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

Abstract

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.