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.