UV photodissociation of C2F5Br, C2F5I, and 1,2-C2F4BrI

Abstract

The photodissociation reactions of C2F5Br, C2F5I, and 1,2-C2F4BrI have been studied at 248 and 193 nm using the crossed laser-molecular beam technique. Photodissociation of 1,2-C2F4BrI was also studied at 266 nm. We find that: (i) C2F5Br undergoes C–Br bond fission at 193 nm (but does not absorb at 248 nm); (ii) C2F5I undergoes C–I bond fission at 248 nm (but does not absorb at 193 nm); (iii) 1,2-C2F4BrI undergoes only C–I bond fission at 248 and 266 nm, but at 193 nm both C–I and C–Br bond fission are observed, with a C–I:C–Br fission ratio of approximately 1.7:1. Center-of-mass recoil energy and angular distributions are determined for each of these photodissociation reactions. These results combined with those of other workers, are used to test simple predictions based on molecular orbital theory. The 266 nm data for C2F4BrI provide an approximate value of 19.3±3 kcal/mol for the dissociation energy of C2F4Br to C2F4+Br and also show that all or almost all the iodine produced in the primary C–I bond fission is in the excited 2P1/2 state. The prospect of doing bond-selective photochemistry in the ultraviolet is discussed.