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

Abstract

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.

Photodissociation Mechanism Study of 1, 3-Dibromopropane using Theoretical Calculations

“…Table 3 represented the bond parameters, bond length and bond angles were computed by a PM3 method. The values of C-Br, C 1 -C 2 bond length also, C 1 -C 2 -C 3 angle reached the values computed by the DFT method in recent studies [12].…”

“…1,3-dibromopropane present in three conformations with respect to the rotation along the C-C bonds, these are the antianti, anti-gauche and the gauche-gauche conformation and there is a conformational equilibrium between these isomers [11][12].…”

Real-time investigation of the photodissociation dynamics of p-chlorotoluene and p-dichlorobenzene

“…The line width of the femtosecond laser in our experiment was about 25 nm at 810 nm which can cover rich energy levels in the excited states. The quantum beat, which is a fundamental phenomenon, has been extensively investigated in the photodynamics of the excited electronic states of polyatomic molecules [24,25]. p-Dichlorobenzene is an asymmetric rotor with its a-axis along the C-Cl bond, and its c-axis perpendicular to the plane of the ring [26].…”

Pseudo‐Bimolecular [2+2] Cycloaddition Studied by Time‐Resolved Photoelectron Spectroscopy

“…[16] Recently, we have studied this phenomenon in a series of organic molecules by using both computational and fs time-resolved experimental methods. [17][18][19][20][21] In the Hofmann-Lçffler and Barton reactions, where the key 1,5-hydrogen transfer step of the reaction has to be preceded by a conformational change, no ultrafast reactivity is observed. [19][20][21] In 1,3-dibromopropane, the photo-Abstract: The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented.…”