Femtosecond observation of benzyne intermediates in a molecular beam: Bergman rearrangement in the isolated molecule

Femtosecond observation of benzyne intermediates in a molecular beam: Bergman rearrangement in the isolated molecule

Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125

Contributed by Ahmed H. Zewail

Abstract

In this communication, we report our femtosecond real-time observation of the dynamics for the three didehydrobenzene molecules (p-, m-, and o-benzyne) generated from 1,4-, 1,3-, and 1,2-dibromobenzene, respectively, in a molecular beam, by using femtosecond time-resolved mass spectrometry. The time required for the first and the second C-Br bond breakage is less than 100 fs; the benzyne molecules are produced within 100 fs and then decay with a lifetime of 400 ps or more. Density functional theory and high-level ab initio calculations are also reported herein to elucidate the energetics along the reaction path. We discuss the dynamics and possible reaction mechanisms for the disappearance of benzyne intermediates. Our effort focuses on the isolated molecule dynamics of the three isomers on the femtosecond time scale.

Footnotes

↵ * To whom reprint requests should be addressed. E-mail: zewail{at}caltech.edu.
Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.030524797.
Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.030524797
↵ † The effect of spin-orbit coupling was not considered in our DFT calculations, i.e., the asymptotic channels of Br (²P_3/2) and Br (²P_1/2) have the same energy. To correct for the internal energy, the energy shift (± 5.25 kcal⋅mol⁻¹) of these two states, giving the spin-orbit splitting of 10.5 kcal⋅mol⁻¹, was taken into account for the two Br atoms.
Abbreviations:

DFT,

density functional theory;

amu,

atomic mass unit;

PES,

potential energy surface