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Science 11 January 2008:
Vol. 319. no. 5860, pp. 183 - 186
DOI: 10.1126/science.1150238
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Reports

Imaging Nucleophilic Substitution Dynamics

J. Mikosch,1 S. Trippel,1 C. Eichhorn,1 R. Otto,1 U. Lourderaj,2 J. X. Zhang,2 W. L. Hase,2 M. Weidemüller,1 R. Wester1*

Anion-molecule nucleophilic substitution (SN2) reactions are known for their rich reaction dynamics, caused by a complex potential energy surface with a submerged barrier and by weak coupling of the relevant rotational-vibrational quantum states. The dynamics of the SN2 reaction of Cl– + CH3I were uncovered in detail by using crossed molecular beam imaging. As a function of the collision energy, the transition from a complex-mediated reaction mechanism to direct backward scattering of the I– product was observed experimentally. Chemical dynamics calculations were performed that explain the observed energy transfer and reveal an indirect roundabout reaction mechanism involving CH3 rotation.

1 Physikalisches Institut, Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany.
2 Department of Chemistry and Biochemistry, Texas Tech University (TTU), Lubbock, TX 79409, USA.

* To whom correspondence should be addressed. E-mail: roland.wester{at}physik.uni-freiburg.de

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