45236-AC6
New Theoretical Methods for Reaction Dynamics
The aim of the PRF sponsored research was to develop new theoretical techniques to study highly reactive intermediate species and transition state complexes that occur in chemical reactions. The detailed dynamical properties of such species are critically important in a wide variety of chemical mechanisms. The following is a brief summary of our findings.
(1) A Transition State Perspective of Bimolecular Reactions
We had proposed to investigate the utility of a new view of bimolecular reaction dynamics based on the use of the quantum bottleneck states associated with the transition state (TS) of the reaction. By decomposing the S-matrix into factors representing the passage into and out of the TS, we hoped to glean physical insight the product state distribution and DCS of some A+BC type chemical reactions. The method has proven to work quite well. We have found that the product state distribution for the D+H2 reaction could be understood in detail using our approach.[i] [ii] A new Franck-Condon like approximation was introduced to further reveal the underlying within the state-to-state-to-state picture.[iii] The model was particularly useful in understanding the reactivity of different helicity state of the reagents. It was found that some anomalous molecular beam scattering results of XM Yang and coworkers could be perfectly rationalized using our approach.[iv] We have found that helicity selected differential cross sections along the collision axis could be inferred from unpolarized beam scattering experiments using an approach based on our methods.[v] Recently, we have discovered that some rather mysterious vibrational branching observed for F+HDàHF(v’)+D could be understood using our model.[vi]
(2) The F+HClà HF+Cl Reaction
We proposed to study the detailed reaction dynamics of this reaction using a newly developed potential energy surface.[vii] It was found that the huge disagreement between molecular beam scattering and thermal reaction dynamics could be traced to the rotational enhancement of the reaction probability. Higher rotational states of the HCl reagent were found to be vastly more reactive than the lower rotational states. A simple physical picture of this enhancement was developed.[viii] Furthermore, we have found that 2/3 of the reaction proceeds through reactive resonance states at room temperature. This resonant mechanism is likely the cause of the pronounced curvature observed in the Arrhenius plot of the rate constant. The resonance states have been studied in detail and assigned quantum numbers using a full calculation of Smith Q-matrix (i.e. the lifetime matrix).[ix]
(3) The F+CH4 Reaction
The F+CH4 reaction has been recently studied experimentally by the group of K. Liu.[x] The results strongly suggest an essential role of an F-H-CH3 intermediate complex. We have developed a new approach for the study of high-dimensional reactive resonances which is being applied to this system. The method employs an L2 basis for wave packet propagation in the trapping well. It is found that the dynamics of the complex is greatly affected by the reagent initial state which determines the coherent superposition of overlapping resonance states that are accessed in the collision.
[ii] J. Zhang, D. X. Dai, S. A. Harich, C. C. Wang, S. A. Harich, X. Wang, X. M. Yang, M. Gustafsson, and R. T. Skodje, "The State-to-State-to-State Dynamics of the D+H2àHD+D Reaction: Control of Transition State Pathways via Reagent Orientation", Phys. Rev. Lett. 96, 93201 (2006).
