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Science 3 May 2002:
Vol. 296. no. 5569, pp. 875 - 878
DOI: 10.1126/science.1068053
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Research Articles

A SN2 Reaction That Avoids Its Deep Potential Energy Minimum

Lipeng Sun,1 Kihyung Song,2 William L. Hase1*

Chemical dynamics trajectory simulations were used to study the atomic-level mechanisms of the OH- + CH3F rightarrow  CH3OH + F- SN2 nucleophilic substitution reaction. The reaction dynamics, from the [OH···CH3···F]- central barrier to the reaction products, are simulated by ab initio direct dynamics. The reaction's potential energy surface has a deep minimum in the product exit channel arising from the CH3OH···F- hydrogen-bonded complex. Statistical theories of unimolecular reaction rates assume that the reactive system becomes trapped in this minimum and forms an intermediate, with random redistribution of its vibrational energy, but the majority of the trajectories (90%) avoided this potential energy minimum and instead dissociated directly to products. This finding is discussed in terms of intramolecular vibrational energy redistribution (IVR) and the relation between IVR and molecular structure. The finding of this study may be applicable to other reactive systems where there is a hierarchy of time scales for intramolecular motions and thus inefficient IVR.

1 Department of Chemistry, Wayne State University, Detroit, MI 48202, USA.
2 Department of Chemistry, Korea National University of Education, Chongwon, Chungbuk 363-791 Korea.
*   To whom correspondence should be addressed. E-mail: wlh{at}cs.wayne.edu

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