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Published August 22, 2002 | public
Journal Article

Coherent Dynamics in Complex Elimination Reactions: Experimental and Theoretical Femtochemistry of 1,3-Dibromopropane and Related Systems


Dynamics of the elimination reaction of 1,3-dibromopropane is studied here using femtosecond time-resolved mass spectrometry. It is shown that the complex reaction involving 27 internal degrees of freedom, which is initiated at a total energy of 186 kcal mol^(-1) (corresponding to a n → 5p Rydberg transition), can be described in a reduced space of two coordinates. The first coordinate is the coherent torsional motion involving the two C−Br bonds. The period was observed to be 680 fs. The second coordinate is the C−Br bond-breaking coordinate. The cleavage occurs in 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. The reaction channels were identified with the aid of density functional theory calculations. Analyses of orbital populations, energies, and ionization potentials for the different conformations are entirely consistent with the observation of the oscillatory coherent motion and the phase shifts that are observed between certain transients. The interactions of the lone-pairs of the two bromine atoms are shown to be the key for changing the ionization characteristics along the torsional coordinate and thereby enabling the selective probing of vibrational coherence.

Additional Information

© 2002 American Chemical Society. Received: August 28, 2001; In Final Form: November 5, 2001. Publication Date (Web): December 14, 2001. This work was supported by the US 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. T.I.S. acknowledges Statens Naturvidenskabelige Forskningsraad and Danmark-Amerika fondet for support. We thank Professor John E. Baldwin and members of his group, Dhazmesh B. Patel and Dr. Rajesh S. Shukla, at Syracuse University for the preparation of the deuterium-labeled DBP and for the collaboration.13 We also thank Prof. Joseph Casanova for the preparation of 1-bromo-3-iodopropane and helpful discussions and the summer research fellows, Wee Kang Chua and Frank D. Duchenaux, for their assistance. Finally, we acknowledge the helpful discussions with Dr. Klaus B. Møller and Prof. John D. Roberts.

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