Models for Intrinsic Non-RRKM Dynamics. Decomposition of the SN2 Intermediate Cl––CH3Br

Abstract

Chemical dynamics simulations, based on both an analytic potential energy surface (PES) and direct dynamics, were used to investigate the intrinsic non-RRKM dynamics of the Cl products were fit with multi-exponential functions. The intrinsic non-RRKM dynamics is more pronounced for the simulations with the analytic PES than by direct dynamics, with the populations for the former and latter primarily represented by tri-and bi-exponential functions, respectively. For the analytic PES and direct dynamics simulations, the intrinsic non-RRKM dynamics is more important for the isomerization pathway to form ClCH 3 -Br − than for dissociation to Cl − + CH 3 Br. Since the decomposition probability of Cl − -CH 3 Br is non-exponential, the Cl − -CH 3 Br unimolecular rate constant depends on pressure, with both high and low pressure limits. The high pressure limit is the RRKM rate constant and for the simulations with the analytic PES the rate constant decreased by a factor of 3.0, 5.6, and 4.3 in going from the high to low pressure limit for total energies of 40, 60, and 80 kcal/mol. For the direct dynamics simulations these respective factors are 2.4, 1.4, and 1.2. A separable phase space model with intermolecular and intramolecular complexes describes some of the simulation results, but overall models advanced for intrinsic non-RRKM dynamics give incomplete representations of the intermediate and product populations vs. time determined from the simulations.

Unimolecular Rate Constants versus Energy and Pressure as a Convolution of Unimolecular Lifetime and Collisional Deactivation Probabilities. Analyses of Intrinsic Non-RRKM Dynamics

“…The intrinsic non-RRKM dynamics of the Cl − ---CH 3 Br S N 2 intermediate were studied by trajectories on an analytic potential energy surface and by direct dynamics. 31 Different models were used to represent the phase space dynamics and nonexponential P(t), including a multiexponential function.…”

“…Effects of intrinsic non-RRKM dynamics on k uni (ω, E ) and k (ω, E ) have been investigated in previous studies. ,,− Multiexponential P ( t ) have been used to calculate k (ω, E ) in eq for model HCC → H + CC dissociation , to compare with chemical dynamics simulations and for Cl – ---CH 3 Cl → Cl – + CH 3 Cl dissociation to compare with chemical dynamics simulations and experiments. , Effects of fluctuations in quantum mechanical state-specific rate constants, and their distributions, on the unimolecular rate constants k uni (ω, E ) and k (ω, E ) have been considered. − , An example is the relationship between quantum dynamical calculations of the highly mode-specific decomposition for HOCl → OH + Cl and the Lindemann–Hinshelwood unimolecular rate constant k uni (ω, T ) in eq . In interpreting experimental measurements of k uni (ω, T ) for HCO → H + CO dissociation, Hippler and co-workers found that a mode-specific isolated resonance model represented their experimental results.…”

“…In interpreting experimental measurements of k uni (ω, T ) for HCO → H + CO dissociation, Hippler and co-workers found that a mode-specific isolated resonance model represented their experimental results. The intrinsic non-RRKM dynamics of the Cl – ---CH 3 Br S N 2 intermediate were studied by trajectories on an analytic potential energy surface and by direct dynamics . Different models were used to represent the phase space dynamics and nonexponential P ( t ), including a multiexponential function.…”

Breaking of a bond: when is it statistical?

“…Therefore, it is hardly surprising that several studies have identied reactions which are intrinsically non-RRKM in nature. [7][8][9][10][11][12][13][14] Such studies are leading to much needed insights into the dynamics, and hence aiding in identifying novel mechanisms and raising hopes for targeted control of the reaction. Clearly, it is crucial to develop models for reaction rates that eschew the statistical assumption and provide insights into the mechanisms for nonstatistical behaviour.…”

Unimolecular Dissociation of γ-Ketohydroperoxide via Direct Chemical Dynamics Simulations

“…A similar behavior was observed at 300 kcal/mol energy also, f 1 = 0.47, k 1 = 2.09 ps –1 , f 2 = 0.53, k 2 = 0.59 ps –1 , i.e., a fast and a slow component were observed. To investigate the nonstatistical dynamics, various coupled phase space models have been proposed, but such studies were not performed any further in the present work.…”