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Published March 15, 2001 | public
Journal Article

Direct Observation of Resonance Motion in Complex Elimination Reactions: Femtosecond Coherent Dynamics in Reduced Space


In this communication we report the observation of a resonant, coherent nuclear motion in the elimination reaction of 1,3-dibromopropane (DBP), a system with 27 internal degrees of freedom. The system was investigated using femtosecond time-resolved mass spectrometry, following excitation at a total energy E = 186 kcal mol^(-1) (n→5p Rydberg state). The vibrational coherence was observed with a period of 680 fs corresponding to the torsional vibration involving the two C−Br bonds. The C−Br bond cleavage occurs with a reaction time of 2.5 ps and yields the 3-bromopropyl radical, which subsequently reacts (cleavage of the second C−Br bond and ring closure) to give cyclopropane in 7.5 ps. These results elucidate the elementary steps and the mechanism:  In a reduced space of two coordinates, the reaction coordinate involves a coherent torsional motion and C−Br bond rupture. Density functional theory (DFT) and time-dependent DFT calculations were carried out to detail the potential energy surface.

Additional Information

© 2001 American Chemical Society. Received: November 9, 2000; In Final Form: January 24, 2001. Publication Date (Web): February 20, 2001. This work was supported by the U.S. Air Force Office of Scientific Research and the Office of Naval Research. C.K., a Feodor Lynen Fellow from the Alexander von Humboldt Foundation, acknowledges the foundation and Caltech for support. We thank Dharmesh B. Patel and Dr. Rajesh Shukla of Syracuse University for the preparation of the deuterium-labeled DBP. We are grateful to the summer research fellows Wee Kang Chua and Frank Ducheneaux for their assistance and to Dr. Klaus Møller and Profs. John D. Roberts and Joseph Casanova for helpful discussions.

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