CH3CI Central-Barrier Dynamics

Given the prominence of statistical theories in chemical reaction dynamics and kinetics, it was natural to use these theories in modeling studies of Sn2 reactions. However, more recent detailed examinations of Sn2 reactions of the type in reaction [83] have discovered a range of important nonstatistical attributes, arising from weak couplings between the X ---CH3Y intermolecular modes and CH3Y intramolecular modes. A particularly important feature of this work has been a close relationship between computational, experimental,and theoretical studies.The computational studies include quantum-dynamical and full-dimensional trajectory calculations. 

A potentially very significant finding from the trajectory studies is the trapping of trajectories in the central-barrier region of the potential energy surface, with concomitant recrossings of the barrier. The former suggests 

This MP2 direct dynamics trajectory study was performed with the VENUS96/GAUSSIAN98 package of programs, which includes an efficient predictor-corrector integration algorithm and Hessian updating. A 

Transition state theory assumes that each crossing of the central barrier leads to reaction. A total of 29 central-barrier crossings occurred for the 

10 trajectories. However, since no Sn2 reactions occurred, it is not possible to determine a definitive correction to TST. An estimate of this correction may be made by assuming that each trajectory that started in one C1 ---CH3C1 complex and ended in the other will ultimately be a reaction event. With this assumption the TST correction factor is 0.2, which is similar to the 0.1 correction factor estimated from the previous trajectory study,based on an analytic potential energy function. This latter value is an upper limit,as is the former value, since none of the trajectories reached the C1 ---CH3C1 asymptote and more recrossings of the central barrier are expected before this asymptote is reached for both the backward and forward integrations of the trajectories. 

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Sn2 Nucleophilic Substitution. 8. Central Barrier Dynamics for Gas Phase CD + CH3CI. 

CU-CH3CI decomposition MP2/6-31G- [C1-CH3-C1] central barrier recrossing and intrinsic non-RRKM dynamics [121]... 

If a wavepacket remains localized in the central-barrier region, it may then be possible to resolve its vibrational dynamics and spectrum. A direct probe of non-RRKM dynamics for the Cl ---CH3CI complex would involve measuring the lifetimes of its resonance states. 

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