Photodissociation dynamics of 1-bromo-1-chloro-2,2,2-trifluoroethane at 157 nm

The photodissociation dynamics of the title molecule at 157 nm has been studied using photofragment translational spectroscopy. The molecule was found to dissociate competitively via CHBrCF3 + Cl, CHClCF3 + Br, CHCF3 + Br + Cl, and CClCF3 + HBr channels with the branching ratio 1.0:0.75(-0.00)(+0.19):0.40(-)0.20(+0.40):0.55:(+0.55)(-0.28). This result indicates that the stronger C-Cl bond breaks preferentially over the weaker C-Br bond, similar to the case of the photodissociation of CBrF2CHClF studied previously. Compared with the C-Cl:C-Br branching ratio (1.0:0.27(-0.04)(+0.15)) for the photodissociation of CBrF2CHClF at 157 nm, this branching ratio indicates that a larger fraction of molecules dissociate via C-Br bond rupture in this case than in the case of CBrF2CHClF as a result of a higher transition probability from the initially excited n sigma*(C-Cl) diabatic surface to the n sigma*(C-Br) diabatic surface. This result is qualitatively consistent with the prediction from previous theoretical works that the transition probability from the n sigma*(C-Cl) to the n sigma*(C-Br) diabatic surface increases with decreasing spatial distance between the Br and Cl atoms. The secondary dissociation of CHBrCF3 and CHClCF3 was also observed following 1,2-fluorine migration. This mechanism was supported by an energy diagram obtained by ab initio molecular orbital (MO) calculations at the QCISD(T) level of theory. The dynamics of the triple dissociation, CHCF3 + Br + Cl, is discussed using the potential energy surfaces of CH2BrCl constructed by ab initio MO calculations.