, and have obtained good agreement with trajectory simulations. 18 This suggests that the method may be generally applicable to a wide range of problems.…”