BrCl elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields.

By using a dc-slice imaging technique, photodissociation of 1,2-CHBrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl in an 80 fs laser field. The occurrence of fragment ion BrCl in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl in this experiment. The relative quantum yield of the BrCl channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl and its partner ion CH , we concluded that BrCl came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-CHBrCl. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl detachment from 1,2-CHBrCl has been examined in detail. According to the calculations, two transition state structures tended to correlate with the reactant 1,2-CHBrCl and the products BrCl + CH . In this entire dissociation process, the C-Br and C-Cl bond lengths were observed to elongate asymmetrically, that is, the C-Br chemical bond broke firstly, and subsequently a new Br-Cl chemical bond started to emerge while the C-Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl detachment.